But when these bays were covered by TPO, moisture could not escape. It seems moisture from the bathroom was able to escape through the asphalt roll roofing, even in these unvented bays. Because of the chimney, these bays did not connect to the ridge vent. Within a year the plywood became mushy, but only in the two bays that terminated at a chimney at the ridge. But when the roll roofing deteriorated I had it replaced with TPO. ![]() The sheathing was fine for several years. Underneath the plywood sheathing in the bays was a 3/4 vent channel, 1 inch of polyiso and 7.5 inches of cellulose and then a layer of poly and sheetrock. When I renovated our house, I put asphalt roll roofing on a shed dormer over a bathroom. If you install closed-cell spray foam insulation, and if the insulation is installed conscientiously, the spray foam is an effective air barrier and vapor barrier that prevents the migration of moisture from the warm interior to the cold sheathing. If you add a ventilation channel between the top of the insulation and the underside of the roof sheathing, the ventilation channel removes the incidental moisture that reaches the sheathing, keeping the sheathing dry and keeping the assembly safe. (For more information on some confirmatory field research, see "Filling Rafter Bays With Fluffy Insulation.") But in 5 or 10 years your roof sheathing might be a soggy mess. Įxperience shows (and field research confirms) that it is impossible to create a tight enough air barrier on the interior to prevent air movement through hidden pathways from reaching the cold sheathing if your chosen insulation is fiberglass. The thickness of the fiberglass is irrelevant, because fiberglass is neither a vapor retarder nor an air barrier. Moisture moves from the interior to the sheathing two ways: by diffusion (slowed by a vapor retarder or vapor barrier) and by air movement (slowed by an interior air barrier). The interior of the home is warm and moist during the winter, and the roof sheathing is cold (and therefore a potential condensing surface). If the fiberglass is thick enough, what's the problem?"Ī. "If #4 and 5 work, why does #1 need to have the complex cold roof channel?. 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 GBA readers post questions 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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