Q&A – Managing moisture with ventilation

With condensation forming inside their home, this person came to our experts for answers.

Elizabeth, a homeowner from Tennessee, recently asked our experts a question about ventilation and moisture management:

I have gable vents on my house and my old roof have wirly birds. When they installed my new roof they put in ridge vents. I’m having a moisture issue in my house and the paneling is starting to welp up from condensation. Could this be because the gable vents need to be covered because they are bringing in and causing moisture. And yes, I do have soffit vents.

Here's what one of our experts, John Kenney from Cotney Consulting, had to say in response: 

Elizabeth, the gable vents may be contributing to the problem, but they are probably not the source of the moisture on their own.

When a roof has continuous soffit intake and ridge exhaust, open gable vents can sometimes interrupt the intended airflow pattern. Instead of pulling outside air evenly from the soffits and exhausting it at the ridge, the ridge vent may draw air from the gable vents. That can leave lower attic areas poorly ventilated. This is often called short-circuiting the ventilation system.

However, closing the gable vents without first checking the entire attic would be premature. Condensation usually occurs when warm, moisture-laden air from inside the home comes into contact with a cold surface. Common causes include bathroom or kitchen exhaust fans terminating in the attic, air leakage around ceiling penetrations, inadequate insulation, blocked soffit vents or excessive indoor humidity.

I would have the attic inspected to confirm that the soffit vents are open, ventilation baffles are in place and the ridge vent has been cut and installed correctly. The contractor should also calculate whether the intake and exhaust ventilation are properly balanced. If the roof now has an effective soffit-to-ridge system, the gable vents may need to be closed, but that decision should follow the ridge vent manufacturer’s requirements.

Overall recommendation: Don’t assume the gable vents are bringing moisture into the house. First identify the moisture source and verify that the new ridge-and-soffit ventilation system is actually functioning. Because the interior paneling is already swelling, I would have this inspected soon before the moisture causes more extensive damage.

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