# Does the soil under an insulated slab warm up?

| Posted in Energy Efficiency and Durability on

The point of this question is to see if you really need R20 under a slab or if R10 is good enough.

The water from the street in Denver comes in at 45F in the winter, and 65F in the summer. That tells me the average ground temp is 55F, since that water has traveled many miles at a four foot depth.

From Wikipedia, the ground temperature remains very constant year round below 20 feet.

If you build a 40′ x 40′ monolithic slab with R10 under it, R30 around the perimeter to a depth of 18″, and a house above it that is always 70F, will the average ground temperature under the middle of the slab eventually increase above 55F?

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### Replies

1. GBA Editor
| | #1

Kevin,
Although the ground temperature may well be constant 20 feet below grade, the perimeter of your slab is not 20 feet below grade.

The question you raise is fairly contentious, and has been debated a few times before here at GBA. Suffice it to say that there are many variables, and different data sets can be quoted to prove your point, no matter which way you lean. (If you want to read more on the topic, see the 96 comments posted below this article: Can Foam Insulation Be Too Thick?)

The soil directly under the center of your house should be warmer in winter than the soil at the perimeter of your house. To me, that's not enough of a reason to skimp on insulation. You will only be installing insulation under your slab once. I can't imagine that you will regret putting R-20 of rigid foam under you slab, but you might well regret putting too little insulation under there.

2. | | #2

Martin,

Yes, I plowed through those comments again. It's non-linear and hard to model, the existing models are probably off, but John Straube thinks they err on the safe side of more insulation.

The "Geothermal Gradient" is typically only 1 degree F per 70ft. of depth. That proves that the underslab temperature won't rise significantly because of geothermal heat coming up from below.

3. | | #3

The temperature changes in the potable water can be easily visualized by looking at a wine cellar example: http://www.winecellarinnovations.com/blog/2011/01/05/wine-101-passive-wine-cellars/ the last few feet that the water pipe travels under your foundation is probably insignificant in the water temperature drop/rise -- if the water is flowing.

If you want to model the floor your self, use THERM follow the examples that BSC and others have provided - turn the model 90 degrees and model it. Use 55 as the mean (see link above -- it explains the variation in water temps).

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