Some Factors Affecting Drainage of Moisture from Wet Insulation in Flat Roofs

Abstract

Moisture often finds its way into flat roofs where it causes a reduction in the thermal resistance of the insulations and sometimes leads to the destruction of the roofing system. Various methods for removing moisture have been studied, but effective drying is usually difficult to achieve because the natural forces acting to remove moisture are small. Some moisture may be drained from wet roofs, but because the slope is small or nonexistent and absorptive forces tend to retain moisture it is not usually an effective method. If the retentive forces can be overcome, however, drainage rates can be substantially increased. In one outdoor study carried out on an experimental roof deck with 2 percent slope glass fiber, perlite-fiber, and wood fiber insulations were found to drain very slowly. Of the moisture contained in the insulation at the outset less than 25 percent drained out in the first 4 months of test. In a second, laboratory study tests were carried out on a deck 2.4 m long with slopes of 2, 4, and 8 percent using a variety of underlays beneath wet glass fiber insulation. Drainage was slow when a plastic sheet underlay was used, but rates were higher with soil and vermiculite-asphalt underlays. The fourth underlay, polyester fabric, produced drainage rates that varied with deck, slope, and length of flap allowed to hang down at the end of the deck. This flap provides a suction force that increases the rate of moisture flow. As an example, moisture content was reduced from over 70 percent to less than 5 percent by volume in a period of 4 days with a slope of 2 percent and a 75 mm flap.