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.
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.
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.
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.
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.
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.
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.
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).
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.
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!