A2.4 Attic Roofs With Wood Joists
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 continuous 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:
- Attic roof standard framing: Insulation is tapered around the perimeter with a resultant decrease in thermal resistance. Weighting factors are 85% full-depth insulation, 5% half-depth insulation, and 10% joists.
- 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.
- 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.
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.
Insulation in such roofs shall be permitted to be tapered at the eaves where the building structure does not allow full depth.
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 joists, see Section A2.5.
|Rated R-Value of Insulation Alone||Overall U-Factor for Entire Assembly|
|Wood-Framed Attic, Standard Framing|
|Wood-Framed Attic, Advanced Framing|
|Wood Joists, Single-rafter Roof|
|Cavity Insulation R-Value||Overall U-Factor for Assembly of Base Roof Plus Continuous Insulation
(Uninterrupted by Framing)
|Rated R-Value of Continuous Insulation|