Tiny house on wheels — building envelope (vapor-open) suggestions
I am in the process of building my tiny house on wheels (My home will be in Zone 3b (In between Los Angeles and San Diego) or Zone 3c), with a local builder. He is covering most of the build, but I am helping out when i can.
I have been doing loads of research on passive/breathable homes and want to make sure we do our building envelope correctly! I am extremely allergic to mold, so this is even MORE important to me than the average person.
There is a lot of information online for how to build tiny houses but rarely do you see good information on the building envelope…Which I think is one of the important things to consider (especially in such a small space).
I consulted with a very knowledgeable passive house designer, and he gave me a solid foundation of suggestions to start with.We are going for a completely, vapor open wall assembly to allow to dry exterior and interior. The home will be water tight, air tight, but vapor permeable. Of course, there are plenty of windows and proper Hvac for good ventilation.
Here is what we are thinking for our building envelope:
Wall assembly: Gypsum board OR Wood –> Intello plus (vapor retarder) –> Insulation (Roxul)->Sheathing (Plywood)->rain proof barrier-> air barrier->WRB (mento plus)-> Western cedar siding
Roof assembly: Gypsum board OR Wood –> Intello plus –> Insulation (Roxul)->Sheathing (Plywood)->rain proof barrier-> air barrier->WRB (mento plus) –> ventilation gap (1×3 lumber) –> metal roof
Now, the floor envelope is what I am really struggling with, because my metal trailer (Iron Eagle PAD series) has a galvanized steel bottom pan. In other-words, completely vapor closed.
After speaking with some tiny home companies and brainstorming myself, these are my two options:
Dry towards exterior: Poke small holes in bottom of bottom pan with air gap, to allow moisture to dry and drain out bottom. Order would be: Metal pan (with holes) –> Air vent gap –> rigid foam –> vapor barrier –> Advantech or plywood
Dry towards interior: Completely vapor/water/air tight (like a boat) and a vapor open assembly. Order would be: Metal pan (with holes) –> Air vent gap –> roxul–> Advantech or plywood.
What do you guys think about my current building envelope and what do you suggest for the floor? Is it ok to have a vapor open wall assembly with a different floor/roof envelope? Also, is it common to use a different insulation for flooring, than the rest of the house?
Thank you so much for taking the time to read.
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