How to Insulate a Slab Foundation | South Salem Homes

In Texas where I build the most common foundation type is Slab on Grade. In effect, we pour a big rock of concrete on top of the ground (also mainly rock) then build a house on top.


My Project Manager Ryan on top of this newly poured Slab Foundation.

I just completed my first house with Perimeter Slab Edge Insulation and I thought I’d walk you through the process. First, let’s talk about why to insulate the slab. As we build tighter and better insulated houses the uninsulated slab becomes more of a heat loss in the winter time for these High Performance Homes. Check out this Manual J energy loss chart from a house we remodeled recently.

Check out the BTU loss through the un-insulated concrete slab on this house.

The chart above is from a house with R-19 walls, and an R-40 roof. As we build better that slab becomes a larger % of the loss of heat!

This is for a house with an R-11 Insulated Slab.

Now look at this chart with a house with similar specs for walls/roof but it has an R-11 Perimeter insulated slab. Big difference! As a percentage of heat loss the floor went way down compared to the un-insulated slab house.

So, let’s look at the mechanics of actually insulated the slab. One of our first concerns for this process is Termites! Adding foam to the outside of a slab is a big no-no in Termite country. First, we used Borate impregnated foam from Nisus Corporation. I’ve been using their Bora-Care on my framing lumber for about 10 years now, but I only recently realized they make a termite resistant foam aptly called Bora-Foam. Our local Termimesh dealer sourced it for me and did the install.

I want to thank Joel Roeling from Termimesh for his help in this project. Also want to extend a big thanks to my Energy Rater/Tester Kristof Irwin of Positive Energy here in Austin, TX. His modeling really helps us decide what approaches will yield benefits long term for the houses I build. Last, but certainly not least was the amazing Architect/Client on this house Scott Ginder of Dick Clark & Associates. Scott and his wife Andrea are wonderful clients and it was such a pleasure to build this beautiful high performance home with them!  Here’s some detail photos but be sure to watch the video too.


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