A Word on Moisture

I thought I’d done all the research I could possibly do on moisture control in construction but all this rain has me reconsidering. I had to go back out into the house to get a clamp and took the chance to recheck the status of the tarp and how it had fared with the ceaseless wind and rain overnight. It’s doing great – thanks Cindy and Rose for helping us not only get it up in record time last night but also do so pretty damn close to perfectly! But for all that there isn’t a drop of liquid water to be seen, everything still feels damp. Maybe it’s just the air, the humidity’s creeping up pretty close to 100%. But all the moisture in the air has to mean moisture in the wood and that’s got my jittery about closing in the floor and walls this week without giving them time to dry.

So I’ve just rechecked all my moisture control plans and thought I’d do a short intro post on the importance of paying really close attention to moisture control in a tiny house build and how complicated that can be. I plan to do a longer post explaining all the terms and concepts thrown around when talking about moisture in construction as well as more detail on some of the solutions we’re using in Carriage House, but that will have to wait until the build is done and I have a little more time on my hands. For now, even with the rain, there’s still plenty that I need to be working on in the garage if I’m going to get this house livable in the next three weeks! So here’s the crib-notes version of where we’re at.

The three cardinal rules of moisture control I’ve been working with are:

  1. Keep liquid water out but design for the worst case scenario with ways it can drain if it gets in (think a roof leak or a burst pipe).
  2. You cannot have too tight of an air barrier. Think wind blowing through cracks in your wall, if it stops that then it’s an air barrier. That air can carry a lot of water and cause big moisture problems. We’ve got an air barrier on the inside made of taped rigid foam insulation and another on the outside made of taped asphalt fiberboard so there shouldn’t be any damp (or cold) air getting through our walls.
  3. Vapor barriers must be on the warm side of the wall if you even have one at all. Think two boxes in a sauna, one’s made of cardboard and the other plastic. They’re both sealed up tight with tape so no air’s getting in or out. Leave them in the sauna for a few hours and the air on the inside of the cardboard box is going to be just as humid as the air everywhere else in the sauna but the air in the plastic box will be as dry as the moment you put it in there. The plastic is a vapor barrier, the cardboard is not. The great thing about vapor barriers is they keep humidity out of your wall. The problem with them is when they’re cold and the humid air is warm, then you get condensation. Think single pane windows in winter. So vapor barriers must always be warm!

I’m still pretty happy with our wall design: between rainscreen siding, highly vapor permeable asphalt paper housewrap, very moisture tolerant asphalt fiberboard sheathing and sheep’s wool insulation, and a taped layer of rigid foam on the inside keeping interior moisture (from breathing, showering, cooking, etc) out of the wall, those walls should be unlikely to get damp and dry pretty damn well if they do.

The floor though has always been tougher to design. Because the house is on a trailer, it needs a splashpan underneath that will be able to manage debris thrown up from the road. Unfortunately, pretty much every material that’s good for that is also impermeable to water vapor and so (per rule #3) really shouldn’t be on the outside of the house. To solve this we settled on polycarbonate plastic panels that are lapped and not taped so liquid water should easily drain out through the cracks. But it’s really a compromise, not a solution: vapor is still going to take a long time to work its way out as the gaps are very small (necessary to prevent moisture being driven up into the subfloor when splashed up by the wheels on the road). Like the walls, the floor is currently designed to dry to the outside with a rigid foam vapor barrier under the subfloor. Now I’m wondering if that foam should be removed alltogether and just tape the OSB subfloor for our interior air barrier, letting the floor assembly dry to the inside. Of course, if the interior lets more vapor out of the floor, it also lets more in (that can then condense on the cold splashpan). Guess I’ll just have to decide which is worse, here’s to catch 22!


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