In a home with a slab-on-grade foundation, the foundation slab is the first floor of the home. This slab floor is in direct contact with the ground and the outdoor air. If the ground and outdoor air are colder than the air inside the home, heat will leave the home through the slab. If the ground and outdoor air are warmer than the air inside the home, heat will enter the home through the slab.
Heat loss
An overwhelming majority of the heat lost from homes with slab-on-grade foundations is lost through the foundation slab. In homes with a basement or crawlspace, the basement or crawlspace creates a cushion between the warm indoor air and the colder outdoor environment, ultimately making the main living spaces more comfortable. Homes with slab-on-grade foundations lack this cushion; concrete foundation floors can be a major source of discomfort and heat loss in the home. Homeowners complain of cold feet and high heating bills. The majority of heat is lost through the vertical edges around the perimeter of the slab, because they're exposed to the ground and the outdoor air.
Heat gain
In hot climates, like those of Arizona and Florida, the slab can absorb heat, increasing cooling costs, especially in hot, dry climates. Many builders don’t realize that the slab can absorb heat, as well as lose heat. Just as heat loss through the slab makes a furnace work harder, heat gain through the slab makes an air conditioner work harder.
Cause 1: Lack of foundation insulation
If the foundation crew doesn't install insulation around the slab perimeter, the floor will be cold and uncomfortable. It will also be more expensive to heat and cool.
Solution
To prevent cold floors and reduce heat loss and gain through the slab, insulation around the perimeter of the slab is essential in most U.S. climates. Insulate the vertical edge of the slab perimeter, using a rigid foam product like Owens Corning® FOAMULAR® Rigid Foam Insulation or Dow® STYROFOAM™ extruded polystyrene insulation.
Cause 2: Improper installation of foundation insulation
Often, slab insulation isn't installed correctly. The installer may stop the insulation at ground level, instead of extending it up the entire vertical edge of the slab. A large amount of heat can be lost or gained through the top portion of the slab edge, because this portion is exposed to the air.
Solution
Use a rigid foam product like Owens Corning® FOAMULAR® Rigid Foam Insulation or Dow® STYROFOAM™ extruded polystyrene insulation to insulate the slab. Make sure the insulation covers the slab edge from top to bottom around the entire slab perimeter.