Moisture Content in Protected Membrane Roof Insulations—Effect of Design Features

In protected membrane roofs, the thermal insulation is placed above the main waterproof membrane where it is exposed to precipitation. The severity of the wetting problem can be reduced by protective measures.

Fibrous and closed cell plastic insulations, ranging from 150 to 610 mm square and 51 mm thick, were placed on experimental roofs. They were weighed periodically over periods of up to five years.

The results indicate that moisture gains are reduced by increasing deck slope. Sealing the bottom surface significantly reduced moisture gain in a bead polystyrene, but improvement was offset by placing paving stones directly on its upper surface. Slotting the lower insulation surface on a 100-mm-square grid was not found to reduce moisture uptake. Moisture content of some fibrous specimens sealed on the bottom surface and edges remained at less than 4 percent by volume. Others similarly sealed, but with paving stone cover flashed to divert water away from the upper surface, remained at less than 1.5 percent. In most cases, when specimens were placed wet on the roof in winter, significant drying occurred over a period of several months.