Hello everyone. This MAY seem like a retarded question but I do need to ask it. Being that we insulate a basement floor before radiant heat is installed, would not a portion of that heat radiate to the foundation wall. This would then allow creep to the footing which has no insulation. The ground, even six feet down, would leak warmth. Even using ICF's, the captured thermal mass would radiate out through the footing. I am trying to cover ALL loss bases here. Would using 2" XPS foam under the footing be feasible or even pass building code?

ould using 2" XPS foam under the footing be feasible or even pass building code?
It is feasible and being done in some houses. However, you can isolate the floor from the footing by placing a vertical strip of 2" styro against the foundation wall and on top of the footing so that the floor does not contact the foundation or footing. Isolating the interior bearing points - lally columns - is another matter. Can be done, but an option is to leave a 6" column of concrete between the interior footing and floor only at these points.

If the slab is floated, thermally broken from the walls with 2"of XPS the thermal bridging of the slab is taken care of.

See the details of thermally breaking the slab without under-footing foam in figure 38, p51, or figure 39 , p53:

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

True, some heat is still lost from the room through the walls with non thermally-broken footing, but the room air is going to be much cooler than the bottom of a radiant slab, and the interior EPS of the ICF is still an R8+ break. At 8' above the footing the R value of the concrete column is also about R8- definitely not nothing, even at R1/foot.

But 2lb density EPS or XPS higher under the footing usually has sufficient load capacity to carry the foundation wall + timber-framed upper floors. Sure, it's doable, but it's easier to just thermally break the slab at the walls.

Thank you both. The article Dana 1 linked was is very interesting, especially concerning the ICF. The article mentions it may take years for the concrete to fully cure. Would there be structural problems from foundation loading during this slow cure process? Regards.

Posted By Dana1 on 27 May 2011 11:17 AM
If the slab is floated, thermally broken from the walls with 2"of XPS the thermal bridging of the slab is taken care of.

See the details of thermally breaking the slab without under-footing foam in figure 38, p51, or figure 39 , p53:

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

True, some heat is still lost from the room through the walls with non thermally-broken footing, but the room air is going to be much cooler than the bottom of a radiant slab, and the interior EPS of the ICF is still an R8+ break. At 8' above the footing the R value of the concrete column is also about R8- definitely not nothing, even at R1/foot.

But 2lb density EPS or XPS higher under the footing usually has sufficient load capacity to carry the foundation wall + timber-framed upper floors. Sure, it's doable, but it's easier to just thermally break the slab at the walls.

Thanks a lot for sharing such useful, valuable and much needed information.