Does it make sense to use ICFs for a frost protected shallow foundation?
Ralph Hill
| Posted in Energy Efficiency and Durability on
I will be building a house near Amherst Ma. in an area with a high water table. FPSF seems like the way to go. ICFs are quick and easy. Is there any reason not to use them for this? Any opinions on insulation – xps or eps? Any recommendations on waterproof material under the slab? I have heard polyethylene gets brittle and breaks down over time.
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Hello, Ralph. Last year, I did what you propose on a small building in Maine, a new office for our small architecture firm. You can read about what I did here:
http://www.nesea.org/blog/2009/02/using-insulating-concrete-forms-to-make-a-frost-protected-slab-foundation/
Perhaps you'll find this helpful. Please note that if I were doing this again, I would carefully calculate the structural loads bearing on the perimeter of the slab, and determine if I could place foam insulation underneath the slab perimeter. In some cases, you might be able to do this, which will increase the energy efficiency of the slab.
A shallow, frost-protected (SFP) foundation requires a well-drained soil base. With a high water table and silty or clay soils, I would not recommend it (soil with more than 5% silt is frost-active, particularly with a high water table). You may be better off with a full frost-wall with careful perimeter drainage and gravel backfill.
If you want to use a SFP foundation, be sure to read the Revised Builders Guide from the NAHB Research Center http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf
You can use ICFs for a perimeter grade beam if they are made from XPS (make sure there is sufficient soil-bearing surface of the concrete core to support the structural live and dead loads of the building) and then pour a "floating" slab inside over well-compacted gravel fill with radon vent, TuTuff reinforced vapor barrier, and 2" of XPS (EPS is not suitable for below-grade applications). And then parge the exterior exposed foam with a cementious coating (I use surface-bonding cement with acrylic modifier over galvanized hardware cloth).