Why Frost in the Attic

How to Eliminate the Formation of Frost in the Attic

By Mark J. Donovan

Question: - I went up into my attic last January on a very cold day and observed dripping water, and mold and mildew, on the roof sheathing. I made every attempt to resolve the problem by ripping out all of the moisture sodden insulation and installing new insulation. I also made sure the soffit vents were not blocked and had a new ridge vent installed. Unfortunately I am seeing the same problem again this winter. Do you have any ideas why?

Answer: Sounds like you have a serious moisture problem in your attic and a moisture source that you missed. You did the right things, however, by addressing the attic insulating and ventilation. To find the source of your moisture problem, I would first check the bathroom exhaust fans, vent stacks and then inspect the roof for leaks.

First, make sure your bathroom exhaust fans are being vented outside and not into your attic. Often contractors or DIY homeowners who install bathroom exhaust fans simply let them vent to the attic. This is a major mistake.

Second, make sure there is insulation packed in around any vent stacks that may be coming up from the living area below. Use a can of spray foam insulation to seal any of these types of insulation holes in the attic. The holes cut into the framing for vent stacks are notorious for letting warm moist air from the finished living areas below enter into the attic.

Finally, inspect the roof for leaks. I doubt you have a roof leak problem as you would most likely see much bigger signs of water/moisture problems, e.g. water dripping down into the finished living area of the home, or wet spots forming on the ceilings.

If all of these suggestions fail to uncover a problem, then make sure you have properly sized the amount of ridge vent and soffit vent for your attic. You possibly have an insufficient amount.