BUILDING ENVELOPE: Basement
Questions:
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.
