WAC 51-11-1002   Section 1002: Below grade walls and slabs.

     1002.1 General: Table 10-1 lists heat-loss coefficients for below-grade walls and floors.

     Coefficients for below-grade walls are given as U-factors (Btu/hr • ft² • °F of wall area). Coefficients for below-grade slabs are listed as F-factors (Btu/hr • ft • °F per lineal foot of slab perimeter).

     Below-grade wall U-factors are only valid when used with the accompanying below-grade slab F-factors, and vice versa.

     1002.2 Component Description: All below-grade walls are assumed to be eight-inch concrete. The wall is assumed to extend from the slab upward to the top of the mud sill for the distance specified in Table 10-1, with six inches of concrete wall extending above grade.

     Interior insulation is assumed to be fiberglass batts placed in the cavity formed by 2x4 framing on twenty-four inch centers with one-half inch of gypsum board as the interior finish material. Exterior insulation is assumed to be applied directly to the exterior of the below-grade wall from the top of the wall to the footing. The exterior case does not assume any interior framing or sheetrock.

     In all cases, the entire wall surface is assumed to be insulated to the indicated nominal level with the appropriate framing and insulation application. Coefficients are listed for wall depths of two, three and one-half, and seven feet below grade. Basements shallower than two feet should use on-grade slab coefficients.

     Heat-loss calculations for wall areas above grade should use above-grade wall U-factors, beginning at the mudsill.

     1002.3 Insulation Description: Coefficients are listed for the following four configurations:

     1. Uninsulated: No insulation or interior finish.

     2. Interior insulation: Interior 2x4 insulated wall without a thermal break between concrete wall and slab.

     3. Interior insulation w/thermal break: Interior 2x4 insulated wall with R-5 rigid board providing a thermal break between the concrete wall and the slab.

     4. Exterior insulation: Insulation applied directly to the exterior surface of the concrete wall.

TABLE 10-1

DEFAULT WALL U-FACTORS AND SLAB F-FACTORS FOR BASEMENTS

	Below Grade Wall U-factor	Below Grade Slab F-factor
2-Foot Depth Below Grade
Uninsulated	0.350	0.59
R-11 Interior	0.066	0.68
R-11 Interior w/tb	0.070	0.60
R-19 Interior	0.043	0.69
R-19 Interior w/tb	0.045	0.61
R-10 Exterior	0.070	0.60
R-12 Exterior	0.061	0.60
3.5-Foot Depth Below Grade
Uninsulated	0.278	0.53
R-11 Interior	0.062	0.63
R-11 Interior w/tb	0.064	0.57
R-19 Interior	0.041	0.64
R-19 Interior w/tb	0.042	0.57
R-10 Exterior	0.064	0.57
R-12 Exterior	0.057	0.57
7-Foot Depth Below Grade
Uninsulated	0.193	0.46
R-11 Interior	0.054	0.56
R-11 Interior w/tb	0.056	0.42
R-19 Interior	0.037	0.57
R-19 Interior w/tb	0.038	0.43
R-10 Exterior	0.056	0.42
R-12 Exterior	0.050	0.42

[Statutory Authority: RCW 19.27A.025, 19.27A.045. 01-03-010, § 51-11-1002, filed 1/5/01, effective 7/1/01; 98-03-003, § 51-11-1002, filed 1/8/98, effective 7/1/98. Statutory Authority: RCW 19.27A.020 and 1990 c 2. 91-01-112, § 51-11-1002, filed 12/19/90, effective 7/1/91.]