ASHRAE Journal - April 2023 - 17

COLUMN BUILDING SCIENCES
PHOTO 1 (LEFT) Roof Rot. OSB does not come that color when it comes from the factory.
PHOTO 2 (RIGHT) More Roof Rot. Frost on nails and roof sheathing.
PHOTO 3 A Young Guy. Yes, me in 1982 pointing
out how to seal attic ceilings.
PHOTO 4 (LEFT) Turbine Vents. Typically led to increased air-conditioning bills.
PHOTO 5 (RIGHT) Civilian Solution. Plastic garbage bag handles the turbine vent problem.
" Canadian Home Insulation Program " that came out of
the fi rst energy crisis. Everyone got a couple hundred
dollars to insulate attics. Guess what? The attics got
colder. Stopped the heat from getting into the attic,
but not the moisture. Roofs rotted (Photo 1 and Photo 2).
More oops.
To stop the moisture, we needed to air seal the attic
ceilings. Canadians did it fi rst, and best (Photo 3). Even
more oops. Moved the moisture problem to the building
underneath because air change in the building was
reduced. It sometimes got real bad, caused spillage and
backdrafting of combustion appliances and folks died
from carbon monoxide poisoning. Had to install sealed
combustion or power vented combustion appliances
and provide controlled mechanical ventilation.
In hot climates, back in the day, you were not actually
venting the roof-you were venting the building under
the roof through the roof bringing in outside air to the
occupied space and removing inside air up through the
attic to the exterior. This worked before we had something
called air conditioning.
With air conditioning, venting roofs sucked air-conditioned
air out of the building. The only way to not suck
air-conditioned air out of the building when venting
the roof is to air seal the attic ceiling. Yup, ended up
with spillage and backdrafting in the U.S. Civilians had
a better understanding than most experts. When they
installed attic turbine vents (Photo 4), and they found
that their air conditioning bill would go up, they had a
simple fi x (Photo 5).
You want to have some fun, ask an American about
how well the Weatherization program worked in
the 1990s. Oops. It has to be an old American. Pretty
sure Canada exported the technology to the U.S. to
get even for the Canada-U.S. Free Trade Agreement.
Weatherization folks discovered controlled ventilation
and combustion safety.
Even if you air seal the attic ceiling, and you handle
combustion safety, and you provide controlled ventilation
in the occupied space, venting a roof in a hot-humid
and mixed humid climate causes moisture problems
because the air outside is humid. Note the " humid " in
" hot-humid " and " mixed humid " climate.
Something that should be obvious is not always obvious.
Vented roofs blow off more than unvented roofs.
Signifi cant air pressure differences act across buildings
due to the action of wind. In general, building
assemblies experience positive wind pressures on the
A P R I L 2 0 2 3 ashrae.org ASHRAE JOURNAL
17
http://www.ashrae.org
ASHRAE Journal - April 2023

Table of Contents for the Digital Edition of ASHRAE Journal - April 2023
