Vapor Barriers

May 15, 2015

I have been struggling with a tiny house dilemma over the past few weeks that I think I have finally made a decision on. The dilemma is regarding vapor barriers and whether you need one for a tiny house, especially in California.  After much thinking, and discussion with my tiny house friends, Andrew and Gregory, I have finally decided not to install a vapor barrier anywhere in the house except for the ceiling.

My Tumbleweed plans do call for a vapor barrier, specifically a layer of 6 mm polypropylene sheeting installed over the insulation on the interior of the walls. Most articles I have read say that if you install a vapor barrier like poly, it also has to be airtight which means it needs to be caulked around all of the edges, taped at all of the seams and a somehow sealed around every outlet, window or other opening.  That includes somehow sealing all of the interior holes in every single electrical box, including around every single wire that leads into or out of the box. Obviously, all of this would be very time consuming and a huge pain. However, I’ve also heard a lot of horror stories about people getting mold inside their tiny houses because it is so easy for moisture to build up in such a small area, especially with people breathing and propane being burned for heed and cooking (which evidently generates a lot of moisture). I definitely do not want mold inside my tiny house. However, I am also not sure if a vapor barrier will actually protect me from mold and I don’t want to install one unless it is absolutely necessary.

I have done some research online and while some people say it is necessary, others say installing a vapor barrier can sometimes cause more problems than it solves, especially if moisture gets trapped behind the barrier (thus the reason to make it absolutely airtight if it is used).  Most sites also say that the benefits of a vapor barrier decrease significantly in dryer, warmer climates like where I live in California. In fact, the NAIMA website (North American Insulation Manufacturers Association) specifically recommends against using polypropylene barriers except in climate zones 5 or higher (CA is climate zone 3) – although I realize that NAIMA probably did not have tiny houses in mind when they made these recommendations. In addition, I’m using Roxul insulation which I believe is fairly resistant to moisture damage.  Finally, I’m not planning to live in my tiny house full time so I don’t think that much moisture will actually build up inside of it without being able to dry out.

The information above was enough for me to decide not to install a vapor barrier on the interior walls of the house. There are a few reasons that I decided to still install a vapor barrier on the ceiling. First, I already installed poly on the ceiling to hold the insulation in. Granted, it needs to be sealed to be effective. However, that will not be that difficult since there are no electrical outlets or windows to go around accept a single skylight. Second, if moisture does accumulate in the house, it is most likely to rise and accumulate in the ceiling, not in the walls. Finally, and perhaps most importantly, I have already created what I think is a very good ventilation path in the space between the rafters, leading from the bird blocking all the way up to the ridge beam. That means that if any moisture does somehow get behind the moisture barrier in the ceiling, it should still be able to dry out again. All of that makes me fairly confident about my decision.

I have not closed up my interior walls yet, so if you’re reading this and have some other thoughts are ideas, please feel free to leave a comment. One thing about building a tiny house is that you never know what you don’t know. So if you think I’m missing something, please feel free to let me know.

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