Q: My house is on a concrete foundation and has six air vents in the crawl space, three on each side. The crawl space has a sand bottom. The builder told us these vents should be left open all year round, even in cold weather, and that no insulation should be used under the floor. Salesmen have advised us that adding insulation under the floor is a must if we want to conserve fuel. Can you give us your view?

A: This is one time the salesmen are right. The vents in the crawl space should be left open all year, and there should definitely be insulation applied under the floor, with a vapor barrier on top (up against the under side of the flooring). I also strongly advise covering the sand floor in your crawl space with either heavy roofing felt or heavyweight polyethylene sheets. Overlap them at least 12 inches at each seam and weight them down with rocks or bricks.

Q: We recently had a painter do some work for us and I assume he used an oil paint over a latex paint. It is now peeling down to the bare plaster. He claims no responsibility for this. Can you tell us how to correct the situation?

A: I'm afraid your painter is right. The peeling is not his fault. Since it is peeling to the bare plaster, it is the old paint that is letting go, not the coat he applied. There is apparently something underneath that is not holding, and as it comes off, the top coats come with it. I recommend sending some sizable chips to the paint manufacturer, because this kind of peeling can be caused by many things and cannot be diagnosed through the mail. It is possible that you may have a leak, or that all the paint will have to be removed before new coats will adhere.

Q: My baseboard heating units (I believe they are made of aluminium) are starting to rust. I have tried many abrasive cleaners but nothing helps. Also, some paint fell on these baseboard units and paint thinner or turpentine won't get it off. I would like to clean them then paint them. What should I use?

A: I doubt if the units are aluminium. Most baseboard radiators aren't, and aluminium doesn't rust. Chances are they are steel if rust is showing. If so, the rust can be removed by using a chemical rust remover you can buy in any hardware store. If they are copper the oxidation need not be removed before painting. To remove paint spots, a paint remover - not a paint thinner - will do the trick. I suspect it is only the radiator cover you are concerned with. If so, you can paint it with any conventional alkyd base flat or enamel after you clean off the rust and apply a metal primer for the first coat.

Questions about home repair problems should be addressed to Home Improvement Department, The New York Times, 229 West 43d St., New York, N.Y. 10036. Only questions of general interest will be answered in this column.