Moisture control & insulation in New England tiny house
Hi there! First time posting here.
I am converting a 10′ x 10′ shed into a tiny house in New Haven, CT and trying to figure out how to adequately control moisture in the roof cavity, walls, and floor insulation. There will be no running water, so I am expecting humidity inside to be relatively low, except we were also thinking of only running heat (space heater) half the day in there to save energy, so I’m not sure what impact that cycling might have on movement of moisture and potential condensation.
WALLS
We have a bunch of polyiso rigid insulation that has been cut to fit between the studs, which I will spray foam around to secure in place and seal. People online say that poly sheet for vapor barrier is the old way to go but can trap moisture in the wall and is generally not needed. People say air barriers are much more important, but since the inside won’t be that humid I’m not sure this is essential either. Won’t the humidity in the wall be somewhere in between the outside and inside humidity no matter what?! In New Haven, outside humidity seems to range from 60-70% average throughout the year–in other words, HIGHER than it will be inside, so wouldn’t I want the humidity inside the walls to be closer to the internal humidity? And therefore not have a vapor/air barrier?
FLOOR
I laid 1.5″ of NGX underneath the floor. Trying to decide whether to leave exposed or to paint or cover with poly sheet stapled to the joists. People online seem to leave it exposed from what I can tell.
ROOF
Not sure whether to go with a vented or unvented roof. Again, because I am expecting humidity to be relatively low, I’m thinking we may be able to get away with an unvented roof? Would it be helpful to simply add gable vents without soffit vents? Do I need to air seal the roof off from the interior of the structure?
Thank you for the feedback in advance!
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Replies
Vapor moves based on differences in vapor pressure, not relative humidity. Since cold air isn't able to hold as much moisture as warm air, cold air with a high RH might be drier than warm air with a lower RH. For example:
Outside air at 0 °F and 65% RH:
contains 1.38 grains of H2O per cubic foot
partial vapor pressure 0.117 inHg
Inside air at 70 °F and 50% RH:
contains 4.0 grains of H2O per cubic foot
partial vapor pressure 0.37 inHg
So the interior air contains more moisture and therefore has higher vapor pressure, causing vapor drive from the interior to the exterior. Even with no running water inside, two people breathing in a small room will add a lot of moisture to the air.
How thick is the polyiso you are using, and does it have a facing of foil or something else? If the polyiso has foil facers, the facers are a class I vapor retarder, so no need to add another. If the polyiso is unfaced, then it will likely be a class III vapor retarder. The IRC allows the use of a class III retarder in your climate if the exterior cladding is installed over a vented air space.
I agree that a good air barrier is more important though, and that applies to the ceiling as well as the walls. The foamed-in-place polyiso will help a lot with air sealing, but you still have to address the remaining gaps such as under the wall bottom plate and between doubled-up framing at jack studs, corners, etc.
If you haven't installed all the polyiso yet, I'd suggest installing at least some of it in a continuous layer on the exterior, where it will cut down on heat loss through the framing and also keep the sheathing warmer.
Each resident produces about five pints of water a day, that's your major source of moisture.
You need to be constantly removing that moisture. You don't want it going into the insulation. The general formula in a heating climate is enough ventilation to remove the moisture, a vapor retarder on the interior of the walls to keep the moisture out, and a wall assembly that is capable of drying to the exterior so that any moisture that does get into the wall has somewhere to go.
Are you planning on cooling it during the summer?