I am designing a house for myself and my wife. I have a bad mold allergy, and my wife has multiple-chemical sensitivity. I have seen some knowledgeable forum members advocating the benefits of a breathable house envelope. I am in agreement for the walls. I plan to use Hemcrete, clay plaster and lime stucco. For the foundation floor and the ceiling, I am not sure which way to go.

For ease of cleaning of the floor, I am leaning toward ceramic tiles with epoxy grout, which out-gasses in a few days. This acts as a vapor barrier. I would choose a more breathable floor covering if I could find another that didn’t stink, didn’t grow mold when fruit juice spills on it, didn’t serve as food for pests, didn’t burn and was easy to clean. Given that I live in the Southeast US where high humidity is a problem during three seasons, I don’t see how allowing humidity to rise from the soil through the floor would be better than a vapor barrier as a floor covering. As long as the foundation materials are not damaged by mold, why would advocates of breathable house envelope say my floor covering is a bad idea?

My roof will have soffit and ridge vents. For my ceiling, I plan to use clay plaster over MgO boards and air krete insulation. None of these materials provide food for mold or termites and they all provide good fire resistance. There are several companies offering MgO boards, only one of which I found specified the perm value, which is 2.46. This is about the same as concrete, which is also a vapor retarder. Thus, the MgO boards are not in the spirit of a breathable ceiling. I haven’t been able to find a highly breathable wall board that is code approved and cost effective in the the US. However, if I have good breathability in the walls, would this cause any problem?

For readers who have never heard of the product, “MgO boards” means “magnesium oxide wallboard.” (You can Google it; that’s what I did.)

First of all, I agree with your analysis concerning the tile and grout. (I’m confused by your reference to the “foundation floor,” but I assume you are talking about either a basement slab or a slab-on-grade.) There is no reason to encourage moisture to pass through a slab into your house. That’s why it’s important to include a moisture barrier (usually 6-mil poly, or something stronger) under your slab, along with a continuous horizontal layer of rigid foam insulation.

The foam will keep your slab warm, thereby limiting condensation in the summer. Limiting condensation in the summer is good, because you don’t want to encourage mold or mustiness. A slab without a layer of foam underneath will be cold during the summer, sometimes leading to condensation. That’s unhealthy.

So now that I’ve concluded that you have done a good job thinking about your slab, you can see that the analysis raises another question: why is it that you want your walls and ceiling to “breathe”? I really don’t know of a single reason why a builder would encourage the flow of moisture through your wall or ceiling assembly.

After all, once your indoor conditions are pleasant — something you can ensure with a heating system or a cooling system — why not separate your indoor conditions from the outdoor conditions? Why encourage the flow of moisture?