Adding vapor barrier to existing home

Q. I am slowly making my home more energy efficient. Among other things, I am about to install new red cedar shingle exterior siding. The problem of a vapor barrier has me stumped.

Is there a product or material to go on well-preserved interior plastered walls that can serve as a vapor barrier and accept wallpaper or paint?

Robert Bruce, Essex, Mass.

A. Your concern appears to be an interior room moisture barrier rather than anything associated with outer wall construction under cedar shingles.

The National Forest Products Association, 1619 Massachusetts Avenue, NW, Washington, D.C. 20036, Telephone (202) 797-5800, printed a booklet called ''Improved Paint Performance Through Sound Construction Practices.'' Says the booklet, ''In homes constructed without vapor barriers, good vapor protection can be achieved by applying to inside walls and ceilings two coats of a low-permeability paint system recommended for this purpose by the manufacturer. However, such measures do not provide complete protection; paints of this type are intended to supplement the action of vapor barriers in new homes.''

Apply the new wood shingles to the outside walls in accordance with manufacturer's specifications. The existing walls may already have a vapor barrier whether under the exterior finish or in the wall insulation. For a quality recommendation as to specifications to apply wood shingles to existing exterior walls, telephone Red Cedar Shingle and Handsplit Shake Bureau at (206) 453-1323, Bellevue, Wash.