Insulation against dampness from a crawl space; what to do when your flat roof leaks

Q Would concrete blocks provide effective insulation on the outside cedar-shingled walls of a room? If not, what would do the job? We have a dampness problem there because of the earth crawl space beneath.

Irma Entemann Painesville, Ohio

Adding a concrete-block wall over rigid insulation and a vapor barrier over the shingles would provide additional thermal insulation. However, the dampness problem appears not to be wall-generated but comes from below, as you say.

I'd place a polyethylene vapor barrier over the earth in the crawl space. Overlap, tape, and seal the joints. Run the vapor barrier up the foundation walls for 6 or more inches and then seal the polyethylene to the foundation. Install batt insulation between the floor joists in a thickness that is equal to the vertical dimension of the joists. Do the same thing in the ceiling area, running the vapor barrier down from the top.

Thermal insulation and the crawl space vapor barrier will economically and satisfactorily solve both the insulation and dampness problems.

Q The flat roof on my apartment leaks. If the temperature has any bearing on the longevity of the roof, the thermometer here can drop as low as 35 degrees below zero F. It seems to me there should be something better than constantly putting tar on the roof. What is your advice?

Colson E. Carr Alexandria Bay, N.Y.

Conventional roofing membranes are subject to expansion and contraction when exposed to extreme temperatures. Such movement induces some high stresses, which can lead to leaks.

''Ideally on a dead-flat roof, the first thing to do is to create a slope of at least one-quarter inch for every 12 inches,'' says W. H. Allen Sheldon, Manville's district engineer in Denver. ''This would promote shedding and avoid the ponding of water.

''The most pertinent advice we can offer your reader,'' Mr. Sheldon continues , ''is for him to contact a qualified commercial (not residential) roofer in his area. A reputable roofer having the benefit of local experience is certainly in the best position to offer good advice on how to eliminate leaks, especially after a visual inspection.''

Mr. Sheldon reports on two recently developed alternatives to conventional built-up roofing membranes. Both provide greater flexibility or elasticity. The first is Manville's Dyna-Kap in roll form. The second is a single membrane called EPDM (ethylene-propylene diene monomer) and is said to maintain its elasticity even in a temperature as low as minus 60 degrees F.