R402.1 General. The building thermal envelope shall meet the requirements of Sections R402.1.1 through R402.1.6.
EXCEPTION: | The following buildings, or portions thereof, separated from the remainder of the building by building thermal envelope assemblies complying with this code shall be exempt from the building thermal envelope provisions of this code. |
| 1. Those with a peak design rate of energy usage less than 3.4 Btu/h ft2 (10.7 W/m2) or 1.0 watt/ft2 of floor area for space conditioning purposes. |
| 2. Those that do not contain conditioned space. |
| 3. Greenhouses isolated from any conditioned space and not intended for occupancy. |
R402.1.1 Insulation and fenestration criteria. The building thermal envelope shall meet the requirements of Table R402.1.1 based on the climate zone specified in Chapter 3.
R402.1.2 R-value computation. Insulation R-value shall be determined as specified in Section R303.1.4. Insulation material used in layers, such as framing cavity insulation or continuous insulation, shall be summed to compute the corresponding component R-value. The manufacturer's settled R-value shall be used for blown insulation. Computed R-values shall not include an R-value for other building materials or air films. Where insulated siding is used for the purpose of complying with the continuous insulation requirements of Table R402.1.1, the manufacturer's labeled R-value for insulated siding shall be reduced by R-0.6.
R402.1.3 U-factor alternative. An assembly with a U-factor equal to or less than that specified in Table R402.1.3 shall be permitted as an alternative to the R-value in Table R402.1.1. U-factors shall be determined as specified in Section R402.1.5.
R402.1.4 Total UA alternative. If the proposed building thermal envelope UA is less than or equal to the target UA, the building shall be considered in compliance with Table R402.1.1. The proposed UA shall be calculated in accordance with Equation 2. The target UA shall be calculated in accordance with Equation 1. U-factors shall be determined as specified in Section R402.1.5.
R402.1.5 U-factor reference and calculations. The
U-factors for typical construction assemblies are included in Appendix A in chapter
51-11C WAC. These values shall be used for all calculations. Where proposed construction assemblies are not represented in Appendix A, values shall be calculated in accordance with the ASHRAE
Handbook of Fundamentals using the framing factors listed in Appendix A where applicable and shall include the thermal bridging effects of framing materials. The SHGC requirements shall be met in addition to UA compliance.
Fenestration U-factors shall comply with Section R303.1.3, Fenestration product rating.
R402.1.6 Vapor retarder. Wall assemblies in the building thermal envelope shall comply with the vapor retarder requirements of Section R702.7 of the International Residential Code or Section 1405.3 of the International Building Code, as applicable.
[Statutory Authority: RCW
19.27A.020,
19.27A.045,
19.27A.160 and chapter
19.27 RCW. WSR 20-01-047, § 51-11R-40210, filed 12/9/19, effective 7/1/20. Statutory Authority: RCW
19.27A.025,
19.27A.045,
19.27A.160, and
19.27.074. WSR 17-10-063, § 51-11R-40210, filed 5/2/17, effective 6/2/17. Statutory Authority: RCW
19.27A.020,
19.27A.045,
19.27A.160, and
19.27.074. WSR 16-02-127, § 51-11R-40210, filed 1/6/16, effective 7/1/16. Statutory Authority: RCW
19.27A.020,
19.27A.045 and chapters
19.27 and
34.05 RCW. WSR 13-04-055, § 51-11R-40210, filed 2/1/13, effective 7/1/13.]