Q: We have a one-piece fiberglass shower that was installed against an exterior wall before the walls were insulated. So during our cold Minnesota winters, moist air condenses on the back of the siding and drips out through the clapboards. Is there any way to seal or insulate this area without removing the shower surround?

— Dennis Ofstedal, Eagan, Minn.

A: Tom Silva replies: That's a big problem, all right, one you need to fix as soon as the weather warms up. Otherwise, you'll have paint peeling off the siding, mold growing in the wall cavities, and rot eating away your siding and framing, if it isn't already. To find out, you'll have to peel away some siding and sheathing in the affected area and take a look.

If the insides of your walls are still in good shape, hire an insulation contractor to pump a medium-density foam into each stud bay behind the shower surround. They can do this from the outside through holes in the sheathing at the top and middle of the stud bays. Patching these spots will be easier than moving the shower surround. This insulation stops virtually all air movement, so there's hardly any chance of water vapor condensing on wood. You might even notice that you feel less chilly when you take a shower.

If you find that the bays are filled with rot, however, you'll be better off removing the surround and fixing these problems from the inside. Replace any rotted wood, spray the remaining wood with a borate solution to prevent fungi from coming back, and have an insulation contractor fill the bays with foam. Cover it with a water-resistant drywall before you reinstall your surround. Kinda makes you wish they hadn't been so hasty installing that surround in the first place, doesn't it?

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