Insulating below slab-on-grade floors

Installing 8 inches of Roxul mineral wool below a concrete slab floor



Building Code requirements for sub-slab insulation vary by region, the only consistency is that it is almost always insufficient, saddling homeowners with a choice between having higher heating bills or chilly feet.

It is common to install as little as R5 insulation below slab floors, making that about the weakest point in a building envelope, as walls are generally at least R20 and ceilings R40. There is a misconception that heat rises (hot air rises, but heat travels evenly in all directions) so a lot more heat is lost to the ground than most people would think.

In this case we have gone with 8 inches of insulation for a total of approximately R32. This will help keep the slab temperature more balanced with ambient interior temperatures, and greatly reduce the amount of heat you have to pump into your floor to stay warm. 

The payback period for adding insulation can be very quick if not instantaneous, as added material costs are offset by money saved monthly on heating.

The insulation we used here is a mix of Roxul Comfortboard CIS under the main slab, and Comfortboard IS on the exterior of the slab and high-density Type III EPS foam under the footing as Roxul has not been tested yet for the weight of a footing and load bearing wall. Following that will be a skirt of R8 insulation extending 4 feet out from the slab just below final grade, to ensure the ground below and surrounding the slab stays above freezing.

We are so accustomed to a house having a basement that the idea of building a slab-on-grade creates an instant sense of fear and doom in many. We assure you this is not new, and it works.

The reason we build basements and stay 4 to 5 feet below grade is to avoid frost heave, using dirt to insulate. With a slab-on-grade, the difference is you use 'insulation' to insulate, rather than dirt. This effectively moves the frost line from 4 feet down to just 6 inches, problem solved. 

Building above grade on a slab can be a much cheaper starting place than a basement, and leads to a more comfortable and durable home. For greater details check out some of the following guide pages:

Thanks to our sponsor Roxul.