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Insulate walls to keep paint from peeling

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Q. I have a big problem with the exterior walls of my bedroom in a brick house that is uninsulated. The paint on those walls is cracking and you can scrape it right off. Where the walls are papered, the paper keeps coming off. How can I make the paper and the paint stick properly? There is no problem with paint or paper on the interior walls of the room. No other room in the house is affected.

- Nina Green, Newton, Mass.

A. First, let's define our terms: Exterior walls means walls of the room with the great outdoors on the other side of them. Interior walls means walls of the room with another part of the house on the other side of them. The problem is on the inside of the exterior walls; the side facing the room.

That done, now for the explanation: Those exterior walls are cold because the house is not insulated, and water vapor condenses on them more readily than the warmer, interior walls. It is this condensing water vapor (water) that is causing the peeling of both paint and paper. Sometimes the moisture gets behind the paint and paper, pushing everything right off. That is obvious. What is not obvious is why other exterior walls in the house don't have similar problems. They may be warmer for some reason, but since they do not have the problem, they don't need fixing.

So, one cure is to insulate the house walls. Since the house is brick, the insulation should be installed from the inside; holes are drilled in the wall and the insulation blown to the wall cavity through those holes. This will keep the wall warmer, and water vapor is less likely to condense on it. Those holes must be patched and plastered over after insulation is installed, which is a pain but necessary.

Then, scrape and sand off loose paint and repaint with two thin coats of a latex wall paint. As for papering, put it up with a vinyl paste, which is more resistant to water than any other kind of paste.

Q. I am buying a house with 2-by-3 studs against the concrete foundation, with friction-fit insulation between the studs. I know there should be a plastic vapor barrier on the wall, but where should it go: in front of or in back of the insulation?

- Chris Clancy, Natick, Mass.

A. Any vapor barrier - plastic, paper or foil - always goes on the wall facing the basement, or in the words of insulators, facing the heated part of the room. Put the plastic on the face of the studs, so that when it is installed and you are in the room looking at the wall, you will see the plastic. Cover the plastic with plasterboard or any other wall finish such as paneling, and you will be done with that wall.

The 2-by-3s instead of 2-by-4s are OK because a room below grade does not need as much insulation as a wall above ground.

Q. I notice your warning against using wood filler on outside trim. I presume it means that the filler might melt outdoors. I filled splits in a rake board (exterior trim) with a wood filler that I think is interior grade. What can I do?

- Joe Joyce, Arlington, Mass.

A. Apply a coat of paint or stain and hope for the best. Properly covered, the interior filler will be protected for as long as the paint and/or stain stands up. As for future filling of holes, use an exterior wood filler.

Q. I have peculiar stains on the concrete apron around my indoor pool. They could be algae or a fungus of some kind, but they are brownish cream and about the size of a quarter. I tried full-strength bleach and Triple Action20, which did no good. How can I remove the stains, and with luck, keep them from coming back?

- Bill Jablonski, Braintree, Mass.

A. They may be stains from leaves and other organic matter left on the concrete for too long a time. But why they are quarter-size is a stumper. At any rate, try this: Sand the stains with coarse sandpaper and, if they come off, seal all the concrete with a masonry sealer or a semitranparent stain.