Temps under a slab
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| Posted in Energy Efficiency and Durability on
Does anyone have any links for the earth temperatures under a slab on grade, including under the footer and out a bit from the building? I’ve seen a very few that are close, and some isotherms w/out temps, but I’d like specific information geared to cold climates (14K dd) with 4″ of XPS under the slab, ICF foundation walls, and with/without spacing between the under-slab insulation and the foundation wall. Kind of picky, I know, but that is what I need. Thanks. john
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John K
I know this is not what you are looking for..but it is an interesting graphic
http://www.passivhaustagung.de/dreizehnte/Bilder_Rueckblick/13PHT_Wanderausstellung_Modell2_200904.JPG
Took me a minute, but I finally realized what it was. That is for what I am striving, though. I want to put very little heat into the earth, but don't think I want to just drop 12" of XPS under the slab. I read an article where Passivhaus used about 14 or 16" of EPS under a house, but one needs to look at the economics of that. At some point, the house will fall down before you recover the $$ in insulation, anywhere in a house. I keep reading "2 inches on the perimeter, 1" in the interior, as slabs just don't dump that much heat" (and/or, the temperature under the slab stabilizes in a few years) but I find it hard to swallow that. But, data has no bias, so I want to see STUDIES, not opinions. Thanks for the picture. j
I'm not sure what you're describing. It sounds like a slab poured inside of a full ICF frost wall on footings (at what depth?). And what do you mean by "with/without spacing between the under-slab insulation and the foundation wall"? Are you referring to slab edge insulation?
You're not likely to find measured ground isotherms that match your application, and the soil conductivity, depth to ground water and snow cover determine the site-specific conditions. But I have several ground temperature profile graphics, including one of measured ground isotherms around an unsinsulated full basement and a crawl space with slab on grade in Saskatoon summer and winter 1956.
Email me at HouseWright at Ponds-Edge dot net.
Robert: Thank you for the offer, and sorry for the brevity above. I will have a SOG, poured over the top of a 4-course ARXX block foundation wall, which will sit on footers approx 63"" below grade. I will have at least 4" of XPS under the slab, and the engineer that designed the foundation system suggests a 16" space between the XPS and the foundation wall. He wants heat to leak down to the footer, but that concerns me and I can not find anyone who also suggests that. All folks say "I am not familiar w/ Frbnks building, but I don't see the need for the gap. Run the XPS right up to the ARXX blocks", which is what I want to do. Thus, I'd like to see what temps exist down there, typically. IMO, since the bottom of the footer will be 63" below grade (code is 40"), I'll not have any frost problems by running the XPS right up to the ARXX blocks. I'll email you, and thanks again. john
Dear J,
Check out this blog and the comments on the same page:
https://www.greenbuildingadvisor.com/blogs/dept/musings/can-foam-insulation-be-too-thick
Here is a posted comment from John Semmelhack: "I calculated an average ground temperature for the heating season (approx. Oct. to April) for the Helsinki, FInland climate data set from the Passive House Planning Package (PHPP) and came up with 41.3F. I also used the classic rule of thumb for average ground temperatures: "average annual ground temperature = average annual ambient air temperature" to get 40.2F (again using the PHPP data set for Helsinki, Finland)."
John Straube wrote, "A slab on grade insulated to R-32 in Finland had an average heating seasons soil temperature of 12.5 C (55 F)."
Many more comments, some of them contentious, follow these.
John,
Where are you located? Your foundation design should strive to eliminate any deep earth losses, isolate the heated space from the soil.
Martin: Thank you for the info and link. That is what I am after; will read asap.
Doug: Agreed. I am in Frb, AK. Heating degree days = 14K.
OK, I had previously copied that article, but had not read it yet. Good read. It may be only academic, but it wouldl be interesting to see what the conditions were (temps, insulation) when J. Siegenthaler developed his SOG heat loss formula, which uses only the R value of the slab's edge. I assume the model would change somewhat as the parameters used to collect the data for that model change. Either way, I think I have about exhausted my desire for information on this topic, and it is time to start building! For that, I thank the many folks who have explored this area and shared info.