Posted By toddm on 07/27/2008 12:29 PM
Alas, I belong in the class comprising 98 percent of American householders who do not have unlimited construction budgets, and won't benefit from the reflexive advice of folks who have products to sell, namely that we should buy more of everything. It is instructive that the only payback analysis in this thread was done by a soon-to-be homeowner. I repeat my advice: If you have to make choices, order your insulation priorities starting at the top of the house and work down. If you can't afford to insulate under the basement floor --as opposed to the foundation perimeter -- it isn't the end of the world.
Better yet, build a frost-protected shallow foundation. Here is a link to a joint HUD/NAHB paper on it: http://www.toolbase.org/Home-Building-Topics/Affordable-Construction/frost-protected-shallow-foundations
As its name suggests, these foundations are not dug to the frostline but rather are protected by insulation. You'll find it in 2006 IRC. The Swedes have been building them for 40 years. Spend some of your savings on concrete on extra insulation and you have a house that costs less to build and to operate. Google the phrase and see how rarely it pops up in this forum. Where is the Whole Earth Catalog now that we really need it?

Posted By toddm on 08/01/2008 11:53 AM
Or you could listen to engineering professors who have spent decades devising models and testing them against real basements. Turns out Basecalc, the Canadian government's basement heat loss modeling software runs fine under W2000, and XP as well, except for some extra steps necessary to launch and exit the program. A couple of runs blows a basement-sized hole in the conventional wisdom on this forum.
I modeled a 1,000 SF basement with 8 inch poured walls, and a 4 inch poured floor five feet below grade (roughly translated from the metric measurements preferred by those pesky Canadians.) I conditioned the basement to a year-round 70 degrees. I modeled heat loss using the software's soil and weather profiles for Wash DC, Minneapolis and Denver. Minneapolis makes the strongest case for insulation, not surprisingly, but the others were in line, so we'll stick with the worst case in Minnesota.
With no insulation, our Minneapolis basement bled 61.5 gigajoules of energy a year. A GJ is roughly equivalent to an MCF of natural gas, so it is fairly easy to adjust the loss for furnace efficiency and translate it to dollars. That would be north of $1,000/yr in the case of no insulation in Minneapolis and natural gas at $15 per MCF.
In the second pass, I added two inches of EPS II to the basement's exterior walls in their entirety. The loss fell by more than half, to 28.8 GJ, for savings of $500+ year that are easily justified.
But that would be the end of the no brainers. Adding 2 inches of XPS under the slab reduced annual energy loss to 26.4 GJ from the 28.8 GJ above. So XPS at $1/SF saves $40/year, assuming $15/MCF gas burned in a high efficiency furnace. The payback is an emphatically unacceptable 25 years. It should be noted here that fuel oil at $5/gallon reduces the payback to 10 years, or just within reason. I am not saying don't insulate. I am saying think about it first, including how that $1,000 could be better spent.
Why XPS disappoints is an excellent argument for studying problems rather than throwing money at them. Butting wall and slab foam to the footers leaves thermal bridges at the precise points that soil can diffuse a fair amount of heat: along foundation edges and most particularly at corners.
In a fourth run, I moved the EPS to the interior of basement walls, put a thermal break (RSI=1) between the wall and the slab, and placed 2 inches of XPS under the outside edge of the slab, insulating its first 3.28 feet. The energy loss drops to 23.5 GJ, for a equivalency savings of 5.3 MCF/yr over exterior EPS alone, or $88/year.
In a final run, I extended the two-inch XPS in the model above to full coverage under the slab, vs about 40 percent coverage in the edges-only scheme. Full coverage reduced energy loss to 21.1 GJ, for savings equivalent to about 2.4 MCF/yr, or $40/yr once again. While the payback on the extra $600 of XPS is better at 15 years, it still isn't good. Clearly, basement floors are not major energy concerns, even in Minnesota.
Lest you think I am fudging this somehow, I refer you to a 2005 study by a Canadian goverment task force: "Performance Guidelines for Basement Envelope Systems and Materials" http://irc.nrc-cnrc.gc.ca/pubs/rr/rr199/rr199.pdf The 193 page study presents best practices and uses Basecalc to perform cost/benefit analyses. It ignores slab insulation except in a single word on page 16: "(Optional)"