Insulating a Cathedral Ceiling
Vented or unvented roof assemblies can work, but failure carries a heavy price.
Synopsis: Cathedral ceilings provide a majestic crown for many a great room, but insulating them can be tricky. A poorly detailed cathedral ceiling can lead to energy losses and rotted roofing. In this article, senior editor Martin Holladay describes eight ways to get a cathedral-ceiling roof to R-45 or better using both vented and unvented processes. Holladay says that vented cathedral ceilings can be difficult to insulate well, so it’s important to place the vent baffles and insulation properly. He also says that unvented cathedral ceilings have a mixed track record, but can work when the details are managed correctly. This article includes cross-section diagrams showing four ways to prepare a ceiling from above (if the roofing is going to be replaced) and four ways to access the ceiling from inside. A sidebar outlines various roof-venting myths.
Because older cathedral ceilings are usually insulated with thin fiberglass batts, they are often thermal disasters. These ceilings usually leak air, leak heat, create monumental ice dams, and permit condensation and rot. Roofers sometimes try to solve these problems by improving ventilation openings at the soffits and ridge, but these “improvements” often make every symptom worse.
Fortunately, there are better ways to build cathedral ceilings. Whether they are vented or unvented, such ceilings can perform well, as long as they are properly detailed.
Although many builders still follow the time-tested technique of installing vent channels directly under roof sheathing, a vented cathedral ceiling makes sense only if the geometry of your roof is simple. You need a straight shot from the soffits to the ridge. That’s easy on a gable roof without any hips, valleys, dormers, or skylights, but if the geometry of your roof is complicated, it’s impossible to ensure adequate airflow through all of the rafter bays. Until recently, building codes required that insulated sloped roofs be ventilated. For complex roof designs, the International Residential Code (IRC) now allows unvented cathedral ceilings.
Vented cathedral ceilings can be difficult to insulate well
Ventilation channels maintain an air gap between the insulation and the roof sheathing. The component that creates the gap is called a variety of things, including a ventilation baffle, a ventilation chute, a ventilation channel, or a product such as Proper-Vent.
The first vent baffles to hit the market — the classic Proper-Vents of the 1970s and ’80s — were flimsy items made of thin polystyrene. These baffles don’t work well. Being thin and flexible, they can’t resist the pressures from dense-packed cellulose; they don’t ventilate the entire width of the rafter bay; and as usually installed, they allow air to leak out the top of the insulated assembly.
Eventually, manufacturers began offering stiffer alternatives that were able to resist the pressures of dense-packed insulation. Many of these products are no longer available. At one time or another, it was possible to buy baffles that were made of cardboard, vinyl, and compressed cellulose fibers. These days, the best available vent baffle is probably the AccuVent baffle, which is made from stiff vinyl.
Some builders make their own site-built baffles. According to the IRC, “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.” Such a vent space can be created by tacking 1-in. by 1-in. “sticks” in the upper corners of each rafter bay, followed by stiff cardboard, thin plywood, OSB, fiberboard sheathing, or panels of rigid-foam insulation. Many experts advise that 2-in.-deep vent cavities are even better.
When installing any type of vent baffle, it’s important to pay attention to air-sealing, especially if you plan to install fibrous insulation in the rafter bays. Seal the edges of each panel with caulk, and tape the seams between panels with a high-quality tape.
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