How far below grade should I insulate my house's exterior? What material should I use?
How far below grade should I insulate the exterior of my house? What material best withstands the elements? Or should the insulation be covered regardless of the material used?
Insulation of below-grade walls is a relatively new practice motivated by higher standards for residential energy efficiency. Insulation can be installed on the interior or exterior of basement walls, or even on both sides.
- Interior insulation doesn't have to be waterproof if the walls have been sealed on the exterior
- Exterior insulation should be moisture-resistant, even if it is covered by a waterproof membrane. The advantage of exterior insulation is that it helps prevent condensation of moisture from humid inside air on the interior surface of the walls. (Moisture can cause mildew or damage to interior insulation and finishes.)
If this is new construction, take exterior insulation all the way down to the footing. For renovations, take it down at least two feet, to a point below the frost line. (You can go deeper, of course, but this will add to the cost of excavation and materials.)
For exterior insulation, you should use rigid foam board made of extruded or expanded polystyrene rated for subgrade applications. Owens Corning Foamular (150 or 250) is an example of this type of product.
- The R-value should be at least R-10*, which is achievable with 2-inch-thick board and generally adequate for any climate.
- Minimum compressive strength of 15 psi is recommended to prevent insulation from being crushed by backfill and soil pressure.
- A separate waterproofing membrane is recommended, and you'll generally get a better warranty (20 years) if the membrane covers the exterior of the board insulation—consult your contractor.
As noted above, you may choose to place insulation on the interior of the basement wall as a supplement or alternative to exterior insulation. We like to have at least 1.5 inches of rigid insulation on the exterior to reduce the threat of mildew. We use additional insulation on the interior (up to a total value of R-25) for exceptional energy efficiency and improved thermal comfort. The interior insulation can be batt or spray-foam insulation applied to the cavity of a finished wall.
* R-10 is required by the 2006 International Residential Code for continuous insulation on basement walls. This applies only to conditioned basements and crawl spaces, however.
If the basement or crawl space is not conditioned, then the floor or wall dividing it from conditioned spaces must be insulated, generally to R-13 (3.5 inches of fiberglass batt).
For more information:
Read John Messerschmidt's Q&A "We’re building a new home. Should we insulate both sides of the foundation?"