Tiny house building — mold and off-gassing prevention
Hi, I am going to be building a tiny house DIY. I have not built walls myself, but have done most everything else in a house I had owned, taking it down to the studs to rebuild. One of the reasons I am building a tiny house is that I have extreme health needs. Because of lung disease and a mast cell disorder, I react to small amounts of mold, environmental pollutants (formaldehyde, cigarette smoke), foods, etc. Since these issues have become graver, I have lived in just one place I would consider relatively safe, my current home. These illnesses became graver specifically after I spent two years deeply ill living in a condo with a hidden mold infestation (the technical term the mold inspector used, after growing cultures from samples, was “yuck”) and had frequent infections (and rashes, etc.)
I am much more well read than the average lay person about these issues as a result. I figure that in building my own tiny home, I can afford whatever measures I need to ensure quality and safety, and I can do the work myself. I have had many issues with contractors cutting corners, perhaps thinking I wouldn’t know better, and so hiring is out of the question unless an absolute necessity.
Questions I have (the last is the most important, so if answering all these is just too much, please chime in on the last):
1. Last time I had to make choices in remodeling, some years ago, the primo option to prevent off-gassing of formaldehyde in building materials was marine grade plywood, more expensive than OSB or even exterior grade plywood, but worth it. I used it for subflooring. Still true?
2. Can you suggest calculators for figuring out sizing of bathroom exhaust, kitchen exhaust, and air-to-air exchangers? I don’t know where I am going to live yet, but likely either the Pacific Northwest or a milder four-season state than the northernmost state I now live in. Should I just plan on a heat recovery ventilator rather than an air-to-air-exchanger?
Constantly circulating air is the best thing for me. I have the furnace (well, magic-pack) constantly on, running the fan so I have constantly refreshed air. When I don’t have the electric to do that in my tiny house, I should at least passively vent. Any book recommendations for designing excellent passive ventilation (besides sticking windows across from one another).
3. I would like to make this off-grid friendly, but I cannot think of an efficient but safe-to-me method of heating for cold climates. I would likely react to propane. Wood fireplaces are completely out of the question — they make me pretty sick. The only option I can think of is gonzo insulation plus solar and electric heating, but from what I understand, solar can’t power constant electric heating, is that right?
5. The biggest question of all, the one most important to me: which building materials to use to prevent mold. I understand that low-slope roofs are, in a way, asking for trouble, but to make this tiny house legal on the road, so I can take it wherever, it can’t exceed a certain height. Yet in such a small home, I need all the headroom and space I can get. So I’m concerned about insulating the roof. I am also concerned about building materials for the walls and subfloor. From what I understand, the safest materials I can choose are metal wall studs, since mold can’t grow on metal (unless there’s dust on it), and blow-in closed-cell foam insulation. Is that correct? If I used blow-in closed-cell foam insulation, I should not use vapor barriers either in the walls or the roof, is that right? (And my roof would be unvented.) But I would use a wind barrier, on the exterior of the walls, over the blow-in foam? What choice of siding would you recommend? What choice of roofing materials? And what interior wall cladding? Drywall is not recommended because, of course, it can crack during transport. Usually tongue-and-groove or car siding is. What could I do to make the wood mold-resistant, back and front? I would spend forty hours painstakingly applying a product to each piece if necessary.
As to the subfloor, I have no idea what I’d do. Tiny homes are on trailer beds. I don’t know if two feet off the ground is high enough to prevent issues. If I could, I’d build a home on huge stilts. The bed of the trailer is metal. Would I spray foam insulation over that metal and then lay subfloor (marine grade plywood) over that?
Thank you so much for any feedback.
Emmy
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Replies
Emmy,
Others will hopefully have more specific answers to your question, but I think you might find it helpful to concentrate on reducing the conditions in which mold can occur, rather than using materials that resist it. As a general approach that means concentrating on building assemblies, rather than specific materials, and reducing indoor humidity, which mold requires to spread.
Thanks, Malcolm. I totally agree; I have a hygrometer and keep an eye on indoor moisture. In such a small space as a tiny house, I wouldn't cook very much (I eat a lot of smoothies, protein bars, and salads; my oven will consist of a microwave/convection oven and my stovetop, a two-burner induction plate), so that helps a ton, and I would hang all my wash outside to dry. But the best laid plans...so I want to be sure to have all my bases covered. (I recall living in an apartment that regularly had ice forming on the inside of the window frame. The problem was as much our extreme climate and inefficient windows that were not properly weatherproofed -- I could feel the air coming through one corner. Expensive apartment, too -- let's just say I've had a lot of bad luck. :) )
Emmy,
Q. "Last time I had to make choices in remodeling, some years ago, the primo option to prevent off-gassing of formaldehyde in building materials was marine grade plywood, more expensive than OSB or even exterior grade plywood, but worth it. I used it for subflooring. Still true?"
A. If you want a subfloor without any formaldehyde, I recommend softwood boards (spruce, fir, or pine).
Q. "Can you suggest calculators for figuring out sizing of bathroom exhaust, kitchen exhaust, and air-to-air exchangers?"
A. Most builders use the ASHRAE 62.2 formula (7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area). For more information on this issue, see Designing a Good Ventilation System.
Q. "Should I just plan on a heat recovery ventilator rather than an air-to-air-exchanger?"
A. Those two terms -- "heat-recovery ventilator" and "air-to-air exchanger" -- describe the same thing. These are two names for the same type of appliance.
Q. "I would like to make this off-grid friendly, but I cannot think of an efficient but safe-to-me method of heating for cold climates."
A. In a cold climate, there are only two ways to heat an off-grid house: with a wood stove or a propane space heater (one that does not require electricity). If you can't use either heat source, you'll have to live on the grid.
Q. "Which building materials should I use to prevent mold?"
A. To prevent mold, keep your indoor relative humidity under control. Building material choices have nothing to do with preventing mold.
Q. "If I used closed-cell foam insulation, I should not use vapor barriers either in the walls or the roof, is that right?"
A. No, that's not right. Closed-cell spray foam insulation is a vapor barrier. If you want to use closed-cell foam, you'll end up with a vapor barrier, whether you want one or not.
Q. "What choice of siding would you recommend? What choice of roofing materials?"
A. Your choice of siding or roofing won't affect indoor mold levels or IAQ.
Q. "Drywall is not recommended because, of course, it can crack during transport. Usually tongue-and-groove or car siding is used."
A. You can install tongue-and-groove boards to finish your interior walls if you want, as long as you remember to install an air barrier behind the boards.
Q. "What could I do to make the wood mold-resistant?"
A. Keep the interior relative humidity levels low.
Thanks, Martin! It's been a long time since I've had to be my own GC. Thanks for the corrections and information. I didn't mean that I was worried my building materials would cause mold, but rather that, should my construction somehow fail, and water get into the building materials, I'd have problems on my hands. I have found mold in too many cavities -- failed caulking, failed roofing. I have been reading your articles about the PERSIST method of insulating on the outside of sheathing, which make a lot of sense! I think I can handle doing that myself. When I understand the technical aspects better, I'll post separately if I have any questions. But just a quick one: though I know you aren't a fan of steel studs too much, because of thermal bridging, doesn't using the PERSIST method eliminate concerns about that?
Thanks so much. I appreciate that you take the time to answer people's questions, even when they (meaning, me) get basic terms wrong. :)
Emmy,
Q. "Though I know you aren't a fan of steel studs too much, because of thermal bridging, doesn't using the PERSIST method eliminate concerns about that?"
A. If you use the PERSIST method, and you install all of your insulation on the exterior side of your wall sheathing, then there is no thermal penalty associated with using steel studs.
Wonderful, thank you, Martin!
An HRV with a HEPA filter, and humidity under 60% (install a dehumidifier with a digital setting) should make a huge difference. Also spray foam will off gas, i would choose a different insulation, even rigid foam would likely be better from a sensitivity perspective
Oh yes, with the PERSIST method I'll be using the rigid foam instead. Like you, I started to re-think spray foam because of off-gassing (plus, I couldn't do it myself, I don't think). Unfortunately, i can't use a dehumidifier -- I tried using it in two different bathrooms, when I lived without exhaust fans, and would get sick around it. I am sure that bacteria and mold would build up in the coils (I would clean the pans with vinegar, so not there). Such a sensitive body, grrrr. Anyway, thank you for the input, Alan. HRV with HEPA for the win!
Hi Emmy,
You are asking all of the right questions and it baffles me that most tiny house builders aren't asking them at all!
I'm currently in the process of building my own tiny house and mold has been one of my top concerns. Living in the PNW for 6 years and with black mold in every house I rented, I needed a way to completely prohibit the growth of mold in my tiny home. So, I partnered with a passive house design company who sources high performance materials from Germany. They taught me the science behind wall systems, and they build houses that can be heated by only a hair dryer! The key points to consider, are building a wall system that breaths. It's sealed everywhere, but if moisture were to get into the wall system due to thermal bridging or what not, the vapor membranes would allow the moisture to escape. Additionally, to address thermal bridging, there needs to be enough of a buffer between the metal of the trailer and the subfloor. And last but not least, an air exchange/heat recovery unit like the Lunos, which we source from Germany. It's super quiet, compact and uses as much energy as charging a cell phone. It can connect directly to solar panels and if it's 0 degrees outside it brings the air through it's ceramic core and heats it up to 62 degrees. I'm putting one in my house and it will exchange the air in my entire space every 38 minutes without depressurizing the building.
The materials you use is important, but if you were to build a wall system like I am and have the Lunos ventilation unit, then you would really be setting yourself up for success! We have actually put together a kit with all the materials you need so the research is done and they would arrive as a package. I figured that I wanted to share the info I learned and make it easier for DIY builders.
I'm sensitive, but I also have friends who have sever chemical sensitivities and have had to live in old cars and tents their whole lives. We need to change the way we build!!!
If you want to continue the conversation and email me directly you can at [email protected]. Also, my website atinygoodthing.com highlights some aspects of my build. I'm about to write a post on mold:)
Good luck!!
Isabelle
Emmy,
It's worth mentioning that one of Isabelle's points -- "The key point to consider [is] building a wall system that breathes" -- is a minority position in the building science community. Most experts would disagree.
For more information on this topic, see How to Design a Wall. In that article, I wrote:
"Many green builders think that a wall has to “breathe.” While “breathing” is a poorly defined concept, it usually refers to a wall that is vapor-permeable. ...
"This type of vapor flow can sometimes cause problems. ...
"There is no truth to the belief that vapor-permeable walls perform better than walls that include a vapor barrier. In fact, limiting the flow of vapor through floors, walls, and ceilings is often essential. (A wall with a vapor barrier can perform very well or very poorly. In general, vapor barriers that provide R-value — for example, foil-faced polyisocyanurate — usually cause fewer problems than vapor barriers that have no R-value — for example, polyethylene sheeting.)
"Wood-framed walls with an adequate layer of exterior rigid foam do an excellent job of preventing problems associated with vapor drive. The exterior rigid foam keeps the wall sheathing warm and dry during the winter, so moisture can’t accumulate in the sheathing. Moreover, the rigid foam prevents inward solar vapor drive during the summer. Everything on the exterior side of the rigid foam dries to the exterior; everything on the interior side of the rigid foam dries to the interior.
"As noted earlier, the sheathing in a double-stud wall is far more likely to get damp during the winter than the sheathing in a wall with exterior rigid foam. This wintertime moisture accumulation won’t necessarily lead to problems, however, as long as the builder remembers to include good details."
It looks like Isabella is doing it right, even though she's describing it using terminology that can be confused with discredited ideas: She's using air tight construction with OSB sealed with European air sealing tape on the outside and Intello Plus, similarly sealed on the inside, with carefully installed mineral wool insulation between. The vapor permeability of those layers allows the walls to dry. Calling that property "breathing" is ambiguous, because some people think that means it needs to have air flowing through. Since she sealed everything meticulously, her walls are not breathing in that way, which is good. I am guessing she understands this well and was trying to reach an audience who doesn't know building science terminology, but I agree with Martin that it is best to avoid the term "breath" because of the confusion it creates.
Charlie,
In addition to the point you're making, I'm making an additional point -- namely, that a wall assembly that doesn't "breathe" -- for example, one with a thick layer of exterior polyiso -- can perform very, very well. So a wall doesn't have to be able to "breathe" to perform well.