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Ask the Carpenter: Dryer vents, damaged ceilings, and frosty attics

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Q. My wife and I are looking into replacing our basement windows with glass block, and the contractor said it would be best to vent the dryer in a different location, through one of the glass-block windows. But that window is located under a 3-foot outcropping, our kitchen. I question whether it is a good idea to vent that much hot, moist air underneath the structure. I think it would cause too much condensation.

MIKE

A. This is an interesting question. Dryer vents that terminate in crawl spaces or attics are not allowed, because they allow moisture to deposit and encourage mold growth, a health hazard. Dryer vents must terminate outside the building.

By venting outside, and under your cantilevered first floor, you will be introducing moisture to the underside of the house floor system. I would ensure that the cantilevered floor is well-sealed. I would also make sure that the ductwork is constructed of galvanized steel and that the fire-resistance rating is maintained in accordance with the International Building Code.

Have other readers dealt with the same problem? Let me know.

Q. We moved into our new two-story Energy Star home in 2010 and discovered an attic problem during and after the very first ultra-cold nor’easter we experienced a couple of years later. The next time it happened, it was much worse.

Frost built up on the inside of the roofing nail points on the north side of the interior. The moisture apparently ran down the inside of the outer walls and onto the floor joists above the first-floor bedroom and kitchen. There is slight water staining on one of the second-floor ceilings, but there are several stains on the first floor that are large and brown. I’m pretty sure the problem came from the condensation on the nails, since that was the only frost I could detect in our well-insulated attic.

We do have a full ridge vent and the requisite soffit venting on all areas of the house. How do we keep this from happening again, and what is the best way to fix the stains?

STAINED AGAIN

A. I’ve seen a lot of frost under sheathing but never stained walls from it. Are you sure you didn’t have ice dams? The cause of the frost is most likely a massive warm-air leak into your attic coupled with an extreme cold snap. The fact that there is frost buildup tells me the attic isn’t insulated as well as you think it is. The key here is to seal all air leaks, add insulation, and ventilate properly. I would have Mass Save evaluate this.

To repair the ceilings (assuming the plaster is intact), put on gloves and protective eyewear and clean the stains with a sponge and a mixture of one part bleach to one part water. After it dries, apply a coat of a stain blocker like Zinsser’s B-I-N. When that dries, apply paint that matches your ceiling.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.