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Attic ventilation plays a
critical role in moderating the temperature within your home during summer
months and expelling warm moist air during winter months. Improper attic
ventilation, along with inadequate attic insulation, can cost home owners
hundreds of dollars a year in wasted heating and cooling costs. Inadequate
attic ventilation also reduces roof shingle life and allows moisture damage
to occur in the attic. The moisture damage can ruin insulation and other
attic building materials, as well as cause mold and mildew growth. |
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Inadequate attic ventilation can also
cause ice dams to form during winter months.
A properly ventilated attic should
have soffit vents along the roof eaves, as well as a ridge vent on the roof peak.
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moist air can easily be expelled from the attic. A properly constructed
ridge vent should traverse nearly the entire length of the roof peak.
Likewise, properly constructed soffit vents should traverse nearly the
entire length of the roof eaves. The standard formula for calculating the
minimum ventilation in a continuous vent system is 1 square foot of vent
opening for every 300 square feet of insulated attic ceiling with a vapor
barrier. |
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So for example, a 26’ x 36’ colonial style home needs 3.12 square feet of
total attic ventilation (Av = [26 x 36]/300 = 3.12 sqft). Typically between
50-60% of the calculated total vent area should be allocated to the soffit
vent and the remainder to the ridge vent. If the attic insulation does not
include a vapor barrier on the warm-in-winter side of the ceiling, the attic
ventilation area required doubles to a ratio of 1:150.
An alternative to installing ridge vent along the roof peak is to use cap
vents such as turbine vents, pot vents or mushroom vents.
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These types of roof vents are passive
attic ventilation devices that also expel heat and moisture from the attic.
However, the formula for calculating the minimum amount of attic ventilation
changes when using a non-continuous vent system. When using a non-continuous
vent system you need 1 square foot of vent opening for every 150 square feet of
insulated attic ceiling with a vapor barrier. Thus you need to double the amount
of ridge and soffit vent area. As a result, you wind up with quite a large
number of pot vents on your roof. Consequently this is why ridge vents are most
commonly used on homes. Cap and gable vents do, however, offer ventilation value
and should be considered to help complement a continuous ridge vent system.
Photos by Mark
Donovan
There are also power attic vent systems
that run off electricity and solar energy. Again, these types of vent systems
can help supplement a continuous ridge/soffit vent system but should not be the
total solution for attic ventilation. In addition, the electric ones consume
energy.
Another area that needs to be
considered when ventilating an attic is how the attic insulation is installed.
In particular, it is important that the attic ceiling insulation is not shoved
up tight against the soffit vents such that it restricts airflow. Particularly
on shallow roof lines, this can be a common problem. Consequently rafter babbles
should be used to prevent attic insulation from blocking soffit vents and
restricting air flow.
So the next time you are inspecting
your roof or attic, make sure to take a look at the attic ventilation system and
to asses is if it is adequate in capacity and functioning properly.
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Related Attic Information
Attic Insulation Vent Systems from Amazon.com
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