How To Save Your Tiny House From Mold

How To Save Your Tiny House From Mold

How To Save Your Tiny House From Mold

Tiny houses are especially prone to moisture issues that lead to mold growth

Interior moisture and condensation control is very important in any structure but this topic has special concern to those of us building and living in tiny houses. Because the volume of space we are dealing with is so much smaller, any moisture added has faster and more profound effects on the building, leading to serious issues such as mold. Unfortunately there isn’t one simple solution to all condensation issues as there are a lot of factors that can cause problems. In this article, we cover how to save your tiny house from mold and moisture issues.


How To Save Your Tiny House From Mold

Our bathroom fan is nothing special but it does the job well. We had to make a custom cut for it to fit our venting.

Bathroom fans are a great way to vent moisture when showering; however, without a moisture sensor switch, they typically don’t do their job adequately. Most people leave the fan on while they bathe (and perhaps while they dry off), but that is most likely not long enough. Installing a moisture sensor switch in lieu of a standard light/fan switch vastly improves the effectiveness of the fan because as soon as the indoor relative humidity reaches a certain level, the fan turns on automatically. Conversely, when your indoor humidity drops below the set level, the fan shuts off. There is no need to guess when the levels are too high or low enough.

How To Save Your Tiny House From Mold

Our moisture dial allows us to set what our internal humidity levels depending on the external temperature

Our moisture sensor switch in hOMe has a handy chart to help us determine our setting. In our winters in southern Oregon, we normally keep our dial in the 30% range and that is enough to abate any condensation issues. The exact setting is something you can adjust day by day or seasonally (we just change ours seasonally). Most fans do not come with a sensor switch and so they must be purchased separately. They typically run anywhere from $25 to $60 depending on the features you choose. It’s money well spent.

This one measure alone eliminated all of the condensation issues we were experiencing our first winter in hOMe. Even when it’s 10F or colder outside, our metal window frames remain dry. It worked so well that is also completely eradicated a mold issue that was just starting underneath our kitchen sink (condensation was building on our water filter lines). Moisture sensors are something I recommend for every tiny house builder. If you already live in your tiny house and don’t have one installed, don’t fret, they are very easy to install retroactively. You don’t need a special fan or different wiring, so you can simply exchange the sensor switch for the old switch. The primary disadvantage of this system is that it vents out our heated air, requiring us to spend more money on propane for heating our hOMe.

HRVs and ERVs:

Heat Recovery Ventilators (HRV) and Energy Recovery Ventilators (ERV) are great options for regulating interior air quality. Which one you get depends on the climate you live in. There are many maps available online that show recommendations for where each should be used.

How To Save Your Tiny House From Mold

The Panasonic FV-04VE1 is and ERV, is pretty small and measures 29.6 x 18 x 13.5 inches, installs on the ceiling, weighs 24 pounds and costs about $350. This unit does not work well in very cold climates though so make sure to take that into consideration.

A general rule of thumb is that warmer areas with high humidity should use ERVs while dryer and cooler climates should use HRVs. As the names imply, HEAT recovery ventilators are able to recover heat from the exiting, stale air and then add it to the fresh air coming into the home. This is better for areas where cooler outdoor temperatures are the norm. An ENERGY recovery ventilator also recovers heat from the exiting, stale air when the exterior temperatures are cold, thus heating the incoming, fresh air. Further, an ERV removes excess heat during the summer months from exterior air, keeping the interior space cool, making it ideal for climates that experience both hot and cold temperatures. Finally, an ERV does a better job of regulating ambient internal humidity levels so that the home doesn’t dry out too much. This is just a basic description, so be sure to contact a manufacturer and ask them what system is best for your area.

HRVs and ERVs are active around the clock, providing a constant exchange of stale air from inside the house with fresh air from the outside. Air entering a house is conditioned through a passive exchange with the air passing out of the house making it much more energy efficient than simply using a fan. In fact, because of their exchange systems, HRVs and ERVs can recovery up to 90% of the energy that would otherwise be lost. They work slightly differently, but the concept is the same: they recover heat from the conditioned space for use in conditioning the fresh air being drawn into the house. In order for these systems to be effective, it is essential you let them work on their own “schedule”, meaning don’t turn them on and off manually.

There are some units that can work in small spaces, like the Panasonic FV-04VE1 (WhisperComfort) which is an ERV. It measures 29.6 x 18 x 13.5 inches, mounts on the ceiling, weighs 24 pounds and costs about $350. The WhisperComfort doesn’t do well in very cold climates though so please research if it is suitable for where you live.

How To Save Your Tiny House From Mold

The Lunos E2 is uber small, uses very little power, and was used on the Yukon climate tested Leaf 3 tiny house

For an HRV option, we’ve heard good things about the Lunos E2 which was used on the Yukon climate tested Leaf 3. It is extremely energy efficient and petite measuring just 9.5″ in length and 5 7/8″ in diameter. What’s more, it uses a ceramic core which absorbs heat from the air as it is pumped through the fan, rather than two sets of space hogging ducts that are typical on traditional HRV options. There is a minimum wall thickness for installation.  The site “475” which sells the Lunos for a hair above $1,000 states, “For projects with walls thinner than 12″, you may want to go with the Lunos e² Short (6.8″) version, which requires a minimum wall thickness of 7.5″; however, the short version comes with a slightly lower efficiency (~85%). Otherwise, furring the tube toward the interior is an option for your project to obtain the maximum (90.6%) efficiency in a shorter (ie 2×6) wall assembly.”

If you are off the power grid, the energy burden of these systems will need to be factored in. Some are more power hungry than others and for those of us generating our electricity from the sun, electrical systems must always be considered before any purchasing decision is made.


How To Save Your Tiny House From Mold

Our IKEA range hood in hOMe has a sleek profile maximizing space between it and the range

Kitchen fans are essential in a tiny house. I’m often asked if it is really necessary to have both a bathroom fan and a kitchen fan in a tiny house since the space is so small. The answer is yes. You don’t want to pull cooking grease and moisture through the house by exhausting it through the bathroom fan and you don’t want to pull excess shower moisture through the house to the kitchen either. Instead, you want to remove the moisture at the source and as quickly as possible. The longer it is in the house, the more likely it is to condense and cause issues.

I recommend that you start the fan before you turn on the stove (if gas) and leave it on for at least five minutes after you see any evidence of steam. Many people don’t realize that the combustion of gas contributes moisture to the interior (unless the appliance is direct vented) so turning the fan on before the gas appliance is ignited is a simple, smart step.

Something to consider with kitchen fans is the space afforded for their installation. Because kitchens are sometimes placed below a sleeping loft in a tiny hosue, the headroom is lower than in a standard home. There may not be enough space for your duct work to exit the building when using the provided materials. As such, you may need to work with an HVAC (Heating, Ventilation, and AC) professional to make custom ductwork for your fan. I would not expect that to cost more than $100 or so.

Another consideration is how much air a fan moves. Because conventional homes are getting bigger and custom kitchens now include 9 burner, commercial ranges, kitchen fans have gotten bigger too. The problem is that the larger fans move so much air, that they can “suck a house empty” in seconds. For example, the International Residential Code (IRC) states that the minimum exhaust rates for a kitchen fan are 100 cubic feet per minute (cfm) (intermittent) and 25 cfm (continuous), yet there are fans available that move 1200 cfm; twelve times the minimum requirements and way more than one could possibly need in a tiny or small house.


How To Save Your Tiny House From Mold

Passive make-up air ducts are very inexpensive, easy to install, and provide much needed make-up air in a well sealed tiny house

If your home is built tightly and your climate prevents you from living with open windows year round, you will have to mechanically manage the moisture in your home. That is what I’ve been discussing above. There is another piece to this puzzle though: make up air. The importance of make-up air cannot be overstated. Let’s jump back to the last point about kitchen fans “sucking a house empty” as this is a great way to explain the need for make-up air. Imagine turning on a 1200 cfm fan in a tiny house. For every minute that fan is on, 1200 cubic feet of air is dumped to the exterior. How is that air replaced inside the home? The reality is that if make-up air is not provided, it will be drawn in from leaks in the building envelope. This means areas where the seal is not tight around windows and doors, electrical outlets and switches, and even through the walls where there are gaps in the insulation and vapor barriers.

The real issue here is safety. If you have a tight house and you have a gas water heater, gas fireplace, or a wood stove, the negative pressure created by the exhaust appliance can actually cause backdrafting: the act of pulling gas, carbon monoxide, and/or smoke back into the house. Obviously, this is not a good scenario.

How To Save Your Tiny House From Mold

Though some active make-up air units heat incoming air, they come at a high price not only in purchase cost but also by taking up precious space and using electricity.

There are two main ways to properly provide make-up air: passive make-up air ducts and active make-up air systems. The IRC requires that you include a motorized damper on any make-up air system that is designed to come on automatically when an exhausting appliance with a cfm of 400 or higher is turned on. In a tiny house, you hopefully won’t need the automatic damper because you won’t have a kitchen fan (or bathroom fan) that moves 400 or more cfm.

In tiny house applications, a passive system is completely adequate and there really is no need for an active system; however, if you prefer to use an active system and you want to heat the air being drawn in, consider a “make-up air heater”. They are big, so you need to consider where you will place the unit. They also use power to heat that air so factor that into your decision, as well as the high price tag (about $1,000 a pop).


How To Save Your Tiny House From Mold

hOMe post house wrap installation

For those currently building or in the planning stages: be sure to understand how to properly instal insulation, vapor barriers, and house wrap. If you don’t get your systems right, you might end up with your dew point (the point where warm air condenses when exposed to cold temperatures) occurring inside your walls. If it does, you may not know there’s an issue until it’s too late. Be sure that any vapor barriers are placed according to code and manufacturer specifications. There is a lot of confusion around the use of vapor barriers, even within the industry itself, so make sure you know what to use and how to use it. For example, if using plastic as a vapor barrier, be sure to understand how to seal the joints properly. The vast majority of moisture problems are caused by air leakage, not diffusion (the process of moisture moving through building materials), so if your vapor barrier has gaps in it, air leakage can drive moisture through those gaps and cause problems inside the wall.

How To Save Your Tiny House From Mold

Foam gaskets for plugs and switches

Another area to consider are the light switches and plugs. You can buy foam seal gaskets and place them under the cover plates which helps seal up the air gaps. Plugs and switches are actually a major source of air leakage in a home, so this is an easy and inexpensive measure you can take. Another great idea is to caulk the faceplate to the wall as well as the openings in the back of the rough in box to stop air leakage. Also, be sure that your insulation job is done well and that no uninsulated gaps exist in the wall cavities. It sounds easy, but a proper insulation installation is actually quite challenging. Do your best to avoid rushing and take the time to pay extra attention to details. It is worth it.

Climate plays a huge role in all of this. The colder it is outside, the worse it will be for interior condensation. As warm/conditioned air comes into contact with the cold window frames (or other areas with inadequate insulation), it will condense. Be sure to plan your project well from the start with attention to moisture control.

Having the right systems in place and doing each job well and to specification can save you thousands of dollars down the road (sorry for the THOW pun). If your tiny house is already built and you are having moisture issues (condensation, mold, etc), it’s not too late. The good news is that there is nothing in construction that you can’t fix, so if you are having issues, don’t worry. Discover the problem and make the necessary fixes. You can do it!


Want to learn more about tiny house living and how to build a tiny house? Want to do so for FREE? Sign up for our totally free 7 Day Tiny House eCourse! Find out more HERE.

88 Responses to How To Save Your Tiny House From Mold

  1. Valerie January 20, 2016 at 4:20 pm #

    Thanks for that comprehensive article! Giving me lots to think about…

    • Andrew January 20, 2016 at 4:40 pm #

      You’re welcome!

    • saundra January 12, 2018 at 2:52 pm #

      Same here! Thank you!

  2. ezra January 20, 2016 at 4:26 pm #

    What about off grid tiny houses that use solar without many watts to spare?

    • Andrew January 20, 2016 at 4:40 pm #

      That can be a challenge for sure. Some of the smaller HRV units, like the one we show, use a lot less energy, but even a small wattage over 24 hours can drag a system down. We are off grid and use the sensor switch on the bathroom fan for most of our needs. We don’t currently use an HRV. Passive make-up air does not impact the power source, it’s really the vents and the HRV or ERV. In our experience, the vents work just fine off grid and with a small bump up in our batteries, the HRV would work for us too.

    • Craig January 24, 2016 at 1:47 pm #

      You could try an essential oil burner. A recent study tested a number of essential oils on molds and oregano and thyme were the most effective of the ones tested. Other studies in hospitals have shown they can also prevent antibiotic resistant biofilms forming.

      • Helene Cusson April 11, 2016 at 6:05 pm #

        Tk you for this information.

  3. Aaron January 20, 2016 at 4:39 pm #

    Fantastic, and thank you! Do you recommend any additional considerations when Zip system sheathing and tape were used as opposed to the traditional house wrap?

    • Andrew January 20, 2016 at 4:42 pm #

      You’re welcome Aaron. Not really, You basically have a tight system so any and all of the recommendations will be good for you to employ. I would not bother with the active make-up air system as the passive air is all you need.

      • Dustin February 1, 2017 at 8:22 pm #

        Awesome article! Answered many questions ive had thank you! I’m planning a build and concerned about window condensation, what are some ways I can prevent this? Gotta love wet ol washington!

        • Andrew February 2, 2017 at 10:46 am #

          Thanks Dustin. The biggest thing is to make sure you have the ability to remove excess moisture from the air. You can do that by opening windows (not so much in Washington since it’s so moist outside as well) or using ventilation systems, recovery ventilation systems (ERVs or HRVs), and/or dehumidifiers. Get the moisture out of the house through a path of your choosing, not through the moisture’s choosing.

    • Aaron February 1, 2016 at 7:57 am #

      Thanks Andrew…I am still struggling with whether or not to use any sort of barrier on the interior walls. As I mentioned, I used zip system sheathing and tape on the exterior, then a 3/4 rain screen and then novelty pine. My rafters are vented and I will be leaving an air gap between the roof and insulation. in my 2×4 walls, I will use Roxul insulation and I live in NJ/NY so we have 4 distinct seasons. Do you think proper ventilation is sufficient and no vapor barrier is needed on the interior walls? No matter how much I read, all I seem to find are 1,000 differing opinions. Thank you!

      • Andrew February 1, 2016 at 8:08 am #

        It is such a confusing topic Aaron and one that, as you have seen, even the experts can’t seem to agree on. Your Zip system will provide for air sealing (if installed properly) from the exterior and manages moisture as well. It basically ensures that you don’t need house wrap. The rainscreen will help alleviate any issues with your siding as well. As far as the vapor barrier on the interior of the walls, I have become more and more a believer that it is up to the builder/owner to decide if they want it. It seems like there are solid arguments for it and against it. I would say that it does a great job of helping to seal air gaps in the envelope that can stop water laden air from being pushed into the wall cavity. That requires the addition of caulking at wall penetrations and floor and ceiling transitions as well. That said, it also has the ability to trap moisture in the walls that does manage to find its way in. As you can see, it is a split decision for me too. I wish I could give you a solid “this is what you should do” answer, but I don’t think that exists in regards to vapor barriers anymore.

        • Aaron February 1, 2016 at 12:53 pm #

          Great information and much appreciated. Do you mind if I ask what you did in hOMe? It seems my research returns the same results. Either good or bad luck, but what is common is that these small spaces are more prone to moisture problems. This is where your article on avoiding mold issues is so important.

          Thanks again.

          • Andrew February 2, 2016 at 11:51 am #

            We did not add a plastic vapor barrier. We sealed the joints of our wall sheathing with vapor tape. Our situation is a little different because our sheathing is on the inside of the studs (weird, I know) to provide backing for our finish wall material.

          • Aaron February 3, 2016 at 9:28 am #

            Thank you!!

  4. Kelli B. January 20, 2016 at 5:24 pm #

    This has become one of my very favorite websites. It is SO informative and relevant.

    • Andrew January 21, 2016 at 8:46 am #

      Hi Kelli. Thanks so much for the kind words. We are so happy to hear that, in your eyes, we are offering good information. That’s certainly our goal: to help out and inspire!

  5. Alan January 20, 2016 at 5:49 pm #

    Excellent article! I have the Lunos e2 installed and running in my tiny. I live in Maine so climate IS a factor. I’ve been heating my house this winter and running the HRU, not living in it yet, but soon.

    I’ll let you know when I’m in. Feel free to contact me later.

    • Andrew January 21, 2016 at 8:48 am #

      That’s great Alan. I installed the Lunos E2 in a house in Australia last March and was really excited about it because of its size and simplicity. I look forward to hearing how you like it and I need to check in with my friend in Australia who has it in her house too.

  6. Billy January 20, 2016 at 5:55 pm #

    Call me a quixotic “enviro commy”, but in yurts, people roll up the bottom and vent through an operable skylight. Sure, energy is lost, but supposedly the principle of moving air prevents moisture from condensing. I am building a Gifford and don’t want to cut any more conspicuous holes than I have too; am limited by a planned one panel 12 volt system, and oh yeah, simply lack the budget. Besides, I’m really aiming for the most primitive/organic homestead possible. Is this naive?

    There are givers and takers in this world, and you guys have given greatly. Thank you.

    • Andrew January 21, 2016 at 9:01 am #

      Hi Billy. There is no doubt that condensation and air exchange can be handled through opening windows in a tiny house, but that becomes impossible to do in a good way when temperatures are too low or high. The loss of conditioned air makes living very uncomfortable, which is why “the collective we” have moved towards conditioned spaces over time.

      Depending on how you run your systems, a one panel solar system may be too small. We have 6 panels at 270 watts each and 4 large batteries running at 24 volts. We run ta lot fo stuff on our system, and when the sun is shining, we do just fine. We had a smaller system prior to this one and we could not get by. Everything adds up quickly: the bathroom fan, kitchen fan, lights, plugs for everything from computers to Vitamix, our fridge, and our washing machine and gas dryer (the tumble part uses electricity). I don’t know what you plan to run on your system, but I strongly encourage you to be as realistic as possible as it is more expensive to upgrade the system than to have it installed right the first time. We learned that through experience on our own place. At the very least, buy an inverter that can have a larger system of panels and batteries added later as that is perhaps the most expensive piece of the puzzle (unless you plan to run everything as 12 volt, then the inverter doesn’t come into play).

      Sorry for the long winded answer. Just want to share our experience in hopes it is helpful.

  7. Rick January 20, 2016 at 7:03 pm #

    Thank you for a most informative article.

  8. Rob January 20, 2016 at 7:23 pm #

    The foam gaskets are not a good option.
    They are expensive and they do not cover all of the boxes that you plug or switch is built into.
    Use some caulk on the outside and do not forget to caulk the opening where the wiring enters/exits the box.

    Cheaper and more effective.

    Otherwise your article is very good.

    • Andrew January 21, 2016 at 9:05 am #

      Good tips Rob. I have used the foam gaskets for years and have bene happy with them. I like that they insulate around the inner opening of the plugs and switches (the gap that the switch and plug pass through). I do like your idea about caulking the box edges and the hole in the back of the rough in box. I should have mentioned that in the article. In fact, I may add it in now for future readers. Thanks for the reminder!

  9. mike mitchell January 20, 2016 at 7:52 pm #

    Also, let’s not forget about de-humidifiers…they work really well here in the East….

    • Andrew January 21, 2016 at 9:07 am #

      Absolutely. My only issue with dehumidifiers is that they are one more machine to fit in a tiny space and that’s all they do: manage humidity. I like that HRVs, and ERVs provide healthy, conditioned air exchange and that the kitchen and bath fans provide point of use extraction (and they are small). That said, YES, dehumidifiers can be a great option as well if they fit the design.

  10. TJ Houston January 20, 2016 at 8:31 pm #

    Hello Andrew,
    Glad you brought this up. I receive information from Zehender about HRV/ERVs’ in my email. To me, heat recovery ventilators are the way to go, but I haven’t been able to find any info about anyone who has used one in a tiny home. I emailed them that I was interested in HRVs’ for a tiny home, but I didn’t receive a reply other than their ebooks that you can download to study. I believe you have to send your plans in to them and they will give you a free systems design and quote.


  11. Doug Campbell January 20, 2016 at 8:36 pm #

    We cant decide if we need a vapor barrier or not, of course we have house wrap and we are doing a tight solid rigid foam insulation. We live in Vancouver, BC. Can you give me some advice.

    • Andrew January 21, 2016 at 9:12 am #

      Hi Doug. Rather than speak directly to your situation, I have included a link to a great article that speaks to the general need (or not) of vapor barriers so that other reads can also benefit from the answer. I will say that the truly big issue is air leakage, rather than moisture diffusion, and that vapor barriers while sometimes useful have been over used in many situations. I myself have been guilty of that. I’m still learning (as is much of the industry other than the true building science guys and gals). Here’s the link.

      • /bob January 21, 2016 at 9:35 am #

        Now see there, I learned something. Always be ready to learn more. Everyone can. No one knows everything.
        Thanks Andrew. 🙂

        • Andrew January 21, 2016 at 9:45 am #

          That’s right! Never stop learning. I’m always amazed at how much there is to know!

          • Andrew January 21, 2016 at 10:36 am #

            I just got a link to another article about mold from another tiny houser, Mac G Clann. There are some great points about passive options for people who already have mold and moisture issues and are looking for ways to handle them. I particularly like the DampRid product. Although I’m not a fan of disposable products, I do like the simplicity of this and it can be a good option for someone with moisture issues while a long term fix is put into action. Here’s the link to that article.

      • Laura May 1, 2016 at 1:10 pm #

        Thanks to you and Gabriella both for invaluable advice.

        I didn’t see the link in the blog, can you post it? I have been reading many informative articles today on and recommend. I did a search online for roofing ventilation and wall rainscreen advice, both times greenbuildingadvisor website came up with extremely wise advice, very careful advice on vapor barriers and ventilation in roofs especially, what to do and what not. Not hard to find the links. While they are more geared, of course, to “big”, not tiny, homes, the advice is still helpful and one blog was centered on a Ryan Reed tiny house, quite helpful for anyone considering a purlin roof structure (not typical, but still interesting). Laura J.

        • Andrew May 1, 2016 at 1:17 pm #

          Hi Laura. Thanks for the advice about Green Building Advisor. I have seen some good articles from them as well. the other link I referred to was to a Facebook post and I have not idea how to find that now. Sorry. I should have included it the first time. My bad.

    • Alan March 16, 2016 at 7:05 pm #

      Hey Doug! Just inacse you’re still following this comment thread, i’m up in Tofino and about to embark on the same conundrum. What conclusion did you get to with the vapour barrier? I was planning to use 30# paper (doubled) on the outside of the plywood (I am also told that plywood acts as an air barrier anyway, especially if you use silicone at the joins!!) and then sheep wool in the walls, with nothing on the inside other than TnG panelling?

      I still have no idea if this is right or wrong, or if it matters? Would love to know how you tackled it. Thanks.

      • Andrew March 23, 2016 at 10:19 am #

        I would prefer to see house wrap on the exterior in lieu of the roofing felt. Roofing felt does a great job of keeping water out, but is not a good air barrier, nor does it allow vapor that does enter to the walls to easily escape to the exterior. Housewrap allows for all of that.

        • Laura May 1, 2016 at 1:17 pm #

          Just a p.s. on the roofing felt comment. I just read a warning about using synthetic roofing membranes on walls or uninsulated attic systems, the warning was, essentially, “don’t do it” due to the, if I understood right, impermeable nature of the new synthetic-type “felt” or membrane. Older-style roofing felt was less concerning to the author of the article, a Mr. Guerin, but it sounds like Andrew, you seen to have concerns.

          • Andrew May 1, 2016 at 1:20 pm #

            Hi again Laura. Roofing felt and roofing membranes are designed for roofs and are meant to keep water out. They are not good at allowing vapor to pass out through them. Further, roofing felt is not good at stopping air flow. House wrap stops air movement, allows vapor to escape, and keeps water out. This is what you want on your walls.

          • Karl Hussong January 4, 2019 at 5:25 am #

            Do yourself a favor and research Joseph Lstiburek of Building Science Corp. He has many books on the subject.

  12. Pauline January 21, 2016 at 6:52 am #

    Great article. I would love to know also your thoughts when it is best to place a vapor barrier on the interior vs exterior (and what the you recommend, as well as when, if ever, to use a vapor barrier for the floor.

    • /bob January 21, 2016 at 8:27 am #

      My understanding has always been that the vapor barrier always goes on the (predominantly) warm side of a wall. The key detail here is dew point. The barrier is to keep moisture from passing into the wall from the side that is usually warmer so if the other side reaches the dew point temp condensation is inhibited from forming. Reducing humidity per this article (very good information BTW) helps reduce the work of a vapor barrier but if your insulation is not inherently a vapor barrier by itself (such as *some* rigid foam boards) you will need this barrier. Up north, in the northern hemisphere, that means the barrier should be on the inside since most of the year outside temp is cooler than inside temp. Closer to the equator outside temp tends to be warmer most of the year so a vapor barrier may need to be put on the outside of the wall. Another detail is that you want moisture that is in the wall to find it’s way out of a wall and it will flow toward the cool side since that is generally dryer than the warm side.

      For wood framed (or trailer framed) floors the same applies as for walls… barrier toward the warm side. I insulated the floor that was over a vented crawl space in my first house in Iowa (read very cold sometimes) 30 years ago and the barrier went on top of the insulation that was fitted between the floor joists.

      I’ve been looking into FPSF slabs per IRC (2009-R403.3). This is the foundation I would use for a tiny house. A vapor barrier would be placed between the concrete and any finished flooring on top, or if building up a wood frame on top of the slab (sleepers) on which to place finished flooring then a vapor barrier would lay between the concrete and the sleepers.

      Having said all that I confess I am definitely not trained in construction practices and only know what I’ve taught myself for my own needs. I’m sure Andrew is far better than I for providing a correct answer on this question… and educating me in the process 🙂

      • Andrew January 21, 2016 at 9:20 am #

        Great response Bob and very much what I have learned over the years as well. I have to confess that I am not as skilled in vapor barrier technology as I once once. The main reason for this is that in recent years, much of the science behind the technology has started to question the value of the vapor barriers in general. It is now clear that the bigger issue is air leakage and vapor barriers don’t necessarily address that. I’ve linked to a couple articles in other comments here which do a much better job of explaining this all than I can in a short response.

    • Andrew January 21, 2016 at 9:15 am #

      Pauline. I just posted this article below in response to another comment about vapor barriers. There is so much confusion about how they work and whether they are necessary or not throughout the industry. I also found this other article very helpful in explaining things and I think you’ll like his opening joke about whether to put the vapor barrier to the interior or exterior…

  13. Christine January 21, 2016 at 6:49 pm #

    Excellent and very informative. Thanks Andrew. I’m in the process of seeing how I can create a Pocket Neighborhood of small footprint homes, so this information is very helpful. (THOWs are not allowed where I live.)

  14. Andrea January 22, 2016 at 10:08 am #

    Thanks for this article Andrew! Super helpful information!

  15. Jacob And Nancy Tritt January 23, 2016 at 8:27 pm #

    Hi Andrew! 🙂

    Sorry, we posted this in the wrong place, so you may have read it already. 🙂

    Not sure you will remember us from the first Tiny House Jamboree. We lingered after everyone was gone and were volunteers. Jacob and Nancy Tritt. Sooooo… we moved our Tiny Slide-In (approx 60sq ft) up to the “cold white north” in Saskatchewan (Jacob looked into that luthier school, pretty affordable!). We’re about 12 hrs by train from polar bears and balooga wales… SHEESH! We lugged our Tiny Slide-In to the jamboree camp grounds at the time we met you and “camped out” in it so to speak since it wasn’t finished. We are continuing to live in it full-time since that day onward even up here in Canada. Both the border control officers for the US and Canada were very interested in our little home and we had no problem crossing.

    Jacob is working on the cabinets now. We don’t have a sink yet but I take our dishes to work and do them there or spray them with a homemade castile soap all purpose cleaner and wipe them clean. We don’t have our dream wood/chimney oven stove yet and have just been using a plain oil heater we picked up at a second hand store.

    EVERYONE is shocked up here that we are surviving so comfortably!!! Tiny homes were such a foreign concept to these folks and everyone thought we were nuts until shortly after we moved a circus came to town from Quebec and they brought three tiny homes with them for the performers 🙂

    MOISTURE – We had a nasty horizontal rainfall one day before the dreaded winter and the rain leaked in around the window and onto our plain wood floors (at the time). A bit of black mold started growing almost instantly. We since have painted with some mold resistant floor paint. As far as moisture in the EXTREME cold winters up here it is so fantastically dry that has not been an issue. I’ve been boiling my kettle just for steam and we have a humidifier though we have no vents put in yet anywhere at all!!! CRAZY. Our door is an antique with lots of cracks though and lets in air and a place for carbon monoxide to get out 🙂

    LOVING TINY LIVING!!! SO THANKFUL for the hope and inspiration that the Tiny House movement brings and for the way people post things to help others reach their dreams too!! A very sharing and giving community. We do plan to build a 24′ THOW one day as we can afford it little by little. First we need a good trailer. It is so sweet to live a simple live debt free finally. It’s worth doing without for a season~ and oddly enough, it doesn’t seem like your really “doing without.” After a while it becomes almost normal to live life where you are at, one step, one board at a time ~ LOL ^_^ We hope to get the “Sanitizer Toilet” from the States shipped in to us here in Canada one day. We are grateful for our porta-potti in the meanwhile 🙂

    All the best to you and Gabriella!

    • Andrew January 24, 2016 at 11:33 am #

      Nice to hear from you guys and congratulations on your progress!!! That must be very exciting. Best of luck moving forward and keep loving it! 🙂 On a safety note, be sure to get the vents in soon as cracks in the door are likely not enough to keep you safe from carbon monoxide poisoning. I can tell you with 100% certainty that you don’t want that. Be well…

  16. Oriane February 6, 2016 at 12:55 pm #

    Hi Andrew !

    I’m planning my tiny house at the time, and I plan to put a window in the bathroom and one directly above the cooking area, and to open them when I’m showering/cooking. Is it enough ? Because I’m on an off-grid home, I am trying to use the less electricity possible.

    I also read/heard that the water can also cause the wood to mold ???! is it enough just to varnish the wood surrounding the shower area ?

    Thank you for this article, and already thank you for your answer !

    Best wishes, Oriane.

    • Andrew February 9, 2016 at 10:48 am #

      Hi Oriane. If you live in a climate where you can have your windows open year round then this approach would be acceptable; however, if your climate makes it too cold or too hot to open windows for long stretches of time then it would not work to use this approach. Keep in mind that in order to really keep a house free of moisture, you need to keep the air dry and that means leaving fans on during and after cooking and showering. It can also mean having a fan come on to mitigate excess moisture from breathing and houseplants. I would instead recommend that you size your off grid system to handle the requirements of the home, including ventilation. We are off grid and we have a fan that runs quite often to keep our moisture levels down. It works just fine if the system is big enough (solar panels and batteries for us). Also, if your climate is humid, opening the windows won’t actually lower moisture levels in the house because the outside air is damp too. In that case, you would likely need a dehumidifier to keep things at a safe level.

      In terms of wood, having adequate protection is key as is keeping the area dry. I would not recommend having wood in the shower area unless it is a) really well sealed and 2) a moisture resistant wood. For example, ipe and cedar are much better than pine and fir for resisting moisture, and so on.


  17. Pat February 9, 2016 at 6:07 pm #

    Wow! What a great article and great repies. So much info. I couldn’t finish it all right now.
    I purchased a manufactured home in FL lat April, 2015. I haven’t been able to move down there from IL becaise of water damage causing mold. I’m just waiting for insurance to send me check for $7-8,000 for half of the mold remediation. Very frustrating but at least it will be mold free by the time it is done.
    What I am concerned about is keeping moisture and mold out once it is done. My house is on a crawl space and the vapor barrier had to be fixed in the crawl space before I purchased it. That being said I’d welcome any and all suggestions on keeping my house safe from moisture, mold and any other microbial and air contaminants. I’m from IL so don’t know a lot about this. I’ve found that it s ems like different people and companies say different things. For ex. the mold assessor said my house had to have all items that are in the house HEPA and/or microbially washed and taken out of the house. The house is to be enveloped and only the mold remediators can go in. When I asked the mold remediator today if I should get a pod put on my driveway for them to put everything that is taken out he said we don’t need that. He said they will microbially wash one of the rooms and put everything in their. This isn’t what was in the assessor’s protocol for the mold remediator. Should I be concerned that he said this? All the floors and all lower cabinets have to be taken out and thrown away. Many walls have to be cut 24″ above the floor and taken out and replaced. I am a 63 yr old single female and I’ve taken advantage of before.
    I’m sorry this is so long. Thanks for and help y’all can give me.

    • Andrew February 20, 2016 at 11:40 am #

      First of all I am sorry for your situation Pat. That is surely not a great feeling, I would imagine. I honestly do not have experience in professional grade mold remediation, so I can’t offer much there. I will say that having remove asbestos from a house I remodeled some 15 years ago, I know that it is possible to move room by room without contaminating previously cleared rooms. The contractor will likely hang plastic in specific patterns to stop any infiltration of mold spores as the house is cleared. That said, I would ask the contractor directly and let him know that you are getting differing opinions and want to make sure he is positive his system will work. If he is a good contractor, he will respond with clarity and understanding. If he becomes defensive (beyond what might be considered normal) you might want to hire someone else. Good luck.

  18. Anna February 13, 2016 at 11:28 pm #

    Grazie! Veramente molto utile e interessante!!!

  19. Zach Petruso March 15, 2016 at 11:13 am #

    hello tiny house community!
    First and foremost, I just want to thank you all here for the hard work and dedication to great information!
    I’ve been in the planning process of a 22′ THOW for about a month now and the educational process has been overwhelming (in a good way)! I Plan on taking my house through many different climates and temperatures for different amounts of time, and though I know there is no 1 way to rule them all, I was wondering if you glorious folk here could give me some splash back on what would be a decent process (order of installation, products etc.) to attack my tiny abodes’ vapor barrier installation. At the moment I live in Orlando FL, which will be my primary residence, so humidity control is a must. I plan on utilizing the LUNOS e2 passive heat recovery ventilation system, as well as a bathroom exhaust fan, and range exhaust vent. This should be more than enough to control the temperature and humidity of my tiny space, but i could use some help designing the interior envelope, as well as the exterior vapor barrier (at least that’s the general consensus I’ve gathered as to where those two layers go). I’m leaning toward ROXUL concrete wool insulation as it is easy to manipulate, thermally insulative, easy to seal and acts as a sound deadening barrier as well. From what I understand, knowing the climate your house will be in will help in choosing products which is where I have my dilemma, the climate is a variable. Thanks in advance for any help!

    • Andrew March 15, 2016 at 11:36 am #

      Hi Zach. Thanks for the kind message. There is a lot of information out there about vapor barriers and how they do or do not work well in the overall building envelope. One thing for sure is that the bigger issue is air infiltration. If you can make sure to seal up the envelope from air, you have a much better shot at success. This means caulking electrical boxes to seal them up, using foam gaskets at all plugs and switches, properly installing and taping the joints of a house wrap on the exterior, and providing adequate ventilation to the house. The use of a vapor barrier (i.e. plastic on the interior face of the wall) is less important. In fact, many believe that the use of a vapor barrier like that actually traps moisture in the walls and can be MORE detrimental than not having a vapor barrier at all. This is an evolving science and I have learned a lot about it myself in the last months (and I’ve been building for twenty years!). Because the vapor barrier is typically installed against the warm side of the wall assembly and you will be traveling to different climates, AND there is evidence showing that vapor barriers are less important than a proper air seal, I would personally avoid installing one at all. I would stick with the house wrap and other items detailed in my comments above. Good luck.

    • J. Anderson June 3, 2016 at 8:19 am #

      Hi Zach – was just reading though the article and noticed you are in Orlando… and that you are considering building a THOW. We are from Orlando and my sister is now in a company called Kokoon Homes and they build THOWs and other homes as well – steel framed insulated. You can find more information if you go to the same name website. Good luck on your project – I know they are very aware of the sealed envelope and air exchanging etc.

  20. AnnaLevis May 20, 2016 at 12:08 pm #

    That is a great article. Bravo!! I really appreciate the way you define the mold and moisture issues in this article. I also want to share some Truth about Mold.

    I go through several articles but i really find your article helpful and i am very much impressed with the facts which you mentioned about mold.

  21. Lara Rabideau September 13, 2016 at 5:51 am #

    Hello, my name is Lara. My wife and I are building a 14’x38′ tiny house in the Adirondacks that will be using water propelled spray foam for installation, though our R values won’t be as high as required here. We are using a continuous run Panasonic bath fan and a kitchen direct exhaust. We have a 24″ propane stove and a tiny propane fireplace, direct vent. Plan on using Brian nutone for fresh air that pulls from screen porch and drops from ceiling into both the living kitchen area and bedroom. This will be both manual and pressure controlled. Big question is do we also need to have an hrv system as well. Thank you for your thoughts and response.

    • Andrew September 13, 2016 at 9:06 am #

      Hi Lara. The big difference is that the systems you plan on at this time will exchange air, but they will do nothing regarding conditioning the incoming air. You will have a lot of heat loss or heat gain (depending on the season) that would be avoided with an ERV or HRV. Be sure to do research on which system is recommended in your area. There are differing opinions out there about what system (ERV or HRV) should be used in cold climates.

      • Lara Rabideau September 15, 2016 at 2:21 am #

        Andrew, thank you for the response. I know ERV’s are better with humidity control but HRV’s are recommended for our area. These are there own units that take air out and replace it with conditioned air. My next question is since we will have a constant exhausting of air for moisture control due to humidity in region and our LP appliances, how do we address pressure issue since hrv is its own contained unit. I’ve heard passive air intakes really don’t work that well. Do we keep the pressure system and set it up on a trigger so it only turns on if the pressure causes to and then add the hrv system too?

        • Andrew September 15, 2016 at 9:18 am #

          Hi again Lara. The HRV and ERV systems both exchange air with conditioned air and precondition the fresh air coming into the house. They are a closed system, so you won’t need to add them to the pressure equalization system. In other words, they bring in air and exhaust air at equal rates so there is no loss or net gain. In terms of other impacts (exhaust fans), I personally like the passive air approach. It’s not for everyone though. You can hook your make-up air vents to your exhaust fan motors so that they open when the fans are in use. It’s a bit more fancy than a passive system, but it creates a certain exchange when the fans are in use.

  22. Curt November 11, 2016 at 2:05 pm #

    Hi, I have a HRV installed in my home and just recently noticed that there is no duct in the walls, the wall cavity is used as the duct. The HRV is installed in my basement and the ducting only goes so far into the 2×4 cavity, noticed this when I took the air diffuser off the wall to clean. Just wondering if this is acceptable way of doing this? I have checked both intake and supply returns and there isn’t any sign of mold.

    • Andrew November 11, 2016 at 5:32 pm #

      Hi Curt. This does not sound right to me. I don’t know which system you’re using; however, I can’t think of any that would use the stud bay as the duct. If you had the system installed professionally, I would contact the service provider and ask them to show you where this is an acceptable practice in the manufacturer’s specifications. To me, that sounds like a recipe for trouble. Good luck.

  23. Rene Bell November 20, 2016 at 11:37 am #

    Hi Andrew. I am living full time in a THOW that I built. Due to a bad ankle break, I ended up installing my water pipes along the interior floor. I reside on Vancouver Island, B. C. which at this time of year is very humid outside. My home is very well insulated, poly vapor and 3/4″ pine boards. Out is Tyvek and cedar siding. I have a proper kitchen fan for my induction burners and a proper bathroom fan. Also I run a dehumidifier during the day for 12 hours. Its just myself and two pets. I don’t cook much at all. The problem is that my water pipes are getting a lot of condensation on them at this time of year. Last year it was the toilet pipes (which are fine this year). Its the kitchen cold water pipe which is under my kitchen cabinets now. It was not installed until this summer.

    My water comes from my cistern 2000 gallon, which is about 75 feet away from my THOW. We have the pipe going underground until it reaches the THOW. Then I built an insulated box for the pipe that is coming out of the ground and into the house. The toilet water pipe and kitchen pipe are branched off with a tee. I have no water heating system at this time due to cost. I am not sure if I should try to enclose the inside water pipe with Great Stuff foam insulator or if this is even an option.

    If you have an suggestions, that would be great. Thank you for your wonderful articles.

    • Andrew November 22, 2016 at 11:54 am #

      Hi Rene. I would suggest you use foam pipe insulation similar to this one around the cold water lines. In fact, I would use it on hot water lines too when you install those. The issue is that your water is coming in very cold and the warm air of your home hits that cold and condenses on the outside of the pipe. Insulating the water lines from the conditioned space will help reduce that condensation. As far as the hot water lines go (future lines) the insulation will help them retain their heat. It’s less about condensation and more about efficiency. Good luck.

      • Rene Bell November 24, 2016 at 9:46 am #

        Hi Andrew,
        I have used the pipe insulation foam last year on the toilet pipes with tape on the seams but it still had enough condensation to make the floors wet (sub floor). I
        had to pull the shower out and the wall was starting to produce mold. I cleaned all that up and then removed the pipe insulation as it was spring at that time. I did not
        replace the pipe insulation and this year it is fine.

        What confuses me is why the kitchen water line is producing condensation but not the toilet one now. The cold water is one pipe through the wall with a tee, one to the toilet and one to the sink in the kitchen.

        I will place the pipe insulation onto the kitchen run and I will let you know if it worked.
        Thanks so much for all of your help. Its great to be able to work THOW issues out with knowledgeable people.

  24. Niels Larsen November 22, 2016 at 2:30 pm #


    Utilizing a ERV, do you still recommend using exhaust fans in both the bathroom and the kitchen. I will be using an LP stove top which will be approximately 3-4 feet away from the bathroom/shower.

    • Andrew November 22, 2016 at 5:04 pm #

      Hi Niels. Yes, I absolutely recommend the use of the location specific fans. In fact, you absolutely must have a kitchen fan if you have a gas appliance (not sure if you mean liquid propane by LP or the company name LP). I think it’s cheap insurance to have fans specific to each use, personally.

      • Niels Larsen November 24, 2016 at 10:23 am #

        Thank you!! Yes, I meant Liquid Propane… Further thoughts are that since I’m using an ERV, I can cut down a little on the CFM capacities of the other fans…. Opinion?

  25. Diana January 11, 2017 at 12:13 pm #

    What do you recommend for insulation in a tiny house 12×16 without heating and air in Hawaii with tons of humidity? I am desperately trying to figure out how to make my place stay dry so I don’t ruin my cabinets and wood, etc.

    • Diana January 11, 2017 at 12:13 pm #

      Also off grid~

    • Andrew February 6, 2017 at 12:06 pm #

      Hi Diana. So sorry for the long delay. I missed this message when it first came in. You may want to consider either Roxul (steel wool) or closed-cell spray foam. the advantage of closed-cell is that it completely seals the insulated cavities so there is no way for moisture laden air to enter. That, along with some time of dehumidifier (and a solar array large enough to power it) would be sufficient. If you want to go with Roxul, it is a good option because it isn’t effected by moisture; however, you will have to work hard to seal all potential air leaks into the insulated cavities such that moisture doesn’t find its way in. Hope that helps.

  26. Nicole January 16, 2017 at 3:25 pm #

    What bathroom fan did you install that fit in between your 4×4 joists?

    • Andrew February 6, 2017 at 2:38 pm #

      We built a custom dropped soffit (box) for our fan unit as it didn’t fit in the 3.5″ bays.

  27. Brandi February 6, 2017 at 5:58 am #

    Thanks so much for this amazing resource! Any opinions on insulation choices to combat moisture and mold? I’ve heard that sheep’s wool is good for this (in theory), but I’m curious to know your thoughts on wool vs. cellulose, closed-cell spray foam, Roxul products, etc.? Again, thanks!!

    • Andrew February 6, 2017 at 11:37 am #

      The biggest concern is about air tightness in the wall. Air brings moisture with it into the wall cavity and that is where mold tends to take hold. The best system for preventing moisture intrusion is a fully sealed, closed-cell foam insulation; however, that has other problems that come with it such as health issues. Wool, cellulose, steel wool (Roxul) are all reasonable options, but they aren’t as good at providing air seal nor Rvalue as closed cell spray foam. If I had to do our house again, I would likely use sprayed closed-cell foam and just make sure that it is done perfectly to keep it healthy and safe.

      • Brandi February 7, 2017 at 3:17 pm #

        Thanks very much!

  28. Frank Foley June 22, 2017 at 7:52 am #

    Great responses, thanks Andrew . Building a small house near Ottawa (950 sqft) , with 2017 Ontario Building code , we need an HRV according to building inspector, and the code. Heat is wood stove, and baseboard, on grid.
    Can I use a lunos e2 or similar in this situation. Any other Suggestions welcome

    • Andrew June 22, 2017 at 10:48 am #

      Hi Frank. You should be able to use the Lunos E2; however, smaller units like that have difficulty in very low temperatures. You may want to see what the working temperature range is for the unit to make sure it matches your climate.

  29. Alyssa P. September 19, 2017 at 8:03 pm #

    Hi Andrew! For a 5×8 Teeny-tiny camper (this would be just a bedroom: no kitchen, bathroom, laundry, etc.) with 2″ walls, for someone with chemical sensitivities living in the D.C. area (hence four seasons), what would you recommend for ventilation, heat and A/C, etc.? Thanks!

    • Andrew September 20, 2017 at 8:40 am #

      Hi Alyssa. That’s a very tiny space, to be sure. I would recommend that whatever system you use, it be a multi function system. For example, a mini split can provide both heating and cooling where as a Dickenson heater (designed for sailboats) only provides heating. I would also consider building thicker walls. 2″ will not provide very much insulation at all. In fact, you will have a hard time finding materials to fit in a 2″ space as the thinnest batt insulation I am away of is 3.5″. Rigid foam would work, but even the foam with the highest Rvalue (polyiso) would only give you roughly R13 walls. For ventilation, you could use a bathroom fan sized heat recovery ventilator (HRV). This is an equal exchange system (air in = air out) which will provide conditioned fresh air. Being that you don’t have a kitchen, bathroom, or laundry facility, this would be adequate ventilation.

  30. Mo October 31, 2018 at 3:11 am #

    What can you do if the tiny house frame you purchased doesn’t have osb or plywood between the siding and the 2×4 frame?
    I’m trying to figure out how to insulate the home, vent it properly, and seal it when there’s essentially no barrier between the walls I will be putting up and the siding.
    There’s also no osb between the roof rafters and the metal roofing so I’m trying to figure out what to do there as well as if I need to install roof vents like a proper home attic has.

    • Andrew November 14, 2018 at 9:10 am #

      That’s a tough one Mo. In my opinion, without knowing more about the details of the home, it seems that the shell may not have been properly built. The plywood or OSB is used not just as backing, but also as the shear strength for the building. It’s possible that there are angle braces in the frame, but nothing is quite as good as plywood when it comes to shearing and providing backing. If there is no housewrap installed under the siding and no roof felt below the metal, you are likely to have issues with moisture as well. Sorry to be the barer of bad news.

  31. Rockstar December 3, 2018 at 10:28 pm #

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  32. Stacie March 12, 2019 at 6:18 am #

    Great article, thank you! So just to confirm, we need a bathroom fan with a humidity sensor, a kitchen fan (Hood), and an ERV…and a make up air? We are moving our tiny home to Oregon in a couple of months so it seems the lumps e2 would work? Although pricey, will the Panasonic do the trick? This is our next step in the building process and we planned for a shower fan and kitchen fan with vented rafters and no gas appliances but I’m concerned about the wet climate in Portland!

    • Andrew March 14, 2019 at 8:26 am #

      Hi Stacie. The word “need” is questionable, because everyone has different wants and hopes for their home and each home, being in a different region, will require different things. The venting of the rafters is absolutely essential (unless you used closed cell spray foam) and is not technically part of the home ventilation system. That is managing moisture that builds up against the underside of the roof sheathing, causing rot. Be sure to use the proper size vents and provide both a ridge (top) and eave (bottom) vent to provide adequate circulation.

      A bathroom fan and a kitchen fan are both something you will want to include. Some folks opt to use an operable window in the bathroom instead of a fan. This meets code, but doesn’t do as good of a job…especially in the winter when you aren’t likely to leave your window open while showering! I’m also a fan of using a moisture sensor control switch on the bathroom fan. It will turn the fan on whenever the room/house exceeds the moisture level to which you have set the switch. This is very helpful as it can stay on after you leave the house and will turn itself off when the levels drop again. The kitchen fan is an obvious one to install and is required by code. So that leaves the ERV and make up air. I am a firm believer in supplying make up air so you don’t suck air through gaps in your insulation envelope, potentially loading the walls with moisture.

      The ERV is a great addition as well, but is likely the one that could be left off if the climate is right. ERVs are great when a house is REALLY tight an doesn’t have a make up air option. They also maintain the energy efficiency of a home by pre-conditioning the incoming air. This is important in a big house, but less so in a tiny house. The reason being that whenever you open a door into a tiny house, a large percentage of the conditioned air is lost right away, as is the case when unconditioned air is pulled through a make up air vent. The advantage is that the tiny space heats up (or cools down) very quickly with the mechanical systems so that loss is quickly managed by the heating/cooling systems.

      Hope that makes sense. Have fun!

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