Slab Edge Insulation (or lack thereof)
Cold edge of Zone 4. When I built my house a couple of years ago, I didn’t understand the purpose of slab edge insulation as explained by the sub doing the foundation waterproofing. It was “optional” as the energy code is not enforced around here. Fortunately, I came to understand it before the slab went in so, along the back walkout wall, I had R-10 board laid horizontally at a 2 ft width instead of vertically.
My neighbor built at the same time with the same custom builder (who never advised us about slab edge insulation). As we are now just past a couple of weeks of record sub-zero temperatures, I was talking to my neighbor. The base molding on the back drywall of their walk-out basement built up ice dams during this period. I had no such problem with the same basic design and materials. I also have a 15 mil vapor barrier compared to their 6 mil and better insulation.
Unlikely my neighbor has slab edge insulation. Would this explain the situation? What can be done at this point? Would installing 2 inches of foamboard 2 feet deep on the outside of the foundation do the trick? Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Bruce, most of the heat loss through the slab edge is directly to the outdoor air, so I don't think laying foam horizontally below grade would do much for your neighbor. Concrete is a terrible insulator (~R-0.08/in) so at the baseboard it's probably close to the outdoor temperature. They should install foam insulation vertically on the exterior, preferably EPS or recycled XPS.
That makes sense. But, I only have 2 ft wide, R-10 horizontal foamboard under my slab; nothing protecting the slab edge from the exterior. I had no problem but my neighbor did. Is there some other issue involved? Thanks.
It could be several things: difference in interior humidity and/or temperature, orientation (direct sun vs no sun), rain or other exterior moisture source, air leaks (under wall plates often leak air), etc..
My money's on the air leaks, nothing bonding sheathing or housewrap to the concrete.