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Community and Q&A

Frost skirt insulation.. 8”!??

Northof60 | Posted in General Questions on

Hey everyone,

location is Whitehorse, Yukon, climate zone 7a.

I have recently received some information back from a local geo-tech company about our proposed frost protected shallow slab. Based one photos and a written description that I had sent in, he had suggested I use 200mm (8”)of rigid foam insulation that extends 2.4m (8’) beyond the edge of the slab.

I have looked at many different publications about FPSS, including the one that has been referenced many time here on GBA, for nothern indigenous communities. I’ve been reading all sorts of blogs, and have never heard of anyone going up to 8”.

Granted it is known that we do indeed have frost susceptible soils ( clay) around the build site. The previous owner has already done a significant amount of dirt work, removing clay and replacing it with rock and road mix. 

Are there other charts I can reference for frost skirting insulation thickness? Our slab will have R20 under it, so I’m thinking I might want to reference the building as if it is an unheated building. Due to the fact that “hopefully” not too much heat is lost into the earth from the slab.

Thoughts? Last night I priced out 8” of rigid insulation and it came in just north of 9,000$

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  1. Expert Member
    Michael Maines | | #1

    I'm guessing that "northof60" means you're at a very high latitude? Here in climate zone 6 I have only used 2" frost wings, or somewhat thicker for European-style raft slabs. Did your engineer use the ASCE 32-01 document that explains how to do the necessary calculations, or did they "wing it"? (No pun intended...) There is another, free document put out by HUD but it's more limited in its options.

    1. Northof60 | | #4

      I will have to ask the engineer how he came up with his calculations. I just wanted to touch base with the GBA community before asking the engineer further questions.

  2. walta100 | | #2

    Foundation design is very local.

    The quote could be correct if you are in antarctica but I did not see us a clue.

    This design seems like it was intended to be unaffordable in an attempt to force you to except some other design.

    General the best advice will come from the old guy that has thousands of working foundation in the region.

    1. Northof60 | | #3

      My mistake, I am located in Whitehorse, Yukon, 7a climate zone.

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