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Helping You Make Your Home Your Castle |
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Proper Attic Ventilation
If you travel throughout the northern New England
states or other northern states during the winter
months you can’t help but notice the amount of
icicles hanging from houses. Approximately 70% of
the homes have large icicles hanging from the
eaves. This is caused by two reasons, insufficient
attic ventilation and attic insulation.
The importance of proper attic ventilation of your
home cannot be overlooked.
Improper attic venting can lead to moisture
buildup which causes mold, wood rot, and ice to
form on the roof, and icicles forming at the eaves.
And nearly just as bad, during the summer
when stagnant superheated attic air can dry out
your roof rafters and roof sheaving and increase
energy costs for air conditioning. The bottom line
is that most attics have insufficient venting and
or insulation.
Although most homes have attic venting, it is
inadequate or the vents are plugged with insulation
which prevents air flow through the attic.
Installing attic fans will not necessarily solve
the problem but they may help in some situations.
Hot air rises due to a differential in density
(mass) of the air. This causes an upward movement
of the air. You
would think that it would be easy for hot air to
rise out of your attic and the movement of the warm
air would bring in fresh air due to the density
differential of hot and cool air. In a static
situation there is no differential pressure between
the attic and outside air unless there is a slight
wind.
As the wind blows across the roof it creates a
positive pressure on the windward side and a lower
pressure on the lee side. This pressure
differential allows air to flow through the attic
and out the vents. For adequate venting there
should be a minimum of approximately 10 square feet
of passive venting for every 1,000 square feet of
attic floor space. A house with an attic that
measures 80 by 30 feet should have 24 square feet
of unobstructed passive roof venting with 50% at
the soffits and 50 % at the peak. The peak should
have full ridge venting and gable end vents.
If the house has a hip roof it may require dormer
vents in addition to full ridge and full soffit
venting. The dormer vents are attached
approximately two thirds of way up the roof.
To determine if your roof is improperly vented take
a trip to the attic on a cold winter day and
inspect for frost on any protruding nails or metal
braces. During the summer look for moisture
droplets. In addition, look for dark gray or black
areas on the roof sheathing. This is a clear
indication of inadequate venting and you should
correct it before serious damage occurs.
Correcting Improper Attic Venting:
1.
Determine the square footage of the attic space to
determine if there is adequate venting. If there
isn’t adequate ventilation, add vents to the soffit
and/or add additional gable end roof vents.
2.
Check to insure that the insulation is not blocking
the soffit vents in each bay between the roof
rafters. Install Styrofoam soffit vent. They are
formed to lie in the narrow space on top of the
insulation where the roof meets the wall. They are
four feet long and will fit between roof rafters
that are spaced sixteen inches on center. They
prevent the insulation from blocking the outlet to
the soffit.
3.
Make sure that the attic insulation is adequate and
that it fits tightly to the ceiling below. If you
need to add insulation I recommend using cellular
insulation. Just add it on top of the fiberglass
insulation.
An attic does not necessarily need attic fans of
any type as long as there is sufficient
unobstructed ventilation and insulation.
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