Anyone have a reference/link for soil temperatures under a slab? I've been reading, this forum and elsewhere, about insulation under the house slab. It seems that the folks recommending it don't differentiate between a heating dominated climate or a cooling one. Is it needed/recommended for a climate such as central Texas? Thanks.

The US Department of Agriculture has some sites that report air temp and soil temp versus depth for a variety of locations. Here's a link. See if you can't find a site near your location and view the data. I suggest looking at soil and air temps, historical, calendar year, and choose the "chart this data" option to view data for a whole year.

http://www.wcc.nrcs.usda.gov/scan/

Without slab insulation you'll likely see some heat gain in summer and some heat loss in winter even in a very moderate climate like central TX. Whether the net losses are worth insulating is a somewhat difficult question. The guys at Building Science Corp. seem to think slab insulation is worth it. Check out this report: The Building Science Corp. publication RR-1005 High-R Enclosures for All Climates, Table 0.2 on page 10 lists recommended insulation values for various portions of residential structures, and is delineated by ASHRAE climate zones.

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view

I'd at least consider insulating the slab edges as this has been shown to typically have a more substantial effect in heat gain/loss.

The canadian govt has some freeware called Basecalc that can calculate basement or slab heat/gain loss over a year for a variety of foundation and insulation configurations. If you're really interested, you might check it out. It's a bit rough on the user interface, but the results can be interesting.

http://canmetenergy-canmetenergie.nrcan-rncan.gc.ca/eng/software_tools/basecalc.html

Some will say that sub-surface soil temps are "constant" year round. I imagine this probably only applies once you reach the water table. The USDA site data shows significant sub-surface soil temp changes (at least at depths of 3-4 ft) year round. Keep in mind, you won't get the solar gain under your slab in the summer, but the soil adjacent to your slab will.

For what it's worth, after reviewing all the above, I'm planning to insulate at least my slab edges in Southern New Mexico (a slightly heating dominated climate).

Ground water temperatures in your area (Austin/San Antonio) are around 76 F (http://mb-soft.com/solar/soilmap.gif), so you could probably do with just insulating the slab edges at most, but that is just a guess. Do your homework.

Soil temps are also not constant as soon as you start adding or removing heat. I can't count the number of times I've seen people do calculations that assume that the earth is a steady and infinite supply of heat/cooling at x degrees. Soil has thermal mass and acts as insulation.

According to the Building Science guys (snoslidr's reference above), recommendation is R-5 on slab edge and nothing underneath the slab in that area. R-5 would correspond to 1" of XPS Styrofoam.

Posted By Lee Dodge on 18 Sep 2011 02:51 PM
According to the Building Science guys (snoslidr's reference above), recommendation is R-5 on slab edge and nothing underneath the slab in that area. R-5 would correspond to 1" of XPS Styrofoam.

The exact zip code counts.  Central TX (including Dallas/Ft.Worth, Abliene, Big Spring) is US climate zone 3, where the BSC folks' target is R7.5 slab edge + R5 sub-slab.

Austin & San Antonio are on the cool edge of Zone 2, where the target would indeed be R5, slab-edge only.

http://www.architecture.uwaterloo.c..._zones.jpg

Mind you these are targets based on energy, site, and cost assumptions. The economics can and will vary in viability with the specific site-conditions & local construction costs, a "one size fits most" situation.  Going with cheaper materials & methods there can often be an economic rationale for even higher R values, or more expensive methods maybe the other way around. Eg:  Using reclaimed XPS or EPS from commercial re-roofing  going 2x those numbers can make economic sense, but using 2lb or 3lb spray polyurethane even R7.5 at the slab edge becomes uneconomic.

If hydronic radiant floors are going to be used for heating, add at least R5 to that table.

Dana1-

The original poster was not very specific in this post except to say "Central Texas," but in his previous post titled "References...." he mentioned "Southwest Texas ... between Austin and San Antonio ." Central Texas usually refers to that area, bounded on the south by San Antonio, the west by Junction, the east by Austin, and the north by maybe Brady (at least, in my opinion!). Dallas/Ft. Worth area is called Dallas/Ft. Worth area. Big Spring, Midland, and Odessa are in the Permian Basin area, or more generally the High Plains. I think Abilene would be considered High Plains and almost in the Panhandle of Texas. As you have said, the zip code would be more specific.

So I would say definitely Zone 2, and the insulation recommendations by the BSC guys are already pretty high in the amounts specified, so R-5 on the edges would likely be sufficient. As you say there might be some site specific things that would change this recommendation.

The big energy user in that area is cooling, so figuring out a shading strategy is probably a more significant issue.

Aha facts of location were known if not in evidence. (And I guess I needed the local geography nomenclature primer too...)

Sounds right to me then!

Thanks to everyone for the information and my apologies for 'disappearing.' I've been away from the computer for a while. Location is correct (78655) as guessed/surmised. I need to do my homework with the links provided. Thanks again.