A brief history of cathedral ceilings
Insulated cathedral ceilings are a relatively recent phenomenon. The craze for insulated cathedral ceilings (and great rooms) really took off in the 1970s and 1980s, when examples began popping up like mushrooms after a warm rain. In those days, most builders stuffed cathedral ceiling rafter bays with fiberglass batts. Sometimes they included flimsy Proper-Vents between the fiberglass and the roof sheathing, but often they just specified thin batts to ensure that there would be an air space above the batts for ventilation.
The cathedral ceilings of the 1970s and 1980s were thermal disasters. In most cases, these ceilings leaked air, leaked heat, created monumental ice dams, and encouraged condensation and rot. In many cases, roofers tried to solve these problems by improving ventilation openings in the soffits and at the ridge; these “improvements” often made every symptom worse.
Until recently, building codes required that insulated sloped roofs include ventilation channels directly under the roof sheathing. Many builders still follow this time-tested technique.
As building scientist Bill Rose has shown, code requirements for roof venting were never based on research or scientific principles. In a well documented JLC article on roof venting (“Roof Ventilation Update”), Rose explained, “For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. … Once this is accomplished, roof ventilation becomes pretty much a nonissue.”
Because of their unscientific origins, code requirements for venting roofs are often misunderstood. It’s worth establishing a few basic facts:
- Roof ventilation cannot be used to lower indoor humidity levels.
- Builders should not encourage the migration of water vapor through a cathedral ceiling.
- During the summer, roof ventilation does not significantly lower the temperature of asphalt shingles or other types of roofing.
- While roof ventilation can lower the risk of ice damming, it’s essential for builders to limit the flow of heat into roof ventilation channels by including one or more ceiling air barriers and by installing thick insulation, so that as little heat as possible escapes from the home.
- While roof ventilation can help dry out damp roof sheathing, it’s essential to limit the flow of water vapor escaping from the home so that the roof sheathing never gets damp in the first place.
- In the absence of an airtight ceiling, roof ventilation can do more harm than good, since air movement in rafter bays can encourage indoor air to leak through ceiling cracks.
As I noted earlier, researcher Bill Rose has exposed the unscientific nature of code requirements and formulas for calculating roof ventilation openings. Unscientific or not, these code requirements must be followed.
Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.
Manufacturers of soffit vents and ridge vents usually specify the net free vent area of their products on product packaging or in specifications available online.
This information is sourced from; http://www.greenbuildingadvisor.com