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Many homes are
improperly or inadequately ventilated even when some form of ventilation
is installed.
Signs of improper
ventilation include:
- Damp insulation
- Unnaturally
dark or discolored decking
- Dark streaks
on roof boards around nails
- Orange resin
beads on rafters signaling sweating wood
- Moisture
on roof boards & rafters
- Crumbling,
curled roof shingles
In drastic cases
of dampness, soft and crumbling boards will be evident, and structural
repairs may be needed. In less drastic situations, the problems
may be solved simply by installing additional ventilation.
To determine your ventilation needs, find the square footage of
your attic floor and use the number to calculate how much Net Free
Area your ventilation must provide. Net Free Area is the approximate
clear opening of the ventilator through which air may move.
As a general rule, the space ventilated should have a minimum Net
Free Area of 1/150*. This means that for each 150 square feet of
attic floor space, 1 square foot of Net Free Area is required for
ventilation.
The ratio can be reduced to 1/300 if:
- there is
a vapor barrier installed in the attic over the living space,
or
- there is
a difference of at least three feet in height between the intake
(soffit under-eave vents) and the exhaust (Ridge).
The total Net
Free Area of the intake and exhaust must be balanced. Any excess
Net Free Area should be at the intake.
* Check local codes.
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