Facts About Moisture and Fiberglass Metal Building Insulation (cont)
What are some installation guidelines to protect against condensation?
Controlling the condensation in a metal building depends on several design, installation and maintenance factors.
1. The insulation must have vapor retarder on the “warm-inwinter” side for the more severe operating conditions. For most buildings, this is the inside surface for protection in the winter months when the inside ambient air temperature is higher than the outside ambient air temperature. This is a design parameter confirmed during installation.
2. The insulation must be thick enough to maintain the facing surface temperature above the inside ambient air dew point temperature to prevent condensation. This is a design parameter. System thickness for condensation control may be different from the thickness for energy conservation.
3. Seams, joints, damage or punctures in the vapor retarder and penetrations should be sealed to retard the passage of water vapor around the insulation.This is primarily an installation requirement for good workmanship.
4. Ventilation to the outdoors on the cold side of the insulation will help remove moisture that gets past the facing.This is a design consideration.
How important is the perm rating of the vapor retarder in controlling condensation?
The rate at which moisture or water vapor passes through a flat material induced by unit vapor pressure difference is defined as water vapor permeance. One perm is defined as 1.0 grain of water vapor transmitted per hour per square foot per 1.0 inches of mercury vapor pressure difference.The lower the perm rating, the more effective the retarder in controlling moisture vapor transmission. When choosing a vapor retarder, remember that the lower the perm rating means the less water vapor that can pass through the material. Materials with a perm rating of 1.0 perms or less are considered to be vapor retarders. Materials rated at .02 perms are recommended where high humidity interior conditions exist.Materials whose perm ratings are greater than 1.0 are not considered to be vapor retarders.General perm ratings for the facings most commonly used on metal building insulation are shown in the table above.
References
1. Fungal Contamination of HVAC Surfaces: The Role of the Dew Point in Microbial Amplification, Jerry J.Tulis, and Wayne R. Thomann; Occupational and Environmental Medicine Division, Duke University Medical Center, Durham,N.C., 1997
2. Ibid.
