I created a side entrance to my basement by excavating down to the footing a 20’x4′ well, wrapping it with a 10″ wall on a 12″x24″ footing with a hi-density styrofoam under the footing. The perimeter drain enters the well at one end under the footing and and exits at the opposite end under the footing into a storm sewer connection. A 7’x4′ section of the well has been enclosed to form a heated lobby. The remainder is outside staircase and landing.
I want to pour a 4″ slab in the well. and am worried about frost protection. The 4″ drain is essentially level so there will always be water in the crushed stone. I also need to direct surface water from the staircase/landing via a floor drain to the 4″ drain. Can I put enough styrofoam under the outside slab to protect from freezing ?. Would it be advisable to excavate a couple of inches below the footing base to gain more space for additional crushed stone or styrofoam ?. I live in Eastern Canada roughly the same climatic zone as the US North East. Thanks
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After a Google search I found a relevant document at
http://www.cs.arizona.edu/people/jcropper/desguide.html
The document is "Design Guide for frost-protected shallow foundations" published by US Dept of Housing. The acronym is FPSF (frost protected etc). It was originally developed in the Nordic countries and Canada. There are also some documents published by Canadian Mortgage and Housing Corporation (CMHC) http://www.cmhc-schl.gc.ca
These documents detail amount of insulation to put under the slab. One detail not discussed is cold bridging caused by the surface drain connecting to the O drain under the slab. Surely this open airway will work against all the other details that keep ground under slab frost free. Only thing I can think of is to position the floor drain as close as possible to the house wall to take advantage of heat leakage from the interior heated space. Anybody have any other ideas ?