Moisture Control in Building Wall Retrofit

Abstract

For the Residential Conservation Service Program (RCS), the Department of Energy (DOE) in 1979 promulgated standard practices for the installation of various energy conservation retrofit measures, including insulation. Various standard provisions and recommendations exist for moisture control in insulated walls of new constructions. These provisions in general require a vapor retarder on the warm side of walls and ceilings in all but the warmest climate areas. To apply these existing requirements to retrofit applications of insulation would be burdensome and costly. Since field experiences did not indicate conclusively that moisture problems in insulated walls can be prevented by the application of vapor retarders, nor that the absence of vapor retarders necessarily leads to moisture damage in insulated walls, DOE, with the assistance of the National Bureau of Standards (NBS) and outside consultants, developed new standard provisions specifically designed for retrofit wall installations. For roofs, the vapor retarder and attic ventilation requirements contained in the Housing and Urban Development (HUD) Minimum Property Standards (MPS) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Guidelines were adopted unchanged. This paper thus deals with exterior wall retrofit. The provisions were developed based on the premise that the relative humidity of the interior air is a good indicator of the potential for moisture condensation in walls. The RCS provisions stipulate a vapor retarder only in high-moisture rooms of houses in ASHRAE condensation Zone 1, but such rooms also require the sealing of cracks on the interior face of exterior walls to prevent moist indoor air from penetrating into the wall cavity. In addition, the standards recommend the same provisions in Zone 2. Finally, the provisions identify houses by size, occupancy rate, and other parameters for which additional precautions may be justified.