Q: My home has a crawl space with insulation installed between the joists under the floor. This insulation was installed with the vapor barrier facing down instead of up toward the floor above. Would the work involved justify my turning the insulation over so the barrier is on top?

A: I think it would - there is too much chance of condensation forming above the barrier and wetting the insulation, which would greatly reduce its efficiency.

Q: Water comes into my basement through a few mortar joints a low in the cement block walls, and at times one of the window wells fills up from below. The soil to the basement seems to have settled, so surface drainage may be contributing to the problem. Can the leaks be fixed by sealing from inside, plus regrading outside?

A: You can try using one of the two-part epoxy waterproof coatings sold for sealing basements. These often work from the inside if cracks are not large, and if the water pressure on the outside is not too great. Regrading would definitely be advisable if the ground is lower near the basement walls than it is a few feet away. The soil should be graded so that it slopes away from the house on all sides. Also, check the gutters and downspouts to see if the roof runoff is being carried away from the foundation, either to an underground drywell at least 10 feet away, or to a surface several feet away from the house where it is carried away from the foundation.

Q: Last spring I had a new concrete driveway poured. As soon as the cold weather set in it cracked in various places. Is there any cement I can use to fill the cracks and do a bonding job that will prevent future cracking?

A: You can fill the cracks with vinyl concrete patching cement, but this will only fill them, it will not stop future cracking. The cracking is caused by a poorly prepared base or foundation under the slab, a poor concrete mixture, or improper troweling and mixing techniques. No patching material can make up for these deficiencies.

Q: One small section of aluminum flashing on the roof of my beach house corroded and I want to replace it with copper flashing. A contractor has advised me that copper and aluminum cannot be used on the roof. He says that if copper is used all the flashing must be replace with copper. Otherwise I must use aluminum to replace the damaged section. Is this true, or can the copper and aluminum be coated to prevent a reaction such as he describes - or would I be better off replacing with aluminum again?

A: There is some chance of a reaction when two dissimilar metals are brought and kept in contract with each other under moist conditions. However, you can prevent this by coating the surfaces where they overlap with a heavy coat of asphalt roofing cement, applying it to both pieces where they touch each other. Or you can replace with aluminum - it should last for quite a few more years if the rest of the flashing is still in good condition.

Q: The joints and corners in our aluminum gutter system keep leaking. We have tried several kinds of sealer, but so far without much success. What can we do to correct this problem?

A: I don't know what materials you have tried, but there are several products on the market that work well on leaking gutters.

One of the easiest to use is a type of self-adhesive black neoprene tape that come with or without an aluminum facing. You press it over the leaking joints to seal them, after first cleaning the surface thoroughly to insure a good bond. This is sold under various brand names in many hardware stores and lumberyards.However, it is only effective when applied on th e inside - not on the underside. This, is fact, may have been your problem with the sealers you tried; they should be applied on the inside. Others that can be used are silicone rubber caulking or ordinary roofing cement troweled on the inside and then covered with aluminum foil, which in turn is covered with another layer of the same cement.

Questions about home repair problems should be addressed to Home Improvement Department. The New York Times, 229 W. 43d St., New York, N.Y. 10036. Questions of general interest will be answered, but unpublished letters will not be answered individually.