Old attic ventilation is incredibly
inadequate — and easy to fix
Dear Sheila: A hot attic increases your
air-conditioning costs and makes attic
and roof materials deteriorate faster
than they should. Having just a vent in
each side of the gable was the typical
attic ventilation configuration in older
houses, but today it is considered woefully
inadequate for an efficient house.
Even with enough insulation on the
attic floor, extreme attic heat transfers
through the insulation to the ceiling
below. What makes things worse is all
the structural lumber in the attic also
reaches 140 degrees by afternoon. With
its thermal mass, it can stay hot well into
the evening and continually transfer heat
into your house.
People often think of attic ventilation
as being important only during summer
when a typical dark asphalt shingle roof
can easily reach 170 degrees. It is also
important during winter. No matter how
well the vapor barriers were installed in
your walls and ceiling, indoor air and
water vapor get into the attic.
When it is cold and this moisture-laden
air is allowed to collect in the attic and,
even in relatively mild climates, it can
condense on the lumber and drip onto the
insulation. The R-value of insulation is
much lower when it is damp so more heat
is lost through the ceiling during winter.
In cold climates where the temperature
often drops below freezing at night, it
is even more important to ventilate the
attic to keep the roof cold. If warm air
from the ceiling below stagnates near the
roof peak, it can melt snow on the roof.
This water runs down the roof to a cooler
area and refreezes causing an ice dam.
Over time this dam causes water to back
up under the shingles and leak into the
attic and destroy the lumber. Its weight
can also damage the gutters.
The ideal ventilation flow is coming in
low over the insulation keeping it dry and
cool. Some of it should flow up under the
roof sheathing to keep it cooler and then
exhaust out near the peak of the roof.
This airflow out the roof peak keeps it
cooler during summer and much colder
during winter to minimize condensation
and ice dam formation.
There are various attic venting options
and all of them are better than your gable
vents. A combination of a ridge vent and
soffit vents is most effective and not difficult
to install yourself. Once you install
proper new attic ventilation, you should
block off the gable vents because they
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Dear Jim: It gets very hot in my attic. I don’t think the two end vents are enough.
What types of additional attic vents are effective and how much more should I
add? — Sheila F.
Proper ventilation in your attic is crucial to keeping
energy costs down and increasing the life expectancy of
HERE’S HOW · 6 your roof.