Roof/ceiling members must be properly designed to ensure they provide adequate support as required by the loads that are imposed on them as well as making sure these loads are correctly transferred to the supporting walls and/or supporting structural elements.
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Want to learn how to easily size roof rafters?Ĭhapter 8 of the 2018 International Residential Code (IRC) regulates the design and construction of roof and ceiling systems.
The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building. The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement. The port area shall be greater than or equal to 1:600 of the ceiling area.
An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port. In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics on the top of the attic floor or on top of the ceiling shall meet the following requirements: For calculation purposes, an interior air temperature of 68☏ (20☌) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.ĥ.2.
Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45☏ (7☌). The air-permeable insulation shall be installed directly under the air-impermeable insulation. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control.
In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1.1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing. Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
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