How to handle a moisture problem in the attic

Rob Robillard is a general contractor, carpenter, and editor of AConcordCarpenter.com.

Q. I’m hoping you can help me with a problem I’m having in my attic. My condo is only two years old, but there is quite a bit of condensation on the roof boards (they are damp/wet almost all the time). I had an insulation person look at it. He said the baffles that are supposed to direct the air up from the soffit vents were not installed properly, so there is not sufficient air flow. He said it’s hard to take them out and reinstall them correctly, however, as the drywall for the ceiling is already in place.

Any advice as to how to remedy this? We do have a furnace/air-conditioning unit in the attic for the second floor as well as two bathrooms that have venting running up through the attic to the outside (west side). The south wall is our neighbor’s, so there is no ventilation possible on that side.

CARL

A. You’re describing a common problem that leads to mold and sometimes rot. The primary cause of your attic-moisture problems results from warm air escaping from the heated portion of your home into the unheated attic space. Heat escapes around penetrations in the wall and ceilings made for items such as light fixtures, wires, vent pipes, and fans. This warm air condenses on the cold roof sheathing, causing frost and moisture issues.

The first step is to control moisture in the house by ensuring that the dryer and all baths vent to the exterior. Ventilation plays some role in keeping attics dry, but controlling indoor humidity is more important.

Dryer exhaust pipes running through unheated spaces should be installed inside an insulated sleeve, and ALL bathroom fans should be on timers to ensure that the fan runs a least 15 minutes after someone takes a shower. I have written a lot on proper bath venting and attic sealing on my website, AConcordCarpenter.com.

Let’s go back to the ventilation question. Increasing the attic ventilation is not always the right solution because it would allow more air to escape, pulling in additional warm, moist air from the living areas. The real solution is to seal off your attic access points, install adequate levels of insulation, and plug ALL air leaks.

It’s possible your ventilation could be improved, but you should really be looking to eliminate the warm air leaking into your attic. This can be done by locating and sealing all open penetrations with expanding foam, high-temp insulation, (Roxul, Rockwool, or ceramic) and sheet metal, if necessary. Sheet metal and high-temp insulation are commonly used to cover the two-inch framing gaps around masonry chimneys. These gaps are a major source of air leaks.

Focus on chimneys and recessed lighting. Chimneys should be air-sealed to stop a home’s moisture from getting into the attic. If you have a metal chimney, check the manufacturer recommendations.

After air sealing, you can insulate around masonry chimneys with high-temp insulation or fiberglass batts — do not put foam or cellulose directly against the chimney. On metal chimney flues, you will need to use sheet metal and high-temp caulking around the flue. You can’t insulate against any metal chimneys that I know of, and that is why the sheet-metal shield is required. It stops air leakage but maintains the two-inch open space required by code.