ASHRAE-90.1-2022- - Page 232

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then compressed when the metal roof panels are attached. A minimum R-3 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor .

A2.3.2.5 Filled Cavity. The first rated R-value of insulation represents faced or unfaced insulation installed between the purlins. The second rated R-value of insulation represents unfaced insulation installed above the first layer, perpendicular to the purlins and compressed when the metal roof panels are attached. A supporting structure retains the bottom of the first layer at the prescribed depth required for the full thickness of insulation. A minimum R-5 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor .

A2.3.3 U-Factors for Metal Building Roofs. U-factors for metal building roofs shall be taken from Table A2.3.3 or determined in accordance with Section A9.2, provided the average purlin spacing for systems with compressed insulation is at least 52 in. U-factors for metal building roof assemblies with average purlin spacing less than 52 in. shall be determined in accordance with Section A9.2. U-factors in Table A2.3.3 shall not be used where the insulation is substantially compressed by the bracing between the purlins.

A2.4 Attic Roofs with Wood Joists

A2.4.1 General. For the purpose of Section A1.2, the base attic roof assembly is a roof with nominal 4 in. deep wood as the lower chord of a roof truss or ceiling joist. The ceiling is attached directly to the lower chord of the truss, and the attic space above is ventilated. Insulation is located directly on top of the ceiling, first filling the cavities between the wood and later covering both the wood and cavity areas. No credit is given for roofing materials. The single-rafter roof is similar to the base attic roof, with the key difference being that there is a single, deep rafter to which both the roof and the ceiling are attached. The heat flow path through the rafter is calculated to be the same depth as the insulation. Additional assemblies include continu- ous insulation uncompressed and uninterrupted by framing. The U-factors include R-0.46 for semiexterior air film, R-0.56 for 0.625 in. gypsum board, and R-0.61 for interior air film heat flow up. U-factors are provided for the following configurations:

a. Attic roof, standard framing: Insulation is tapered around the perimeter with a resultant decrease in ther-

mal resistance . Weighting factors are 85% full-depth insulation, 5% half-depth insulation, and 10% joists. b. Attic roof, advanced framing: Full and even depth of insulation extending to the outside edge of walls .

Weighting factors are 90% full-depth insulation and 10% joists. c. Single-rafter roof : An attic roof where the roof sheathing and ceiling are attached to the same rafter.

Weighting factors are 90% full-depth insulation and 10% joists.

A2.4.2 Rated R-Value of Insulation

A2.4.2.1 For attics and other roofs, the rated R-value of insulation is for insulation installed both inside and outside the roof or entirely inside the roof cavity.

A2.4.2.2 Occasional interruption by framing members is allowed but requires that the framing members be covered with insulation when the depth of the insulation exceeds the depth of the framing cavity.

A2.4.2.3 Insulation in such roofs shall be permitted to be tapered at the eaves where the building struc- ture does not allow full depth.

A2.4.2.4 For single-rafter roofs, the requirement is the lesser of the values for attics and other roofs and those listed in Table A2.4.2.

A2.4.3 U-Factors for Attic Roofs with Wood Joists. U-factors for attic roofs with wood joists shall be taken from Table A2.4.3. It is not acceptable to use these U-factors if the framing is not wood. For attic roofs with steel j oists , see Secti on A2.5.

A2.5 Attic Roofs with Steel Joists

A2.5.1 General. For the purpose of Section A1.2, the base assembly is a roof supported by steel joists with insulation between the joists. The assembly represents a roof in many ways similar to a roof with insu- lation entirely above deck and a metal building roof . It is distinguished from the metal building roof category in that there is no metal exposed to the exterior. It is distinguished from the roof with insulation entirely above deck in that the insulation is located below the deck and is interrupted by metal trusses that provide thermal bypasses to the insulation. The U-factors include R-0.17 for exterior air film, R-0 for metal deck, and R-0.61 for interior air film heat flow up. The performance of the insulation/framing layer is calculated using the values in Table A9.2-1.

A2.5.2 U-factors for attic roofs with steel joists shall be taken from Table A2.5.2. It is acceptable to use these U-factors for any attic roof with steel joists .

A2.5.3 U-factors for attic roofs constructed of cold-formed-steel conventional C-shape framing or coldformed steel trusses, where the insulation is located at the ceiling joist or the bottom chord, and where the framing spacing does not exceed 24 in. on-center, shall be determined in accordance with AISI S250.

230 ANSI/ASHRAE/IES Standard 90.1-2022 (I-P)