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Community and Q&A

Frost-Protected Shallow Foundation Insulation and Radiant Heat

Tim_O | Posted in General Questions on

Hello, we are pretty early in the stages of designing a house.  Going for a relatively simple rectangular shape for the house and garage off the side to create an L shaped structure.  Roughly 1200sqft house and 800sqft garage.  I like the idea of a FPSF for many reasons, but one of which being radiant heat becomes fairly simple (I know it’s not particularly popular here, but my diabetic feet like every degree I can get).  I’m reading through the code, and here in South East Michigan it seems like I need between a 49 and 63″ wing of insulation surrounding the garage at R6.3-9.8 in addition to 12″ crushed rock (ASCE 32), while the rest of the building needs no wing insulation and minimum of 4″ of gravel because it is heated.  I read in the “Revised Builder’s Guide for FPSF” and there is mention of superinsulated (R28+ subslab) buildings following the method for unheated buildings.

With that said, in order to follow prescriptive methods, a lot more gravel and insulation are required for R28+ subslab insulated buildings (ASCE 32).  What methods and techniques are the community members finding as a best fit here?  I see a lot of FPSF builds, but I think not a ton follow the “unheated” method.  The code doesn’t mention this R28+ should follow unheated methods, but in retrospect, it makes sense.

Raft slabs seem like potentially a great route as well.  I have seen this from Legallet.  I believe these need to be engineered, or is there a prescriptive path I missed?

One more edit – I do plan for some heat in the garage as it is partially a machine shop.  So technically it could be a “heated” slab construction following the semi-heated building method.  But if I sell the house some day, the next owner may not want to heat the garage, and I would hate to have it heave because of that.


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  1. plumb_bob | | #1

    I think any structural slab foundation will require engineering, and the engineer will want to insulate the slab, with both vertical insulation up against the footing and horizontal wing insulation to prevent frost from getting under the footing.
    Some engineers like to put foam directly under the footing, some do not.
    I would build the garage and house foundations with the same details and same levels of insulation, and plan on supplying some amount of heat to the garage. Or, if you do not want to heat the garage, a control joint of some form may be a good idea to manage differential movement.
    The soils you have and the materials you place under the slab will have a big role in the future performance of your foundation. If the native soil is not free draining and heave resistant, I would over excavate and put a good layer of drain rock under the slab. You will need some rock as a capillary break and radon layer regardless, but I like to use much more than the minimum required amount.

    1. Tim_O | | #2

      I do not think all slab foundations need engineering, there is a prescriptive path that can be followed. It seems Iso-slab, Warm Form, and MonoSlab EZ all follow this path with their products.

      I agree about checking the soil and using plenty of drain rock.

  2. plumb_bob | | #3

    News to me that these types of slab foundations are not required to be engineered in the USA, not that I am well versed in the common codes down there.
    Cool products.

    1. Tim_O | | #4

      Ah, are you building in Canada? I thought I read that it is not part of the code there yet. Soon I'm sure. It's commonly used in Nordic countries I believe.

  3. Tim_O | | #5

    I read through this article, and basically confirmed what I need to do - treat the whole building as unheated. In the article, he states he uses 12" of gravel below the foundation.

    Side note on that - recently came across Glavel, seems like it might be a good fit in a monolithic slab. $75/yard is great, shipping starts to creep in on the price depending where you are. At least from a volume perspective, it's less than EPS it seems.

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