The prescriptive approach is the simpler way to comply with the Standards. Each of the three building systems complies separately from the others. The compliance procedures and documentation are also separate for the three. The prescriptive approach for each system requires that the proposed design meet specific energy efficiency criteria specified by the Standards. If the design fails to meet even one of the requirements, then the system does not comply with the Standards. The performance approach provides the most flexibility to the building designer for choosing alternative energy efficiency features.
Building Envelope. The prescriptive envelope requirements are determined either by the envelope component approach or the overall envelope approach. These two approaches are described in detail in Chapter 3 of this manual. The stringency of the envelope requirements varies according to climate zone and occupancy type.
These sections are also shown in Appendix B of this document. The prescriptive requirements have separate criteria for heavy mass walls. While the Standards recognize both heavy mass and light mass walls, separate criteria are presented only for heavy mass walls and only for Package D. Heavy mass walls are those that weigh more than 40 lb/ft². Where the package indicates “NA” for a light mass wall the assembly must comply with 0.102 U-factor for climate zones that require R-13 for wood-framed walls, or 0.074 for where R-19 is required, or 0.069 where R-21 is required the. The “NA” applies to both heavy and light mass walls for Package C and light mass walls for Package D.
The R-value listed in Standards Table 151-C (also in Appendix B of this document) for heavy mass walls is the minimum R-value for the entire wall assembly, including insulation and both interior and exterior air films. Heavy mass walls require R-2.44 in climates 2 through 10 and R-4.76 in the other climates. Tables IV.12 and IV.13 from Joint Appendix IV have the thermal
properties of hollow unit masonry, solid core masonry, and concrete walls. Choices from these tables that have a heat capacity (HC) greater than or equal to 8.0 have a density greater than 40 lb/ft³ and qualify as heavy mass walls.
To determine the total R-value of a heavy mass wall, the U-factor from Table IV.12 or IV.13 is added to an insulation layer selected from Table IV.19. When the prescriptive requirements are used, the insulation must be installed integral with or on the exterior of the heavy mass wall.
• Because it is difficult to inspect wall insulation behind tub/shower enclosures after the enclosures are installed, insulation of these wall sections should be inspected during the framing inspection.
• Batt insulation should fill the wall cavity evenly. If kraft or foilfaced insulation is used, it should be installed per manufacturer recommendations to minimize air leakage and avoid sagging of
• Wall insulation should extend into the perimeter floor joist (rim joist) cavities along the same plane as the wall.
• If a vapor barrier is required, it must be installed on the conditioned space side of the framing.
Nonresidential and Hotels and High-rise Residential
Exterior walls can meet the component requirements by either using a construction that has an assembly U-factor lower than the specified criteria as shown in Table 3-9, or installing the required R-value of insulation (Table 3-11). For nonresidential buildings, R-11 insulation is required for the middle and south coasts (zones 3 through 9) and R-13 is required in other climate zones. For residential buildings and hotel/motel guest rooms, R-11 is required for the middle and south coast areas (zones 3 through 9); R-13 is required for the valley and desert climates (zones 2 and 10 through 15); and R-19 is required for the cold climates (zones 1 and 16). For public school buildings, R-13 is required in all climate zones.
The U-factor criteria for walls depend on the class of construction. U-factors used for compliance must be selected from Joint Appendix IV. There are six classes of wall constructions: wood frame, metal frame, metal building walls, medium mass, high mass, and other (Figure 3-13). The “other” category is used for any wall type that does not fit into one of the other five wall classes. The following bullets give more information.
• Low mass walls. Low mass walls have a heat capacity (HC) greater or equal to 7.0 but less than 15.0 Btu/ºF-ft². See the definition below for heat capacity. From Joint Appendix IV, Tables IV.12 and IV.13 have U-factor, C-factor, and heat capacity data for hollow unit masonry walls, solid unit masonry and concrete walls, and concrete sandwich panels.
• High mass walls have an HC equal to or greater than 15.0 Btu/ºF-ft². See Joint Appendix IV for HC data on mass walls.