Osmosis and the Blistering of Polyurethane Waterproofing Membranes

Cold-applied, asphalt-modified elastomeric polyurethane waterproofing membranes have been popular for roofing and waterproofing applications for at least the past 15 years. Their relatively low cost and easy application resulted in their widespread use in inverted roof membrane assemblies applied to concrete decks. However, water-filled blisters under these membranes have been discovered on numerous buildings in the Pacific Northwest in recent years. In several cases, the blisters were so large that replacement of the membrane was required. Water leakage into the interior has resulted when the blisters expand to a crack or joint in the concrete slab.

The local building science and roofing industries are aware of the problem but they lack a complete understanding of causal effect or of the physics of moisture transfer. Water vapor diffusion and capillary flow do not adequately explain the pressures or volumes of water contained within these discrete water blisters. However, moisture transfer via osmosis can result in blisters under significant pressure and explains the observed conditions. Results from a series of laboratory experiments demonstrate that the required conditions for osmosis to occur exist in the field in these roofing assemblies. Osmotic-flow rates measured through several of the membranes using a controlled laboratory apparatus are discussed.

This paper was presented at the 2009 RCI Building Envelope Symposium.