How to Eliminate and Prevent Mold in an Attic

Our home only has narrow, 3” wide soffit on the eaves. As a result, the venting in the attic is very poor. We have a massive mold problem on the rafters and underside of the roof decking, but only on the west facing side of the gable roof. What are your suggestions? -Dan

Hi Dan,

It definitely sounds like you have a major ventilation problem in your attic. Instead of circulating adequate outside air through your attic from the soffit vents to the roof ridge then back out, pockets of moist air are getting trapped inside the attic and condensing.

Since only one side of the roof is moldy, it probably receives more shade during the day, especially in the morning when the cool night air is trapped inside your attic. This can cause a temperature difference on the underside of the decking, resulting in the moisture in the trapped air condensing more readily on that side of the roof.

Unless you can increase the amount of venting in your soffits, you’ll need help from power vent fans to exchange the moist air in your attic for dryer, outside air. Attic ventilation fans would help move that moist air out of the attic without giving it time to find a home on the rafters and decking.

Attic vent fans can be hard-wired and equipped with a thermostat and/or humidity sensor so they automatically cut on at a preset moisture level or temperature. You could also install solar-powered attic vent fans, though I’ve found that most solar models aren’t powerful enough to be very effective.

For attic vent fans to work properly, you’ll need to be sure you have the recommended attic intake venting for the fan, either through existing soffit vents, gable vents, or roof vents. You can find out more at How to Size Attic Exhaust Vent Fans for Your Home.

Good luck with your project,



  1. Hi Dan,
    I am selling my home. I had it inspected and black mold was found in the attic. I had it remeditated and test came back normal. I am putting in a dehumidifying fan. Will this be enough to prevent new mold? Also put a new furnace without humidifier.
    Thank you,


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