Insulate Interior or Exterior of Basements

An other view point of interest;

**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.


Good info Marcel

A local insulation company I used, also damped and water proof foundations.
Their waterproofing system consisted of applying tar to the concrete, sealing the pins, then installed 2" rigid foam with slits in them to wick the water down to the perimeter drain.


That’s dampproofing, **NOT **waterproofing. Waterproofing requires a membrane.

The material they applied to the concrete was thicker than roofing cement and with the 2" foam panels, they called it waterproofing and guaranteed it to be waterproof for 5 years.
I’ll take 'em for their word, works for me!:wink:


Unfortunately, the dampproofing and/or waterproofing is only as good as the substrate it is applied to.
Unless it is structurally stable and designed to withstand all the lateral forces of piss poor backfill and reinforced to withstand the hydrostatic pressures of a poorly designed wall, it will fail.

Water resistant and water proof have different meanings in any Product.

One can not substitute the other. But the better of the two can.
The key words here would be resistant and proofing. I will choose the later.

Marcel :slight_smile: :slight_smile: :smiley: