Solving snowstorm mystery of the wet and moldy attic

Inspector's Eye

Inspector's Eye

March 30, 2003

After the heavy snow in February, the owner of an 18-year-old townhouse noticed that part of the roof was wet in his attic. It remained wet during the time that the snow was on the roof and spots of mold began to grow on the plywood roof sheathing.

He asks why the problem occurred this year, but not in years past, and what can be done to prevent a recurrence. A little investigation revealed several factors inside the house that, combined with the severe cold weather, created the moisture problem.

The roof on this townhouse was replaced several years ago. The new asphalt shingle roof surface has adequate workmanship and is performing well. None of the moisture in the attic appears to result from roof leakage.

A problem does exist, however, with attic ventilation under the new roof.

During the roof replacement, the plywood roof sheathing that the shingles were nailed to was entirely removed and replaced due to deterioration of fire retardant treated plywood it contained.

The new roof has a properly installed ridge vent at the peak of the roof. But at the lower edges of the roof, down toward the eaves, the new roof has inadequate ingress venting to allow air to enter the attic.

The old roof had rigid foam vent channels that maintained an air space between the top of the attic insulation and the underside of the roof. But under the new roof, almost all of those vent channels are either missing or crushed flat.

With little fresh flow, the air in the attic is mainly from the interior of the house. This air migration occurs due to the "stack effect" of hot air rising. Interior air entering the attic can cause a serious moisture problem in the absence of adequate fresh-air ventilation.

As air from the interior of the house comes in contact with the cold underside of a snow-covered roof, it cools. If the air is sufficiently humid, some of the humidity will condense into water on the cold plywood. This is similar to the "sweating" on a glass containing a cold drink. The owner of the house recently installed a high-quality, steam humidifier. That humidifier is very effective at keeping the relative humidity in the house as high as you set it. The owner was mindful of the importance of not setting the humidity level too high - he said he had set the controls for 35 percent relatively humidity - normally a safe level given the climate in this part of the country. But it appears to have been too high when combined with the poor attic ventilation.

The recommendation to prevent a recurrence of moisture buildup in this attic is to retrofit vent channels to restore fresh air flow and install an attic vent fan with a humidistat to circulate fresh air. This probably is unnecessary if vent channels are retrofitted.

The second recommendation is to monitor indoor humidity in winter using a humidistat. A combination digital humidistat/thermometer the size of a pack of cards costs about $50.

A third recommendation is to have the mold removed and the plywood treated. Air normally flows from the house to the attic, so there is little likelihood that mold spores have circulated into the living space.

Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys Inc.

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