Should floors over concrete slabs be insulated in warm climates?
michaelbluejay
| Posted in Green Building Techniques on
I remember reading years ago that floors over slabs in CZ-2 (e.g., Austin) shouldn’t be insulated, because the ground temperature is about 68°F, which sucks the heat of buildings in summer (desirable), and in winter, the ground temp isn’t lower than what we heat our homes to, so there’s not much heat loss from the home to the slab in winters. Of course I can’t remember now where I read this, but I must have considered the source reliable because I took it to heart.
However, I just noticed that IECC R402.1.2 (2021) prescribes R13 for floors, and makes no exception for floors over slabs. I looked around for temperature data, and the 68°F figure seems to be for groundwater. According to an open-source weather database (which I can’t link to b/c then the GBA system rejects my post), ground temp in the summer at 7″ deep is about 92°F, which would favor insulating floors
Further, winter ground temp is around 50°F, which would also favor insulating floors.
So, it seems like floors over slabs should be insulated in *all* climates, but I want to make sure I’m not missing anything. Am I missing anything?
ANSWER: Based on the answers from helpful GBA members below, the 50/92°F figures I found are for the ground in general, not directly underneath a house. Directly underneath a slab, the temp should be steady around 68°F, which favors not insulating.
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See this BSC Article, page 10, Table 2:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
In the "Sub-slab" column on the right you see they do not recommend any insulation until you get to CZ3. That said, I would personally still install some Type IX EPS (about 2") underneath my slab if/when I ever build a new home.
IECC2018 did not require any sub-slab insulation, see below link Table R402.1.2 second to last column "Slab R-Value and Depth":
https://codes.iccsafe.org/content/iecc2018/chapter-4-re-residential-energy-efficiency
Thank you for this.
I'm not asking about slab insulation, per se. I'm asking about floor insulation. Even the 2018 IECC specified R13 for floors in CZ-1 and CZ-2. Maybe that's only for exposed floors (above a crawlspace), but it doesn't seem to say that.
I do know that insulating the slab means that the floor is insulated, of course. But if the slab is already poured then the underside of the slab can't be insulated. When I'm remodeling a house in CZ-2, the issue becomes whether I should put some XPS on top of the slab, then plywood, then finish flooring. Based on the ground temperature data I found, and IECC code, it seems like I should, unless I'm missing something. Am I missing something?
Those ground temperatures you found are 7" away from the outdoors. Roughly the same distance as from the edge of your slab to the inside of your walls. The soil up under your slab is going to be pretty close to your ~70* indoor temperature.
Insulating the floor isn't going to gain you much and will remove a significant chunk of thermal mass from the conditioned space. Could actually reduce comfort without delivering any noticeable reduction in energy use.
If the edges of your slab aren't already insulated that would probably be worth doing. I'm in CZ3 and people without edge insulation complain about cold floors up here.
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The soil under the house will probably stay near the year-round average temperature, which you've indicated is 68F. That's so close to room temperature there probably won't be much heat flow, but in both heating and cooling weather there will be heat flow into the ground. Usually you have to look at two questions: first, is the heat loss in cooling weather, when you want it, less than the heat loss in heating weather, when you don't? And second, will the heat loss in heating weather cause comfort issues? If the answer to either of those questions is "yes" then you want insulation under the slab. Otherwise you don't.
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You said 68F is groundwater, that's going to be at the year-round average temperature. Near the surface the soil temperature fluctuates with the season, but once you get about 10' down it stays constant at the year-round average temperature. Going deeper it gets warmer, the earth's core is at about 11,000F.
The reason the surface temperature fluctuates is that it is exposed to the outside air, which fluctuates even more. Ten feet of soil provides enough insulation to dampen the fluctuations. Soil has an R-value of about R4 per foot, so that's about R-40. Soil under a house isn't exposed to outside air, it has a house between it and the outside air. That house may or may not provide the full R40 needed, but the interior of the house is being heated and cooled so the soil under the house isn't exposed to the fluctuation of the outside air. You will see some fluctuation around the perimeter of the slab which is why in colder climates you'd insulate for two feet around the perimeter.
I also think you're incorrect in your interpretation of the code. A slab is not an exterior floor, it's a slab, and needs to be insulated as such. If there is a floor over a slab that's not an exterior floor, that's an interior floor over a slab.
Thank you. You seem to be saying that the code requirement for insulating floors is for floors above crawlspaces, not slabs. That would make sense, but I can't find confirmation of that anywhere in the code (e.g., a definition of "floor"). Do you know any part of IECC which clarifies.