Rigid Insulation Under Rat Slab
MidModHome
| Posted in Energy Efficiency and Durability on
Hello,
This is for Zone 4C regarding a crawl space detail, please see attached. I had looked at several details on this site regarding crawl spaces and someone brought up insulating under the rat slab. Do I need to provide a layer of rigid insulation (R-10) under the rat slab for this zone?
Thank you!
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Requirements are different depending on whether your jurisdiction is using IRC 2021 or an earlier version. I'll assume 2021.
I've seen the details too. They show perimeter insulation of the crawlspace walls, no floor insulation above and none below either. After thinking about it I found that curious. I don't see that it meets code, which requires floor insulation of some kind. Also, the crawlspace ground surface would be around 55 degrees in most places, most of the year, while the floor above would be 15 degrees warmer. The conditioned space above the floor framing would be coupled to the earth and losing heat to it all winter.
Again, I don't see how this meets code. If it does it must be buried in "exceptions" somewhere. Regardless, I wouldn't build this way. Rigid insulation against the ground, with a vapor barrier membrane over, and a rat slab on top seems to me the best way to go. It would avoid vapor issues and make the house more efficient and comfortable.
I hope others chime in, especially if there is some code exception allowing an uninsulated floor. I'd like to understand it.
If the crawlspace is conditioned then the floor above isn't part of the building envelope and doesn't have to be insulated. The IRC also doesn't require insulation on the crawlspace floor because, as you note, the delta T is small and the dirt below the floor usually acts as insulation. Its R-value per inch is low, but there is a lot of it. An exception would be in areas with high groundwater where the flowing water will carry heat away, but the IRC doesn't address that.
To answer this question you'll need to review your local code requirements since they can vary based on your location.
Looking at the 2021 IECC, https://codes.iccsafe.org/content/IECC2021P2/chapter-4-re-residential-energy-efficiency#IECC2021P2_RE_Ch04_SecR402, for a zone 4C it looks like your crawl space wall r-value should be 15ci or 19 or 13&5ci. Now, for the rat slab, I'm a bit at a loss because this wouldn't classify as a slab on grade but if it did the IECC requires 10ci to a depth of 4ft. I think you do not need insulation under the concrete because I think this would be classified as a crawl space and not a slab. I'm not 100% on this but that's how I read it. You would be best to check with your local code official to make sure they agree since ultimately they'll be the ones approving it.
Edit: I see a note "d" in table R402.1.3 of the 2021 IECC and the insulation for a slab is not under the slab but is at the edge of the slab (hence the 4ft depth). So, if you were required to insulate as though this were a slab, you would just have to extend the insulation along the edge to a depth of 4ft below grade. That's not a bad idea but I'd check with the code official to make sure they're good with whatever you plan to do.
Building Science Corp recommends R-7.5 under slab in CZ4.
https://www.greenbuildingadvisor.com/article/r-value-advice-from-building-science-corporation
I would put insulation under the slab now while it's cheap and easy. At some point in the not distant future, the homeowner will want to remove the floor insulation and condition the crawlspace. If the sub-slab insulation is there, the job is almost done.
MidModHome,
You should also consider using foam-board not batts to insulate your rims-joists:
https://www.greenbuildingadvisor.com/article/insulating-rim-joists
You do not have to insulate below the slab. However, the IRC requires min. R-15 continuous insulation on crawlspace walls in zone 4C.