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Poor Attic Ventilation Can Ruin
Your Insulation,
Destroy Your Shingles, And Raise Your Energy Bills!
Why Take Risks?
It is estimated that 9 out of 10 homes
in North America do not have proper attic ventilation. Why?
Because most people are unaware that attic ventilation can
impact the longevity of their entire home!
For example, in the
summer, an improperly
ventilated attic can cause heat to build in excess of 160°F.
This superheated air eventually penetrates the ceiling
insulation into the living area below.
Types of damage that
can result include:
• Premature aging of your roofing system (“fried” shingles)
• Warping, cracking, or breaking down of wood framing
• Damage to siding, exterior or interior paint, and wallpaper
• Higher energy costs
A properly ventilated attic can help reduce the load on your
air conditioner by moving the superheated air out of your
attic before it builds up and causes damage.
In the winter, various
household appliances, bathtubs, showers, and cooking vapors
can contribute to excess moisture build-up. Improperly
ventilated attics will allow this moisture to collect and
cling to the underside of the roof. There, it will condense
and fall, soaking the attic insulation and reducing its
efficiency.
Additional structural damage can include:
• Roof deck warping and rotting of the wood frame
• Mildew growth
• Buckling of shingles and felt
Finally,
attics should be properly ventilated to help prevent ice dams
in cold northern climates. During the winter, ice and snow on
a roof will melt and run down the deck to the cooler eaves.
This run-off can re-freeze, creating an ice dam that will trap
moisture on the roof. The moisture can eventually back up
under the shingles and enter your home — causing hundreds or
thousands of dollars of damage to your ceilings and walls.
Adequate attic ventilation will reduce the amount of initial
melting that occurs on your roof, thereby reducing the chance
that
ice dams will form. |
Proper Attic Ventilation
Proper attic ventilation systems allow
a continuous flow of outside air through the attic (see
illustration
at left), protecting the efficiency of the insulation and
helping to lower temperatures in the living space.
It consists of a balance between air intake
(at your eaves or soffits) and air exhaust
(at or near your roof ridge).
The FHA (Federal Housing Administration) recommends a
minimum
of at least 1 square foot of attic ventilation (both intake
and exhaust) for every 300 square feet of attic space. For
example, if your attic is 900 square feet, you need a total of
3 square feet of ventilation. This amount is generally divided
equally between intake and exhaust ventilation (i.e., 11/2
feet of each), to insure proper air flow through the attic. To
calculate the amount of ventilation you need for your home,
follow the example shown on the next page. |
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Calculating Your Ventilation
Requirements
Example: Calculating the number
of lineal feet of Cobra® Exhaust Vent needed for a 30’x 40’
attic: |
Step 1
Step 2
Step 3
Step 4
Step 5
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Calculate attic square footage
How: Multiply length of attic (in feet) times width of attic
(in feet)
30’ x 40’ = 1,200 square
feet
Calculate NFA (Net Free Area)
needed for this attic by using the “1 in 300” rule
How: Divide attic square footage by 300
1,200 sq. ft. ÷ 300 = 4
square feet of NFA needed
Convert square feet of NFA to square inches
How: Multiply square feet of NFA by 144
4 sq. ft. x 144 = 576
square inches of NFA needed
Split the amount of NFA needed
equally between the intake and the exhaust
How: Divide square inches of NFA needed by 2
576 sq. in. ÷ 2 = 288
square inches of NFA needed equally for exhaust & intake
Calculate # of lineal feet of
Cobra® Exhaust Vent needed
How: Divide the square inches of NFA needed at the ridge by
the NFA of the Cobra® Exhaust Vent
(Cobra® Exhaust Vent has 16.9 sq. inches of NFA per lineal
foot)
288 sq. in. ÷ 16.9 = 17
lineal feet
This attic requires seventeen (17)
lineal feet of Cobra® Exhaust Vent to meet FHA minimum
requirements. |
Intake ventilation MUST be at least equal
to exhaust ventilation. To determine the number of intake
vents needed, select your vent on page 12, and install enough
vents so that their combined NFA is at least 288 square
inches.
Remember:
Always have a balanced ventilation
system. In no case should the amount of exhaust ventilation
exceed the amount of intake
ventilation. |
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