Attic Ventilation

Mucluk

I recently added a gable vent fan to generate more attic ventilation. The attic has 2 gable vents and 3 roof vents, there are no soffit vents. Should I cover the 3 roof vents and let the fan pull the air only from the other gable vent or should I leave them open? Thanks

Windows on Wash

Why did you add the fan?

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Mucluk

The fan was added because of mold growth on sheathing. Mold was removed, but there was not enough ventilation to prevent it from coming back. I also added additional insulation on attic floor.

Windows on Wash

Did you try air sealing some of the attic floor?

I would always recommend getting the low hanging fruit first before depending solely on ventilation to control moisture.

Do you have an overhang? Passive ventilation is always best if you can accomplish it.

Having the vents between one side of the home and the gable can short circuit the productivity of the gable fan.

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Mucluk

I have recently added another layer of R30 insulation to attic floor. Overhang is only about 4". Fan seems to be pulling a lot of air from all open vents. Hoping that added insulation and increase in ventilation will prevent any future mold issues.

Windows on Wash

Insulation does not equal (unless it is foam or dense packed cellulose) reductions in airflow.

You need to air seal that attic floor and any penetrations in the envelope to get a proper seal and reduce bulk moisture transfer into the attic.

The will reduce your necessity for ventilation and make the attic happier.

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Double-A

Well, here's the deal ... the fan might not be doing you much good.

For a solid discussion of attic venting, check out a company called AirVent.

Here's the short version:
If you have gable vents, just stick with gable vents (if they provide enough ventilation).

Don't mix gable vents with roof-top exhaust vents.

Definitely don't mix gable vents with roof-top exhaust vents and a power fan.

First, you need to figure out how much ventilation your attic needs. This is based on the square footage of the attic floor.

What sort of roof type do you have -- hip or gable?