Peripheral Heat Loss

This drawing of the wall section shows that the foundation is insulated on both outside and inside, but the footing is not insulated. Therefore cold will enter (or heat leave) primarily through the footing. The top of the foundation is insulated from the room temperature by only the wallboard and the 2x4 sill that is bolted to the foundation.In the drawing above the slab is insulated from the foundation by 1 1/2” rigid insulation thermal break. My question is this, Is this thermal break necessary? It will be costly to construct, because with the thermal break the foundation and floor slab cannot be poured at the same time. (Or can they?) And this thermal break also provides a structural break between slab and foundation - which may be bad if the ground settles under the house.

How can we determine whether that thermal break is necessary? How much added heat will be lost without the thermal break? I see two ways to determine this:

• estimate the heat lost through the foundation
• measure the temperature of a floor slab that is cast together with the foundation.

Let’s assume that the footing will be 40°F. It is below the frost level. That means there is a 30 Fahrenheit degree difference between the top and bottom of the foundation in winter. The R-value of high-density concrete will be about 0.2/inch. The inverse U-value is 5 Btu/(hr °F sq ft). We have approximately 180 feet of perimeter x 10”/12”/ft foundation x 30°F x 5 Btu/hr = 750 Btu per hour - a trivial heat loss at the perimeter of the floor slab.

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