Q. I am redoing the kitchen in my 40-year-old house and ran into a situation that worries me. When I took the drywall off one exterior wall, the fiberglass insulation was damp and the points of nails holding the siding on were black from moisture. The back of the plywood sheathing was wet, too. I saw no source of water, so what is going on? Is it fixable? There is no moisture in the attic.

A. Whew. It is an old story, oft repeated, but worth repeating because it is so widespread, and often baffling. The Handyman has told this sad tale (and its cure) until he is blue in the face, but it is still worth repeating: water vapor, created by people breathing, cooking, bathing (especially showers), and washing, builds up in the air until it essentially saturates the air, then pushes its way through the drywall, through the staple holes in the vapor barrier, through the fiberglass (or other type) insulation, then hits the cool sheathing, and, presto, condenses into water. Ergo, wet sheathing, corroding nail points, and damp insulation, which is unlikely ever to dry out completely. The biggest symptom of this malady is peeling paint.

Old houses, which are very leaky of air, do not have the problem because of their leakiness; water vapor can easily escape without condensing on anything.

And the cure: Of course you could eliminate or greatly reduce the moisture in the house (not practical), or you can set up a ventilation system to release that water vapor. The best way to do that is with an air-to-air heat exchanger, which releases warm, humid air and transfers its heat to incoming air. Heating and air- conditioning companies carry these exchangers.

Another cure: Take off outside siding and install a three-eighth- inch-thick Styrofoam insulation, with a layer of Tyvek, and reinstall the siding or put up new siding. The kind of siding does not matter. The insulation will keep the sheathing warm and water vapor can fill the space, but will not condense. Simple, really.

In this case, only one wall may have the problem, and problems in other houses may be restricted to north-facing walls. So, unless you suspect other walls are wet, and/or there is a musty smell indoors, you may not have to check the other walls. The lack of moisture in Brett's attic is due to its being adequately ventilated.

Q. I had my fireplace taken down to the base and entirely rebuilt, complete with firebox and liner. I waited one or two months before building a fire, and there have been numerous fires since the first. There was a bit of water in the fireplace when I first set a fire, and it burned with a good loud roar. Recently I discovered a crack over the firebox, going through the mortar as well as straight through several bricks. It happened on the outside as well. At one point I thought I heard the steel lintel over the firebox buckling. The mason who built the fireplace said I burned fires too soon after it was built. What went wrong and how can it be fixed to make it safe?

A. You did not burn a fire too soon if it was one or two months after the fireplace was built, because mortar and other masonry products cure in 28 days. But ordinary mortar is susceptible to heat, and your fires were pretty hot, roaring and all that. Heat will cause cracks as the bricks expand and contract. The fact that the bricks cracked indicates a good job by the mason. I don't think cracks are that bad. You can have a chimney sweep check the flue liner, and if it is intact, you should be able to burn wood in the fireplace. Of course use cured hardwood and not softwoods such as pine and fir. And keep the wood to a minimum; that is, small amounts of wood make smaller fires. Another thing you can try is to reduce the draft (and the roaring) by closing the damper a bit. With the damper closed down a little, the fire will be smaller but should not smoke.

Your idea of digging out the cracked brick and mortaring in new bricks, both inside and out, is a good one. Another good idea is to have the fire department of your town inspect the fireplace.

I think I have covered everything. If anyone out there has any other ideas, Ken from Hingham and The Handyman would dearly love to hear from you.

* Write Peter Hotton at the Boston Globe, Boston, Mass. 02107, or e-mail him at photton@globe.com. Hotton also chats on line about house matters 2 to 3 p.m. Thursdays. To participate, go to Boston.com.

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