The foundation may seem like a simple or straightforward part of a build. In reality, there are many key areas of concern here when building a mold preventative build. There are also many smaller materials that the chemically sensitive will want to be aware of in order to specify the best ones.
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If you are building for the extremely chemically sensitive this article will help, but you also will need a copy of Prescriptions for a Healthy Home. And if you want to build a mold-safe home Cheryl Ciecko’s course on Building a Safe Home is essential.
Does Concrete Off-Gas?
Portland Cement/concrete with no additives, admixtures, or fly ash does not offgas any chemicals. It is hazardous when in dust form, however, since it contains silica.
Which Concrete Aggregates are Chemical-Free?
Natural non-toxic mineral aggregates should be used. Toxic aggregates can include crushed sandstone, concrete slag, fly ash, cinder, and volcanic materials other than pumice, says Paula Baker-Laporte in the previous edition of Prescriptions for a Healthy House.
In the latest edition of her book, Paula explains that recycled aggregates can be a source of contamination. Recycled industrial waste can contain hydrocarbons, sulfur, or toxic metals. Recycled brick is absorbent and could have absorbed chemicals from its previous life, depending on where it was used.
Fly ash and volcanic materials contain toxic metals, Paula prefers not to use them. Metapor Metakaolin is one safe alternative to fly ash.
Form Release Oils
For healthier form-release oils see Prescriptions for a Healthy Home.
There are a lot more details on the foundation like rebar and other details in Prescriptions for a Healthy Home. If you are building new and have a high level of MCS this book is a must.
Under the concrete, there is a plastic layer that acts as a vapor barrier. 6 mil poly is not enough according to Cheryl Cieko.
It should be 10- 20 mil plastic says Cheryl. Stego is a good brand.
Insulation in the Foundation
XPS or EPS are the foam types used under the concrete foundation floor. XPS is more of a vapor barrier but it does not replace the plastic vapor barrier layer. The seams of XPS should be taped. See my post on insulation for the hazards of each type.
Other alternatives that are flame-retardant-free and blowing-agent-free are Rockwool Comforboard, more commonly used in Europe, and Foamglass or Glavel. (More info on those here).
The foundation also needs insulation on the exterior side, below grade – usually, XPS is used here but the same options used under the slab can be used here.
Dampproofing Layer on the Exterior
A liquid asphalt product here is the norm which will be OK for some sensitive people since it’s below ground. However, this is only dampproofing not waterproofing so you can still have trouble with moisture infiltration. A more effective product that is also good for the chemically sensitive is AFM Safecoat Dynoseal (which is a water-proofer).
Mat Risinger explains: “effective foundation waterproofing is more than just one product; it’s a system with three critical components: a membrane to protect the concrete; a drainage mat to relieve hydrostatic pressure and allow water to drain down, instead of in; and a French drain at the footing level to carry water to a daylight drain or to a sump pump.”
Paint that goes on the above-ground concrete foundation part has to be vapor open. See options here.
The sill seal acts as a capillary break between the wooden sill plate and the concrete foundation. This is often a simple polyethylene foam, which is not toxic.
Using pressure-treated wood (PT) as the sill plate is usually code here and usually unavoidable.
Fentrim tape by SIGA can be used to seal the intersection between the sill plate and the concrete foundation on the outside. There are also liquid products that can be used here, there is one by ZIP systems which is likely to be extremely low in offgassing.
Caulking is sometimes added between the sill plate and the concrete. I would avoid liquid polyurethane caulks/sealants there (due to high offgassing). Silicone also won’t last according to Mike Maines. He recommends Pro Clima Contega HF on the interior side which is an acrylic caulk. This is very low in offgasssing but not as low as polyether caulks.
Interior Walls of Basement or Crawlspace
Do not put a vapor barrier on the interior walls of your basement or crawlspace. This wall has to dry to the interior – it can only dry to the interior. Don’t use a “poly” barrier here nor XPS, plastic- or foil-based EPS, or polyiso foam.
Cheryl recommends leaving walls bare for at least 2 years so they can dry out and she prefers leaving them bare forever. There are a lot of details that go into building a mold preventative foundation and you definitely will need to see her course to get it all right.
If you have to insulate your basement walls in the future, use a breathable insulation on the interior like Rockwool Comfortboard.
Rim Joist Insulation
I would not use two-part spray foam here because of the risk of offgassing. I would use XPS rigid foam plus one-part canned spray foam to seal around that. You can also avoid one-part spray foam here and use XPS with a caulking sealant or even a tape, says Ben Bogie from BS and Beer.
If you also need a batting there you can use Rockwool.
Finishes for Basement and Crawlspace Floors
If your floor is smooth and not too dusty, and you just need the most basic sealer for a storage space, you can use a silicate sealer like Radon Seal. This is 0-VOC, odorless and totally benign.
If you will be using the basement as a living space I would go with polished concrete or tiles with breathable rugs. This allows the floor to continuously dry to the inside.
Powdered pigments can be added to the cement before pouring to add more color.
If you need a self-leveling cement, see this article for non-toxic options.
There are basements, crawlspaces, and slab-on-grade foundations.
Both basements and crawlspaces must be conditioned spaces to be mold-preventative. See Building A Safe Home, a foundational course for more info. You should also use a dehumidifier in a basement or crawl space.
The mechanical room must be in a conditioned part of the house.
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