A.Wood technologist Paul Fisette responds: Many builders and material suppliers think that using housewrap under vinyl siding is a waste of money. I disagree. Housewrap is cheap insurance, for two reasons: It protects the building shell from the weather and it helps save energy.

Vinyl siding is a good siding material, but it is loose fitting and somewhat permeable to weather. Wind-driven rain can penetrate at overlapping ends, around windows at J-channels, at corner posts, at the intersections between siding and rake trim, and any other places where joints are involved. A smart installer should expect water to penetrate the siding at times; the housewrap provides a backup weather barrier that allows water to run down and out without penetrating the structure. Proper flashings should be used at doors, windows, and other penetrations, and the housewrap should overlap the tops of these flashings.

The second issue relates to energy performance. Studies have shown that air leakage is an important factor in heating and cooling losses; installing taped housewrap directly over wall sheathing is a good way to greatly reduce air leakage. If your budget allows, a layer of taped foam insulation, instead of housewrap, will not only reduce air leakage but also improve the conductive resistance of the wall.