Frost-Protected Monolithic Slab w/ 2-in. Foam

Foundations, Floors, and Walls Are Critical Green Connections

Air sealing is imperative. The connection between concrete foundations and wood framing is a place prone to air leaks and moisture problems. Wood is often warped, and concrete is rarely flat. There are at least three places for air to leak in and probably a lot more. Leaky connections can mean energy, moisture, comfort, and IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness. problems. Extending the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. past these connections is a good first step whenever practical. Caulks, adhesives, spray foam, and gaskets can seal them up tightly.

Keep water at bay

Unless you live in the desert, the ground is always wet; and that ground water is always pushing its way in. Footing drains can carry away bulk groundwater, but foundations also have to disrupt capillarity. Water in the soil will wick all the way up to the roof framing if you let it. CapillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. breaks such as brush-on damp-proofing, sill sealer, and rigid insulation block this process. A foundation is a bad place to cut corners because problems are expensive and complicated to fix after a house is finished.

Monolithic slabs are fast and economical

Not only do slabs eliminate the need for floor framing, but they also reduce the excavation and concrete needed for a crawl space or basement. And concrete slabs can be used as the finished floor. Monolithic slabs are great choices in any climate, but not any lot—relatively level lots are needed.

Insulated slabs improve comfort. Install a continuous layer of rigid foam insulation around the slab, except in hot climates or in termite-infested areas where local codes may prohibit the practice. The insulation buffers the slab from outdoor temperature swings, keeps the slab warm and dry, and lowers energy bills.

Learn more in the Green Building Encyclopedia

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Nov 23, 2016 6:36 PM ET

No underslab insulation?
by Kenneth Gartner

In a regime which has to meet an aggressive HERS rating (like Massachusetts), I think that one might find there is insufficient thermal decoupling with the main slab, no?

The diagram shows 4 foot of skirt foam, which suggests there is a concern about frost-protection in a seriously cold climate (and one less likely to be concerned about termites right now, one would think). Brrrr. Underslab foam would help meet the Energy Code guidelines and save heating costs and certainly worth a recommendation, albeit at increased cost. Was underslab insulation removed from the diagram because it was deemed unnecessary or because there is some particular negative associated with it in your scenario? I am curious. Also since much of the heat is lost around the perimeter, wouldn't having an extra inch (~R15) at the slab edge provide an affordable advantage worth pursuing in that climate?

Assuming that 'climate change' might broaden the range of termites during the next 3-5 decades, it seems it would be prudent to defend against them even in areas that are currently outside the termite range. I am thinking that adding a second, J-shaped, metal flashing (upside down over the exterior slab wall foam) immediately butted up to the current rain flashing with the drip edge and adhered to the concrete with silicone or similar terminate-unfavorable boundary. It seems a cheap insurance.

I have seen variations of this diagram* and still I have not been able to figure out at what point the 6 mil poly is cut on the outer edge -- 4-6 inches up from the bottom of the grade beam? That is assuming the external wall is dampproofed/waterproofed above that point (and the foam adhered directly to that damp-proofed wall via some kind of mastic or maybe screws), though not shown in the diagram.

* -

Best, Ken