Q: I would like to put wallpaper on the walls of our bedroom. This room is on the north side of the house, and the walls get moldy in the winter. Can you tell me what could be causing this, and what I should put on the walls before the paper is hung?

A: The wall is apparently a cold one and may lack sufficient insulation. I suspect that it also does not have a vapor barrier (if it has insulation). The combination is causing mildew to form. The first thing to do is kill the mildew by washing it with a solution of fresh liquid laundry bleach and water (one part bleach to three parts water), scrub it on, let it dry, then rinse thoroughly. Next, if there is no vapor barrier in the wall, apply a coat of special vapor barrier paint such as the one recently introduced by Glidden.

After this, wait a week or two to see if the mildew returns. If it does, chances are you have a moisture problem inside that wall and the mildew is growing from the inside. It may be that there are leaks on the outside that are letting moisture enter, or it could be a condensation problem that requires venting the exterior siding. Either way, until you cure the dampness problem you can never permanently solve the mildew problem.

When you buy the wall covering, make sure it is mildew resistant, and use mildew-resistant paste to hang it.

Q: We have a number of pieces of furniture that were originally finished in natural oiled walnut or teak (some of each). Over the years the finish on these pieces has been marred by wet glasses and other unknown causes. I have tried scrubbing with fine steel wool dipped into some of the oil normally used on this furniture, but to no avail. The funiture has often been sprayed with various polishes. Is there any way I can get rid of these marks without complete refinishing?

A: As a rule, a true Danish-type finish is done with tung oil or similar penetrating sealer, and this is easily maintained by applying the same sealer or oil when necessary. The stains you describe should have come out with the system you used. Since this did not remove the colored marks, it is possible the problem is caused by the many layers of polished and sprays used on top of the original sealer.

The marks may be in these films, which do not respond to treatment with the oil. This means you have to get all the old wax or polish off to get back to the original oil or sealer underneath. In some cases you can do this by rubbing with a coarser (medium) grade of steel wool until the white marks have disappeared, or you may have to dip the steel wool into paint thinner to get all the old polish and wax off. When the stains are gone, use the oil and fine steel wool again.

Q: This fall I had several aluminum replacement windows installed with double glazing. I am now getting heavy condensation on the lower half of the frames. I am planning to install more of these windows next year. Can you tell me how this condition can be avoided next time?

A: It sounds to me as though the metal frames you had installed do not have a thermal barrier -- that is, they do not have a break in the frame (usually a plastic insert) that keeps heat from being conducted from one side to the other. Make sure your new metal windows have such a thermal barrier.

Q: I live in a 40-year-old cooperative apartment and many times the water temperature changes drastically and gets scalding hot when I am taking a shower. This is directly related to flushing toilets or running cold water in other apartments. Is there anything I can do to prevent this potentially dangerous sudden change in the water temperature?

A: The only way you can correct this, short of having new pipes installed in the entire building, is to have a plumber install a special type of mixing valve that maintains water temperature automatically. Your local plumbing contractor should be able to tell you where to find one and quote a price for installing it.