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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 Rockwool rigid board under the main slab and on the exterior of the slab, and high-density Type III EPS foam under the footing as Rockwool 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 / Rockwool.
Why did you use Roxul rather than XPS? Which one is cheaper?
Our choice of Roxul was born of a desire to move away from foam more than anything. Roxul is a recycled stone wool, and there are no fire retardants. I wouldn’t use XPS under a slab as it is unnecessarily expensive (and a heavy polluter) but EPS insulation is far more affordable that either Roxul or XPS. We were really happy with how the Roxul handled and how it performs, but it did however require the inclusion of EPS under the footing anyway As Roxul doesn’t have the compressive strength to handle the load of a footing and bearing wall.
It was a great experiment, but it was tougher to source the higher density Roxul boards, so it has not taken the slab world by storm, nor has it turned into a big market success for Roxul. Don’t hesitate to try it because it works, but if it’s a hassle I’d go with EPS, or a pre-fab slab foam kit.
One more question: Other shallow frost-free foundations I have seen use rigid foam laid 2-4 feet horizontally out from the edge of the foundation. Is this unnecessary with Roxul? Can you explain why this foundation will will work as shown, without the extra horizontal insulation? If this were an unheated space (garage) with a second floor living suite above, do you see any problems with your method? Thanks.
A 2 or 4 foot skirt of rigid insulation around the perimeter is often necessary to prevent frost heave, but not always, and the type of insulation wouldn’t affect whether or not one is needed. Whether you need a skirt or not, and how much, should be determined by an engineer who factors in the climate and elevation. That said, we did require (and install) a 2-inch thick 2 foot skirt around the perimeter in this case.
As for an unheated garage with a living space above, no problem there, but if you're not heating the garage portion then it's even more likely need a perimeter skirt than if it were heated, and again, I'd check with an engineer to determine how much. Best regards.