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
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
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