Retrofit Slab Perimeter with Insulation

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Old 01-22-10, 10:46 AM
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Retrofit Slab Perimeter with Insulation

Live in 4500 heating degree day climate. House is a 1975 slab-on-grade with no insulation under the slab. I've read that you can retrofit by digging around the edge of the slab and putting down 1" board insulation for 2 feet or so. How does this work? Does the slab go down that far into the soil? Wouldnt digging around the whole house make things a bit shifty? Anyone have any guidance or done this before?
 
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Old 02-04-10, 03:15 AM
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Anyone ever done this as a retrofit? Did it work??
 
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Old 02-04-10, 04:34 AM
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The only perimeter insulation I have installed is on the interior, usually 2'-0" vertical and horizontal.
Digging around the house is fine as long as you do not undermine the footings.
 
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Old 02-04-10, 04:53 AM
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Around Indianapolis a slab home usually has a foundtaion of poured concrete walls or a few courses of concrete block which sit atop a poured concrete footing. Usually 2 inch thick styrofoam is placed against the inside of the walls and backfilled with pea gravel to within 4 inches of the top of the wall. Then around the perimeter of the house, 2 foot wide sections of the same styrofoam are laid horizontally atop the pea gravel. These pieces are dug down so that their top surface is level with the pea gravel. This allows 4 inches of concrete to be poured atop the pea gravel and stryrofoam. The top of the poured slab ends up being the same height as the top of the short foundation wall.
The vertical piece of styrofoam certainly helps insulate the slab, but the horizontal ones that extend 2 feet under the slab really help out as well. I'm not sure exactly how much it would help to retrofit the vertical piece to the outside of the wall without the inside horizontal piece, but it certainly couldn't hurt. If I were doing it, I'd want to use 2 inch foam board and dig down at least 2 feet, which incidentally won't hurt the foundation. While you've got it dug out, why not lay a perimeter drain in the resulting trench as well, to help channel off groundwater?
 
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