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6.5.3 Roofs

Roof Name
Applicability All roof surfaces
Definition A unique name or code that identifies the roof and ties it to the construction documents submitted for energy code review. It is not mandatory to name roofs.
Units Text, unique
Input Restrictions None
Baseline Rules None
Roof Type
Applicability All roof surfaces
Definition One of three classifications of roofs defined in the baseline standard. These classifications are defined in ASHRAE Standard 90.1-2001 and ASHRAE Standard 90.1-2007 along with the associated User's Manuals. The prescriptive U-factor requirements for roofs depend on the type. For green building ratings and tax credits, it is not necessary to specify this information, as the PRM fixes the type for the baseline building to “insulation entirely above the deck.”
This descriptor can be derived from other building descriptors and it may not be necessary for the software user to specify it directly.
Units List: attic and other roofs; metal building roofs; and roofs with insulation entirely above deck.
Input Restrictions This input is optional for the purposes covered by this manual.
Baseline Rules All roofs in the baseline building are modeled as “insulation entirely above deck.”
Roof Geometry
Applicability All roofs, required input
Definition Roof geometry defines the position, orientation, azimuth, tilt, and dimensions of the roof surface. The details of how the coordinate system is implemented may vary between software programs. The data structure for surfaces is described in the reference section of this chapter.
Units Data structure: surface
Input Restrictions There are no restrictions other than that the surfaces defined must agree with the building being modeled, as represented on the construction drawings or as-built drawings.
Baseline Rules Roof geometry will be identical in the proposed and baseline building designs.
Roof Construction
Applicability All roofs, required input
Definition A reference to a construction assembly for the proposed design. See the building descriptors above for construction assemblies.
Units Dimensionless reference
Input Restrictions All roof surfaces must reference a construction assembly.
Baseline Rules Roofs in the baseline building are of the type “insulation entirely above deck.” The insulation requirement is determined by climate zone and is given in Table 6.5.3-1. The baseline building roof construction shall be modeled as layers as defined in Table 6.5.3-2. These tables reflect a construction that is available in the standard list of constructions.

Table 6.5.3-1: "Baseline Building R-value and U-factor Criteria for Roofs"

Applicable Standard Space Category Climate Zone Standard Design
Minimum Insulation Maximum Assembly
90.1 – 2001
 Nonresidential 1-7 R-15 c.i. U-0.063
8 R-20 c.i. U-0.048
Residential 1,2,3,4,5,6,7 R-15 c.i. U-0.063
8 R-20 c.i. U-0.048
Semi-Heated 1 NR U-1.282
2,3,4 R-3.8 c.i. U-0.218
5,6,7 R-5.0 c.i. U-0.173
8 R-10.0 c.i. U-0.093
90.1 – 2007 and 90.1 – 2010 Nonresidential 1 R-15 c.i. U-0.063
2-8 R-20 c.i. U-0.048
Residential 1-8 R-20 c.i. U-0.048
Semi-Heated 1, 2 R-3.8 c.i. U-0.218
3, 4 R-5.0 c.i. U-0.173
5 R-7.6 c.i U-0.119
6,7 R-10.0 c.i. U-0.093
8 R-15.0 c.i. U-0.063

Table 6.5.3-2: "Baseline Building Roof Construction Assemblies"

Construction Layer Thickness (inch) Conductivity (Btu/h ft F) Density (lb/ft²) Specific Heat (Btu/lb F) R-value (ft²·°F·h/Btu) U-factor
(Btu/h-ft²-F)
Roof R-20 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-20 continuous insulation 4.8 0.02 1.8 0.29 20.00
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 20.78 0.048
Roof R-15 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-15 continuous insulation 3.6 0.02 1.8 0.29 15.00
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 15.78 0.063
Roof R-10 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-10 continuous insulation 2.4 0.02 1.8 0.29 10.00
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 10.78 0.093
Roof R-7.6 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-7.6 continuous insulation 1.8 0.02 1.8 0.29 7.6
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 8.38 0.119
Roof R-5 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-5 continuous insulation 1.2 0.02 1.8 0.29 5.00
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 5.78 0.173
Roof R-3.8 c.i. Exterior air film 0.17
Roofing membrane 0.00
R-3.8 continuous insulation 0.9 0.02 1.8 0.29 3.80
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 4.58 0.218
NR. Exterior air film 0.17
Roofing membrane 0.00
Steel deck 0.06 26 480 0.10 0.00
Interior air film 0.61
Total for assembly 5.78 1.282
Exterior Roof Surface Properties
Applicability All roofs
Definition The exterior roof surface properties descriptor defines the characteristics of exterior surfaces. Exterior surface properties include emissivity, reflectivity and roughness. The first two govern radiation exchange from the surface, while the latter governs the magnitude of the exterior air film resistance.
Units Data structure: exterior surface properties
Input Restrictions The default value is a reflectance of 0.30 and an emittance of 0.75. The default value may be overridden when roof materials are used that have been tested by the Cool Roof Rating Council (CRRC) and are called for in the construction documents. In cases where the default value is overridden, the user is required to submit documentation identifying the test procedure that was used to establish the non-default values.
Baseline Rules The default values (see Input Restrictions above) shall be used for roofs for the baseline building.