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Construction Profile - Below Grade Wall Hints


Below grade walls should only be added if the home contained a basement or below grade room that is heated or cooled. A room is considered below grade if at least two feet (on average) is located below grade. 

Framing Material - Finished basements are typically either framed out with 2x4 or 2x6 construction. Alternatively, to save on costs, wooden furring strips are sometimes attached directly to the foundation, and the drywall is affixed directly to those. If furring strips are used, you would select "Framing Material" = None. 

Cavity Insulation - Similar to Above Grade Walls, it is sometimes difficult to determine the R-value of insulation located behind drywall. If the R-value is not know, first you should determine the thickness of the wall. If the basement contains a window or exterior door, you can measure the door or window jam to determine if 2x4 or 2x6 framing was used.

There are two types of insulation typically used in walls (blown-in cellulose insulation or fiberglass batt insulation which comes in rolls). Cellulose Insulation (typically made from recycled paper) has an R-value of about 3.8 per inch. Fiberglass  insulation is closer to 3.2 per inch. So 2x4 walls (which are really only 3.5" wide) typically have R-values between R-11 and R-15 (3.5" x 3.2 = 11.2 and 3.5" x 3.8 = 13.3). 2x6 walls are typically in the R-19 to R-21 range. 

If the below grade walls have spray foam insulation, a good rule of thumb is that closed-cell spray foam has an R-value of about 6.5 per inch. However, most spray foam is only applied in a 3" thick layer since applying more than three inches can cause issues when drying.

Board Insulation R-Values - Board insulation can also be added on the inside or outside of the foundation. Although in recent years builders have moved away from board insulation on the outside of the foundation as it was found to attract termites and other insects.

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