Natural attic ventilation, with 1 ft2 (0.09 m2) of vent opening for every 300 ft2 (28 m2) of attic area, was proven to effectively prevent condensation in attics and as a result the 1:300 rule was born. The outdoor temperature in the experiment was –10°F (–23°C) and indoor conditions were 70°F (21°C) and 40% relative humidity. But these conditions are typical to cold inland climates only. Chances are you do not live in a cold inland climate and, as a result, the 1:300 rule does not apply anymore.

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
13 possible spots for your roof leak
20 point roof inspection check list
How to replace faulty roof shingles

Do I Always Need to Ventilate the Attic?

Most likely not! First laboratory experiments showed that attic ventilation can reduce condensation on roof sheathing during cold weather. But the experiments were carried out in conditions typical to cold inland climates and the result do not hold true to other types of climate such as hot climates or wet, cold coastal climates.

Hot, humid climates

The outside air tends to be much more humid than the inside air, which is cooled and dehumidified by air conditioning. Building attic vents will in fact increase rather than reduce moisture levels in the attic.

When the ceiling is not airtight, attic ventilation may also increase the latent cooling load in the building. Sometimes, air-conditioning ducts are located in the attic space and attic ventilation with humid outdoor air will increase the danger of condensation on these ducts.

Venting in such climates is done for other reasons that moisture control.

Hot, dry climates

Venting may result in dryer attics but there is no reason to expect any serious accumulation of moisture in unvented attics. Venting in such climates is done for other reasons that moisture control.

Wet, Cold Coastal Climates

In such climates, moisture in the outside air that is carried into the attic by ventilation is a major source of moisture in the attic. In fact, higher ventilation rates produce colder attics without lowering the content of water vapors in the attic. As a result the moisture content in the sheathing increases. In such climates unvented attics should be preferred, as long as indoor humidity is controlled by ventilation or dehumidification.

In a nutshell, discovering that you have a moisture problem in your attic does not necessarily means that you have to increase your attic ventilation by adding more vents or installing a mechanical ventilation system. In some instances reducing ventilation will solve the problem, instead. For more on moisture control in attics read How to Control Moisture in Attics.

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
13 possible spots for your roof leak
20 point roof inspection check list
How to replace faulty roof shingles

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