Posted By PanelCrafters on 17 Mar 2007 09:27 PM
BTW, I hadn't thought about the heat loss through the footers. That makes the ICF wall even less efficient than the 5" insulated poured wall.
I don't think that there is any heat loss through the footers(heat won't get there). My point is that the ground temperature is thermally conducted into the wall(from the footers), which is a good thing as it reduces the Delta T(especially in above grade applications).
The key is simply preventing the heat from escaping the 'Thermal Envelope'. Because once it does, I doubt that you'll see any benefit.
Huh? Heat won't move from the house to the concrete, and from concrete into soil? How does that work? That statement seems counter to some pretty well-established physics.
Ground heat is conducted into the wall, in some seasons in some locations, but in most of the US the direction of heat flow is from the wall into the footer, all year long. Heat flow into the wall is from the interior all winter long, but only out of the footer into the wall when averaged daily interior & outdoor temp is below that of the subsoil. Only when the concrete in the wall is above the temp of the subsoil is the flow reversed. In a 70F house with 50F subsoil and an extremely shallow footing, it only starts conducting out of the footer in an all-above grade wall when it's below 30F outside. But when that condition persists over a season the soil below the footing drops too.
Since the footer by code typically has to be below the design freeze depth, with the ICF going all the way to the footer it has to be even colder still outside, in part because the depth of say 4-6' of concrete has the same order of magnitude R as the ICF skins. Concrete is not a refractory material. Soil isn't a refractory material either, but it's less conductive that concrete, but with 50F soil there's a constant heat load 24/365 of heat moving from the interior to the footing and beyond. But it matters what the subsoil temps are- in very colder climates it's worth insulating the footers (even underneath), but in TX, not so much.
The effect of that thermal conductivity pf the footing to the subsoil has nearly no consequence for the above-grade temperature of the concrete or delta-T experienced by the wall. In a conditioned basement with 5' of below grade concrete between grade and the footing you have about R5 of isolation from the footing in just in the thermal resistance of the concrete, and less than 1/5 the surface area/cross section as compared to the area of the R10 interior & exterior foam. It's still highly dominated by the interior & exterior temperatures. Heat flow is an delta-T x R x surface area thing, and just the bottom two feet of foam above the footing account for a bigger heat flow into/out of the footing than the rest of the wall added up, so by the time you're above-grade the effect on the concrete temp is negligible. But the 24/365 heat load going through the lower half of the wall is still readily measurable with 50F subsoil, even if it's not always worth treating. Slab on grade stemwalls in shallow frost depth regions with warmer subsoils may get a measurable net benefit out of the conductivity to the soil, but not in most heating dominated climates.