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A4 Below-Grade Walls
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For the purpose of Section A1.2, the base assembly is 8 in. medium-weight concrete block with a density of 115 lb/ft3 and solid grouted cores. Continuous insulation is installed on the interior or exterior. In contrast to the U-factor for above-grade walls, the C-factor for below-grade walls does not include R-values for exterior or interior air films or for soil. For insulated walls, the C-factor does include R-0.45 for 0.5 in. gypsum board.
C-factors for below-grade walls shall be taken from Table A4.2.1 or determined by the procedure described in this subsection.
Table A4.2.1 Assembly C-Factors for Below-Grade Walls
| Framing Type and Depth | Rated R-Value of Insulation Alone | Specified C-Factors (Wall Only, without Soil and Air Films) |
|---|---|---|
| No Framing | R-0 | C-1.140 |
| Exterior Insulation, Continuous and Uninterrupted by Framing | ||
| No Framing | R-5.0 | C-0.170 |
| No Framing | R-7.5 | C-0.119 |
| No Framing | R-10.0 | C-0.092 |
| No Framing | R-12.5 | C-0.075 |
| No Framing | R-15.0 | C-0.063 |
| No Framing | R-17.5 | C-0.054 |
| No Framing | R-20.0 | C-0.048 |
| No Framing | R-25.0 | C-0.039 |
| No Framing | R-30.0 | C-0.032 |
| No Framing | R-35.0 | C-0.028 |
| No Framing | R-40.0 | C-0.025 |
| No Framing | R-45.0 | C-0.022 |
| No Framing | R-50.0 | C-0.020 |
| Continuous Metal Framing at 24 in. on Center Horizontally | ||
| 3.5 in. | R-11.0 | C-0.182 |
| 3.5 in. | R-13.0 | C-0.174 |
| 3.5 in. | R-15.0 | C-0.168 |
| 5.5 in. | R-19.0 | C-0.125 |
| 5.5 in. | R-21.0 | C-0.120 |
| 1 in. Metal Clips at 24 in. on Center Horizontally and 16 in. Vertically | ||
| 1.0 in. | R-3.8 | C-0.233 |
| 1.0 in. | R-5.0 | C-0.201 |
| 1.0 in. | R-5.6 | C-0.189 |
| 1.5 in. | R-5.7 | C-0.173 |
| 1.5 in. | R-7.5 | C-0.147 |
| 1.5 in. | R-8.4 | C-0.138 |
| 2.0 in. | R-7.6 | C-0.138 |
| 2.0 in. | R-10.0 | C-0.116 |
| 2.0 in. | R-11.2 | C-0.108 |
| 2.5 in. | R-9.5 | C-0.114 |
| 2.5 in. | R-12.5 | C-0.096 |
| 2.5 in. | R-14.0 | C-0.089 |
| 3.0 in. | R-11.4 | C-0.098 |
| 3.0 in. | R-15.0 | C-0.082 |
| 3.0 in. | R-16.8 | C-0.076 |
| 3.5 in. | R-13.3 | C-0.085 |
| 3.5 in. | R-17.5 | C-0.071 |
| 3.5 in. | R-19.6 | C-0.066 |
| 4.0 in. | R-15.2 | C-0.076 |
| 4.0 in. | R-20.0 | C-0.063 |
| 4.0 in. | R-22.4 | C-0.058 |
It is acceptable to use the C-factors in Table A4.2.1 for all below-grade walls.
If not taken from Table A4.2.1, below-grade wall C-factors shall be determined from Tables A3.1-2, A3.1-3, or A3.1-4 using the following procedure:
- If the below-grade wall is uninsulated or only the cells are insulated:
- For concrete walls, determine the C-factor from Table A3.1-2 based on the concrete density and wall thickness.
- For concrete block walls, determine the C-factor from Table A3.1-3 based on the block size, concrete density, degree of grouting in the cells, and whether the cells are insulated.
- If the mass wall has additional insulation:
- For concrete walls, determine the Rc from Table A3.1-2 based on the concrete density and wall thickness. Next, determine the effective R-value for the insulation/framing layer from Table A3.1-4 based on the rated R-value of insulation installed, the thickness of the insulation, and whether it is installed between wood or metal framing or with no framing. Then determine the C-factor by adding the Rc and the effective R-value together and taking the inverse of the total.
- For concrete block walls, determine the Rc from Table A3.1-3 based on the block size, concrete density, degree of grouting in the cells, and whether the cells are insulated. Next, determine the effective R-value for the insulation/framing layer from Table A3.1-4 based on the rated R-value of insulation installed, the thickness of the insulation, and whether it is installed between wood or metal framing or with no framing. Then determine the C-factor by adding the Rc and the effective R-value together and taking the inverse of the total.
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