![]() ![]() Since this is a vented assembly, it can’t be used on convoluted roofs. In most cases, you’ll want to tape the rigid foam seams with high-quality tape, and install 1×3 or 1×4 strapping (perpendicular to the rafters, installed 16 inches on center) on the interior side of the rigid foam to make it easier to install a drywall ceiling. (Note that the R-values shown in the illustration can be reduced in warmer climates.) The illustration shows an assembly that will work in Climate Zones 4, 5, 6, 7, and 8. If your rafters aren’t deep enough to accommodate the R-value you need, you can create a vented roof assembly with fiberglass or mineral wool batts between the rafters, supplemented by a continuous layer of rigid foam on the interior side of the rafters. If your roof is convoluted, you need to use an unvented approach.įor more information on vented roof assemblies, see “How to Build an Insulated Cathedral Ceiling” and “ Site-Built Ventilation Baffles for Roofs.”Īssembly #2: Vented assembly with fiberglass or mineral wool batts and interior rigid foam Note that vented roof assemblies require a simple gable roof or shed roof, without any hips, valleys, dormers, or skylights. The air channel must be at least 1 inch deep-2 inches is better-and must be connected to soffit vents at the bottom of the assembly and a ridge vent at the top of the assembly. Most builders use a ventilation baffle to define this air space. Vented assemblies require an air space between the top of the insulation layer and the underside of the roof sheathing. If your rafters are deep enough, you can create a vented assembly insulated with fiberglass batts or mineral wool batts, as shown in the illustration at the top of this page. The best way to ensure good insulation performance is to make sure that your roof assembly includes a bulletproof air barrier, and to make sure that all electrical boxes and other penetrations are carefully sealed against air leakage.Īssembly #1: Vented assembly with fiberglass or mineral wool insulation Most cathedral ceiling moisture problems can be traced to air leaks. So if you hope to insulate your ceiling with a fluffy insulation material like fiberglass batts, and you plan to install all of the insulation between your rafters, you’ll either need very deep rafters, or you’ll need to scab on some extra framing material (for example, 2x4s) to your rafters with plywood gussets to get enough depth for the minimum insulation levels required by building codes.Īir sealing. If you are insulating with fiberglass, that’s not enough space for the minimum R-values required by building codes unless you live in Climate Zone 1. Note that 2×10 rafters equipped with a 1-inch-deep ventilation space have room for only 8 inches of insulation. That said, local codes may differ from these general guidelines, so it’s worth asking your local building department about minimum R-value requirements in your community.įor more information on minimum R-values for cathedral ceilings, see “Three Code-Approved Tricks for Reducing Insulation Thickness.” In Zones 2 and 3, the minimum requirement is R-38, while in Zone 1, it’s R-30. In Climate Zones 4, 5, 6, 7, and 8, most prescriptive building codes require ceiling insulation to have a minimum R-value of R-49. While these five assemblies are probably the most common approaches used, note that there are other acceptable ways to insulate a cathedral ceiling. This article will cut to the chase by providing simple descriptions of five cathedral ceilings that work. When people post questions on the Green Building Advisor forum about cathedral-ceiling insulation, they are often directed to a 3,800-word article from 2011 called “How to Build an Insulated Cathedral Ceiling.” While the comprehensive information in that article-including detailed discussions of vapor diffusion, condensation risks, and failure modes-is useful, it may represent information overload for many builders.
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