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Q.Should attics and cathedral ceilings have ventilation and vapor barriers?

A.Bill Rose responds: We’re conducting research at the University of Illinois to answer these questions. The advice below is based on recent literature and my experience in a temperate climate (hot summers and cold winters).

Vents do keep attics and cathedral ceilings cooler. With the sun shining, the air temperature in a vented attic may be 10°F to 15°F cooler than in an unvented attic. So the temperature argument for ventilation holds in most of the U.S. The other main reason to ventilate is to prevent moisture accumulation in the attic or roof system. The jury is still out on how effective that is, but for now the prudent course of action is to ventilate all attics and cathedral ceilings.

What about vapor barriers? Our research shows that in a building with a small moisture load such as a church, you’ll never have roof moisture problems. But a house roof must have a continuous ceiling plane. The old way of providing an unbroken plane was to use a poly vapor barrier. The new way focuses on sealing electrical and plumbing chases and holes in the top plates. Holes around ceiling fans and fixtures are the first sites for water spotting on the roof sheathing.

The new way of sealing addresses the fact that most attics are linked to the basement or crawlspace through partition chases.

Therefore, a wet basement or crawlspace will likely cause a moisture problem in the attic, and the cure for some attic moisture problems is better gutters, downspouts, and grading around the foundation.

For cathedral ceilings, we’ve seen both successes and failures with practically every imaginable assembly in every climate. We prefer scissor trusses because the air circulates more freely around the smaller framing members, so they may be less prone to local moisture damage.

But with any roof system, vented or not, you can relax and not worry about moisture if:

  • the ceiling plane is tight
  • the house is kept reasonably dry

In retrofit work, put your effort into providing good insulation and sealing all holes in the surface where the living space meets the attic.

Bill Rose is researching moisture and roofing systems at the University of Illinois, in Champaign-Urbana, Ill., where he teaches building technology.