Q: The last couple of times that I've been in my attic when the temperature is really cold, I've noticed ice on the nail tips sticking through the roof sheathing. Do you know why? Is there anything I should do about it?
—Stasia, Wood-Ridge, N.J.
A: Tom Silva replies: What's happening is that warm, moist air from inside your house is seeping into the attic and the moisture is condensing and freezing on the cold steel. And yes, you should do something to stop the seepage because it can reduce the effectiveness of your insulation and encourage mold growth.
There are three steps to solving this problem. First, seal off any openings for air to leak into the attic. Maybe a bathroom fan isn't venting outside or the pull-down attic stairway needs weatherstripping or the light-fixture boxes aren't sealed. A home-energy specialist with a thermographic imaging device can pinpoint many small leaks you can't see.
Second, try reducing moisture levels in the house by turning on exhaust fans when bathing and vent fans when cooking.
Third, make sure the attic is well ventilated so that any moist air that gets past your defenses has a way to escape. Soffit and ridge vents are more effective ventilators than fans, and they don't use any energy. If you already have these vents, make sure they aren't being blocked by insulation.