Posted By drlebel on 13 Nov 2012 05:01 PM
Most people in the south don't insulate their foundation much but when I input R-11 edge insulation in HvacCalc it reduces my BTU loss by 2/3 thru the floor.(12,000 down to 4000) Does that seem about right? Does this insulation go in between the fill and concrete on the bottom or just on the edges of the concrete on the side of the foundation? Anyone know a good average cost for this level of insulation?
That's right!
For slab-on-grade stemwalls it's usually placed between the stemwall and fill, running all the way down to the footing to the top of the concrete, leaving the slab floating, bounded by a strip of EPS (thermally breaking the slab from the stem wall.)
You can use either EPS or XPS (or compressed rock wool products that aren't really available in the US yet, but which are termite resistant). EPS is usually cheaper, and runs 8-10 cents/R per square foot for 1.5lb density (type-II) EPS. To hit R10-R11 takes about 2.5", and would run about ~$1 per square foot. There are borate-loaded termite resistant versions if that's an issue in your area, but if it's on the interior side of the stem wall you can also just backfill the foot or so adjacent to the EPS with compacted sand as a termite countermeasure, which is also quite effective (probably more effective than termite-resistant EPS.)
Bottom-of-the-line insulated concrete forms start at R16 (2" of EPS on both sides of the concrete) which is a bit overkill for stem walls if you're in climate zones 3 or 4, but not necessarily for zones 5-7, since it goes in quicker than adding the insulation after pouring the concrete.
In climate zone 3/4 it would also be long-term cost effective to put R4-R6 worth (1-1.5") at full coverage under the slab.
See table 2, p10.
[edited to add]
This picture may clarify
how it goes together.
In high-R wall assemblies it's also important to thermally break the bottom plate/sill of the wall, which is sometimes more readily done on the interior side. There is a stripe of thermal bridge of the upper concrete + a couple inches of foundation sill wood that can't be more than about R2.5. This can be a tough one to solve when there is no exterior foam on the stem wall, depending on the wall assembly, but more important to solve in colder climates than in zones 3 or 4.