Frost Protected Shallow Foundation With Brick Ledge
user-952125
| Posted in Green Building Techniques on
Trying to think through a frost protected shallow foundation with a brick ledge for a low wall to be built out of field stone which is available on site. I’ve attached a rough sketch.
My immediate questions are:
1) Using foam at the base of the wall, even with 100psi compressibility — is this OK?
2) I’m assuming the 4″ air gap is OK as long as the proper ties are used and will help with solar and wind driven moisture?
3) Advice on ties going into a SIP?
Any tips or suggestions gratefully accepted.
House is in Northern MI with 7,500 HDD.
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Replies
Michael,
I don't think your plan will work.
It seems that you want to build a fieldstone wall to hide the above-grade section of rigid foam insulation used to insulate the perimeter of your slab.
Builders have developed a list of products that can be used to protect above-grade exterior foundation insulation, and a fieldstone wall is not on the list.
Here's my list:
1. Pressure-treated plywood.
2. Fiber-cement panel siding.
3. Cementitious coating (stucco) -- either reinforced with fiber, or installed over fiberglass mesh, or installed over metal lath.
4. Insul-Cap vinyl covering from Wisconsin Poured Wall Products ( Muskego, WI).
5. Ground Breaker fiberglass covering from Nudo Products (http://www.nudo.com)
6. Insul-Guard 2 fiberglass covering from Diversified Composites (http://www.diversified-composites.com).
7. Surface-bonding cement.
8. Perma-Bond Complete (foam plus factory adhered cementitious coating) from http://www.permabondws.com/contractor.htm.
9. FP Ultra Lite panels (factory coated foam panels) from Styro Industries (www.styro.net).
10. Protecto Bond peel-and-stick membrane (www.protectowrap.com).
More discussion of the issue can be found in this Q&A thread:
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/14555/question-about-finish-over-exterior-foundatio?
Martin,
Thanks for the other posting, I'll work through it. I was working off the diagram on Page 20 (24 in PDF) from the Revised Builder’s Guide to Frost Protected Shallow Foundations (http://www.toolbase.org/pdf/designguides/revisedFPSfguide.pdf) which shows a brick veneer, is this not reccommended either?
Michael,
Sure enough -- you're right. There is a detail in that guide for placing brick veneer on rigid foam, so your proposal might work.
The detail on page 20 of the guide you linked to shows a terrible thermal bridge, though. Look at the wall's bottom plate -- it's adjacent to an air space and brick veneer, with no insulation at all between the bottom plate and the outside world.
Foam under your footer is fine, but 100 psi is likely overkill. You need to figure the load there, then get the appropriate foam. The advertised "100 psi" is likely at 5% deformation. DOW recommends using foam that is rated/advertised as 3x the load on the foam, so you are into the 1% deformation zone. This will avoid "creep" of the foam. If I recall, XPS is advertised at 5% deformation (likely still in the elastic limit) and EPS is advertised at 10% deformation, which is outside the elastic limit of the foam. Why did I not see any foam under your whole slab?
Michael,
I'm going to ask the same question as John K-- Wheres the continuous foam under the slab?
In a 7500 HDD climate, I would recommend at least R-10 under the entire slab.