BUILDING ENVELOPE: Basement

Questions:

• Which is the better method for insulating basement walls: exterior or interior insulation?
  
• How deep should foundation insulation extend below grade?
  
• What is the R-value of soil?
  

Which is the better method for insulating basement walls: exterior or interior insulation?
Both methods can be used effectively to reduce heat loss, and each has advantages and disadvantages.

The preferred method, from a thermal standpoint, is exterior rigid foam board insulation. It allows the concrete to interact thermally with the interior and helps reduce temperature fluctuations.

Exterior insulation for a basement wall must be protected from the sun and physical damage.
A major disadvantage to exterior insulation is that it provides a hidden entry path for termites. For this reason, exterior insulation should only be used in areas where the threat of termites is low.

Interior basement wall insulation is less costly, easier to install and provides a finished living space with room in the walls for utilities. Also, most builders are familiar with the techniques.

How deep should foundation insulation extend below grade?
Insulation should extend all the way to the footing. A heated basement will always lose heat through its walls, no matter how deep they are.

Although heat loss to the soil near the bottom of the wall is not great, heat is conducted up the wall to colder soil near the surface. Insulating the entire wall reduces this bypass heat loss.
Also, keep in mind that the cost of the additional insulation is relatively small compared to the cost of framing and finishing the wall.

What is the R-value of soil?
The resistance of soil to heat flow (R-value) varies a great deal, depending on the type of soil and the moisture content. In general, soil is not a good insulator.
For a fine-grained soil with 20 percent moisture content, the R-value is about 1 per foot, roughly the same as concrete.

Because of this low R-value, it is important to insulate foundations, including slabs-on-grade, crawl space walls and full basements. Insulating the first few feet below grade is the most critical area, but we recommend full-depth insulation.