A cathedral ceiling is a pitched ceiling that goes up through more than one floor. The sloping sides have the same pitch as the roof, and the ceiling is made by covering the bottoms of the roof rafters with ceiling material. Insulating your cathedral ceiling helps prevent heat loss in the winter and keeps your home cooler in the summer. However, a cathedral ceiling, unlike an attic floor located over a flat ceiling, has special requirements when it comes to insulation.
Spacing Requirements
The Model Energy Code, which is now known as the International Energy Code, is the residential energy code most states use. According to this code, cathedral ceilings must provide sufficient space between the roof deck and the ceiling for insulation and ventilation. This can be accomplished by using truss joists, scissor truss framing or large rafters.
R-Value
Insulation is rated on how well it resists the flow of heat. This number rating appears on the insulation and is referred to as its R value. Higher R values denote more efficient insulation. R values vary depending on the type of insulation, its density and its thickness. How effective an insulation is also depends on where and how it's installed. The Model Energy Code recommends that insulation used in a cathedral ceiling has an R value of 25 to 38 for most parts of the country.
Types
Several types of insulation can be used in cathedral ceilings as long as they're installed in a way that controls condensation. The 2009 edition of the International Energy Code includes a table that lists the minimum requirements for different climate zones. How the insulation is installed is also important. Rigid board or sheet insulation must be installed right above the roof sheathing, according to Section R806.4 of the International Energy Code. "Flash and batt" insulation consists of a spray foam that must be applied directly to the underside of the roof sheathing, according to the code, and an air-permeable insulation that's installed between the rafters. Foil-faced batt insulation is commonly used on cathedral ceilings because of its permeability rating. The code expressly forbids installing dense-packed cellulose insulation if you don't have ventilation channels.
Venting
Although the Model Energy Code originally required cathedral ceilings to have ventilation channels under the roof sheathing, builders are no longer required to do so. While ventilation channels prevent moisture from accumulating and reduce the chances of ice buildup, building scientists now think that builders should construct airtight ceilings that reduce the amount of heat that flows into ventilation channels. This can be accomplished by including air barriers and using thick insulation. Don't install fiberglass batts, mineral wool batts, dense-packed cellulose or blown-in fiberglass into an unvented cathedral ceiling unless you also use a layer of spray polyurethane foam or rigid foam panels right above or right below the roof sheathing. Air-permeable insulation, such as fiberglass batts, creates condensation problems that cause the roof sheathing to rot. Only spray foam or rigid foam insulation can meet code requirements for air-impermeability on a cathedral ceiling.
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