A home’s roof takes the brunt of the elements—hot sun, strong winds, rain, and snow. To be durable, it should keep water and moisture out. And the entire roof assembly, which includes the ceiling of the home, should be well-insulated to help keep occupants comfortable year-round.
Choosing a roof insulation strategy depends on the style of the roof and the climate. Entire books can be (and have been) written about insulation, so let’s focus on a roof style that’s particularly tricky to insulate: roofs over cathedral or vaulted areas.
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These dramatic spaces are becoming popular with homeowners. However, typical methods of insulating ceilings don’t apply for this type of roof. With the triangular shape of typical attics, there’s no limit to how much insulation you can install. With typical attics, you’ll have no problem achieving the minimum level of insulation that building codes dictate for roofs. And, to improve energy efficiency and occupant comfort, installing more is easy to do (see the Insulating Your Attic for Performance article for more information.) Insulation for roofs over cathedral areas, however, needs to fit into the cavities between the rafters. For example, if the rafter is a 2X12, the depth of the insulation can be a maximum of ten inches with a one and a half inch airspace. Two best practice options for insulating cathedral roofs are fiberglass batt insulation with an airspace and expanding spray foam. Click here for details of these two options.
Making the roof and ceiling assembly airtight is an important part of the insulation strategy. In some climates, airtightness keeps moisture-laden air from entering and condensing within the insulation or on the underside of the roof sheathing, and in all climates, airtightness is important for improving occupant comfort and energy efficiency. All penetrations through the ceiling should be sealed with caulk or spray foam. Typical penetrations include skylights, speakers, electrical boxes, and recessed downlights. Recessed downlights should be airtight and Insulation Contact (IC) rated. When you’re applying gypsum board where a wall meets the portion of the roof that slopes up, you should take extreme care to seal that connection. An easy way to seal the connection is to glue the edge of the drywall to the framing. And ideally, you shouldn’t run mechanical ductwork through vaulted assemblies. It isn’t recommended, as registers and grilles penetrate the roof. And if the ductwork isn’t 100% sealed, air can enter the cavity of the assembly through the leaks and lead to condensation, which could cause moisture-related problems.
While an airtight roof and ceiling assembly keeps the amount of moisture-laden air entering the home to a minimum, you need a water management strategy to keep water from entering the roof assembly through the exterior shingles. A comprehensive water management strategy should consider correct installation, a drainage plane, and flashing. Learn some of the details of installation, including how the roofing paper should be installed to provide good water management. For details on roof flashing, see the March 2003 issue of the QualityHome® newsletter available at http://www.ibacos.com/newsletter.html.
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