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GAZETTE Newspapers
By: Arthur S. Lazerow
Attic Ventilation a Must this Season
Question: Mr. White emailed a question last week asking
whether or not his home should be improved with an attic fan
through the roof to improve ventilation for this summer.
Unfortunately, my computer ate the email, so I was unable to
respond to him directly. However, his question is certainly
timely, since an attic I inspected recently measured in excess
of 140 degrees.
Answer: A home with a well-ventilated attic is a happy home,
everything else being equal. Understand the three functions of
attic ventilation. First, venting removes the warm, moist air
that migrates from the home into the attic before it can
condense on structural portions of the attic and cause rot or
mold growth. Second, the movement of fresh air into the attic
forces the hot stale air out of the attic, lowering attic
temperatures which otherwise would radiate through the attic
insulation and heat the living spaces below. And finally, the
introduction of freezing cold air into the attic during winter
months prevents ice dams from forming in gutters and water
spilling into living spaces.
What is considered a well-vented attic? Mechanical engineers
base their calculations on the number of square feet of attic
area. A home 30 feet deep by forty feet wide contains 1200
square feet of attic surface. If there is no vapor barrier in
the attic facing the living space, the ratio is one square foot
of open ventilation for every 150 sq. ft. of attic surface.
With a vapor barrier, such as paper-faced insulation with the
insulation facing towards the living space, the ratio is one
square foot for every 300 sq. ft. of attic. The 1200 sq. ft.
home with blown-in insulation but no vapor barrier needs 8
square feet of ventilation open to the exterior.
This assumes the home has a gable roof. Homes with flat roofs
or cathedral ceilings need special consideration and are more
difficult to ventilate.
Half of the ventilation should be high, such as at the ridge,
and half should be low and on opposite sides of the home, such
as front and rear soffit vents. If the home has gable end
louvers, there should be one on each gable end. In both cases,
high/low or gable ends, regardless of which direction the wind
is blowing, the side facing the direction of the wind will
become the active side. This, in combination with convection
currents caused by heating of the cooler air, is the process
that moves air through and out of the attic.
As a generalization, most houses have inadequate ventilation.
It is common during the summer for the attic to be 30 to 40
degrees warmer than the outside temperature. All this heat
increases the heat load that the air conditioning system must
handle, putting added strain on the system and costing more
money to maintain a comfortable temperature.
A power vent installed thru the roof, also known as an attic
fan, significantly increases the amount of fresh air moving
through the attic. Typically, the attic vent fan is installed
for aesthetic reasons on the rear portion of the home’s roof
somewhere near the middle of the home. This location permits
the fan to draw in fresh air from both gable louvers or from the
soffit vents located below on the perimeter of the attic.
As a home inspector working in attics every work day, I have
observed a marked difference between homes with and without
power venting. A 100-degree attic may sound hot, but compare
that to 140 degrees. R-25 rated insulation (six to eight inches
deep) or R-30 insulation (ten to twelve inches deep), regardless
of whether blown or batts, can handle 100-degree attic
temperature. 130-140 degree heated attic air will filter
through any shallow area of insulation, through cracks and
through the attic scuttle door and seriously impact the air
conditioning system.
During winter months with a snow load on the roof, the power
vent helps to control ice dams. Ice dams occur when the heated
attic space causes the underside of the snow to melt at night.
This water drips down the roof surface and lands in the gutter,
which is exposed to the freezing night temperatures.
This water freezes and over time, ice builds up until the weight
brings down the gutter or water melts and runs into the home.
Having the power vent in operation with snow on the roof, there
is reduced heat build up in the attic, which lessens the
opportunity for nighttime melting.
In answer to Mr. White’s question, all good things come to those
who improve their home’s attic ventilation. Operating costs
decline, the home becomes more comfortable during the hottest
summer days, your children will be smarter and better looking,
and you may even win the lottery. That’s a promise. As they
say in sports, just do it!
Have a question relating to a housing or environmental issue?
To contact Mr. Lazerow, click on “Contact Us” at
Albaninspect.com. Mention “Gazette Newspapers” in your
question. Every question will be answered and some will be
included in this column.
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