RDH first discovered a blistering phenomenon in waterproofing membranes while performing warranty reviews and condition assessments. The hidden nature of an inverted roof assembly often causes the issue to go unnoticed for some time until more visible problems occur. In the past, RDH has observed blisters so large they displaced concrete pavers, creating hazardous walking conditions. Leaks can also develop when a blister expands over cracks and joints in the concrete structure. In some cases, the blistering was so severe it was necessary to prematurely replace roof assemblies.    

The cause of these blisters had the building industry confused, producing many theories such as inadequate membrane thickness, outward vapor drive or poor workmanship. After disproving many of these supposed causes, RDH set out to research and determine the physical building science mechanism responsible. This research has included a series of field studies, building monitoring projects, and several laboratory experiments. Ultimately, the team concluded that the underlying mechanism was osmosis.  

Osmosis describes the naturally occurring movement of a solvent, such as water, flowing through a semi-permeable membrane. The solvent travels from a solution with a lower solute concentration to a solution with a higher solute concentration, until equilibrium is reached. In the case of osmotic blisters, the solute may be a salt mixture that entered the substrate during construction. Osmosis does not commonly affect building enclosure assemblies, so discovering it was the process responsible was unexpected. 

As no standardized testing procedure existed to measure osmotic flow across a membrane, RDH developed an experiment to demonstrate this phenomenon. We also conducted further research to establish thresholds for vapor permeance, water absorption, and osmotic flow rates, to be used to predict the risk level for membrane failure. Testing included various types of waterproofing membranes to determine how susceptible they were to osmotic blistering, as well as increasing our understanding of the effects of aging and water absorption on membrane performance.  

Pairing the testing procedure our team developed with existing ASTM guidelines has enabled us to measure the osmotic flow rate of a membrane, a service unavailable elsewhere in the industry. We established a threshold for membrane vapor permeance, long-term water absorption, and osmotic flow, to quantify whether a particular membrane is low risk for blister formation. Several peer-reviewed scientific research papers have been published on the study and findings to date. This work has also been published in the Flat Roof Construction Manual.   

Read more about this research:  


March 5, 2021



Year Completed:


Key Services


Research at RDH Building Science Laboratories (RDH-BSL) is wide-ranging—from R&D projects that help manufacturers with product development to major research programs that help government and industry partners answer fundamental questions or address large-scale issues. Each project is approached as a unique challenge, requiring a tailored solution.

We undertake these projects in our two laboratory facilities and at field research sites in multiple climate zones across North America. Our staff includes highly trained and experienced researchers and technicians who understand the behaviour of assemblies, components, and materials, as well as the construction and design-related issues that impact performance.

In addition to laboratory and field research, computer-based modeling is an indispensable tool. We analyze and extend measured results using various modeling programs, including two- and three-dimensional heat flow programs (THERM, HEAT 3D), hygrothermal modeling software (WUFI Pro and WUFI Plus), and other in-house proprietary software. Many projects include several phases to allow validation of modeled results with measured data.

Graham Finch | Dipl.T., M.A.Sc., P.Eng.

Principal, Senior Building Science Specialist
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Brian Hubbs | P.Eng., BEP

Principal, Senior Building Science Specialist
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