Why insulate stemwalls?
Matt_Michaud
| Posted in Green Building Techniques on
For slab-on-grade construction there are 3 insulation locations – edge, sub-slab, and stemwall (down to the footings). In FHB 245, Martin says it is important to insulate stemwalls to a minimum R10. Code says a minimum of 4ft of R10 in any combination of vertical, sub-slab, or exterior skirt. Building Science shows an edge and sub-slab (no stemwall) insulation diagram here. So why stemwall insulation? Is it to prevent heaving below the slab? Wouldn’t the crushed stone layer do this like it does for raft slabs? It would save a lot of labor on a current project, if its unneeded.
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Replies
Matthew,
The main reason to insulate the interior of a concrete stemwall is that this method keeps the vertical insulation protected from physical abuse and sunlight.
Floating slabs (raft slabs or FPSFs) still need vertical insulation at the perimeter -- but without a stemwall, the vertical insulation is vulnerable to deterioration and abuse, so it needs to be protected with stucco or metal flashing -- an added step, and an inelegant detail.
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Martin,
Thanks for the response. Isn't the vertical portion of a raft equivalent to the slab-edge insulation in your diagram more so than the stemwall? I understand that there is a horizontal equivalent in FPSF. If the edge and horizontal insulation in your diagram take care of the envelope, why the additional stemwall insulation? Why not this:
Matthew,
I can think of two reasons to prefer my detail to your detail:
1. In my detail, the soil under the slab is warmer. That reduces heat loss compared to your detail.
2. In your detail, the soil under the slab may freeze. While a deep enough layer of crushed stone can address the risk of heaving, contractors aren't accustomed to installing 12 to 16 inches of crushed stone under a slab between stemwalls, and they may get this wrong (or unusual soil conditions could trip them up). More importantly, a raft slab can float -- but with the stemwall, the perimeter walls can't float. The result might be differential movement.
Matthew,
Note also that many raft slab details (especially those for cold climates) include wing insulation to keep the soil under the slab from freezing. See the details below.
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I had not considered the differential movement of stemwall vs. slab. Stemwall foam ordered. Thanks for the thorough answer.
Building Science Corporation also typically recommends insulating your stem wall in "cold climates" (4 and higher in this case), or if you're after a really high performance home. They're definitely not against it or anything.
See this article: https://buildingscience.com/documents/insights/bsi-059-slab-happy
Brendan, that is good context - the Building America report my diagram was from just says "for all climates". However, from your cited article, stemwall insulation is simply for anecdotal comfort when you only have 2" below the slab - not particularly compelling when we are talking about 6"+ like most raft slab details we were comparing to.