Current building codes typically call for attic ventilation to minimize condensation under the roof sheathing. This requirement is based on experimental results showing that with 1 ft2 (0.09 m2) of vent opening for every 300 ft2 (28 m2) of attic area will successfully prevent moisture buildup in attics. Other commonly cited benefits of attic ventilation are reducing moisture problems, minimizing ice dams, ensuring shingle service life and reducing cooling load. But ventilation should be part of a broader strategy…

In the same series of articles:

Why ventilate your attic?
How to Control Moisture in Attics
Attic Moisture and Your Climate
How to Fix a Water Stain on a Ceiling
How to Prevent Icicles and Ice Dams on Roofs
Why and how to install an attic vent
How to Prevent Icicles and Ice Dams on Roofs
How to replace faulty roof shingles

Attic ventilation can be beneficial in some circumstances and climates but it should not be viewed as the principal strategy to eliminate moisture from attics.

General things to consider in a broader strategy

It is desirable to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below.

Make sure that bathroom fans, hood ranges, dryer vents and air exchangers are discharged through the roof not through the soffits. Discharging hot, humid air through the soffit is a major factor contributing to the moisture problems in attics.

Climate specific measures

Cold and mixed climates

Indoor humidity control should be the primary method to limit moisture accumulation in attics in such climates. Attic ventilation is recommended as an additional safeguard. The attic can be unventilated, if the amount of snow if small and if measures are taken to control indoor humidity, to minimize heat sources in the attic and to minimize air leakage into the attic from below.

Wet coastal climates and humid hot climates

Ventilation should be treated as a design option in such climates. There is support evidence that in fact attic ventilation will increase the moisture in attics and cathedral ceilings in these climates.

Climates with lot of snow

Ventilation is a must in climates with a lot of snow to prevent icing at eaves, regardless of insulation level or indoor humidity control measures.

For additional climate specific information read Attic moisture and your climate.

In the same series of articles:

How to Control Moisture in Attics
Attic Moisture and Your Climate
How to Fix a Water Stain on a Ceiling
13 possible spots for your roof leak
20 point roof inspection check list
Why and how to install an attic vent
How to Prevent Icicles and Ice Dams on Roofs
How to replace faulty roof shingles

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