There is a large misconception in the waterproofing industry that all cold fluid-applied waterproofing membranes are susceptible to osmotic blistering. But this is not the case. Over the years, costly waterproofing repairs have led to a lack of confidence in using cold fluid-applied waterproofing membranes in damp, wet, and immersion environments. Understanding what osmotic blistering is, knowing why it occurs, and choosing the correct product for each job will help minimize the potential for osmotic blisters.

What is Osmotic Blistering

Osmotic blistering occurs in coatings or membranes when moisture on one side of the coating tries to move to the other side of the coating, which builds pressure. Eventually, the pressure built on the coating can lead to blisters, holes, bubbles, peeling, flaking, cracking and even complete delamination of the coating! This can happen on any surface a coating is applied (wood, asphalt, steel, etc). However, concrete is the biggest offender of osmotic blistering. Concrete is a porous substrate and will always have moisture present at varying levels due to the environment, the thickness of the slab, and other conditions.

Related Article: What Waterproofing Membrane is Right for You?

What Are the Causes of Osmotic Blistering

There has been a substantial effort to identify what physical properties lead to osmotic blistering. For example, RDH Building Sciences developed its own proprietary osmotic testing program to determine what types of waterproofing materials are likely to develop osmotic blisters. Performance properties including permeability and water absorption are used to predict field performance and the likelihood of osmotic blistering in a waterproofing membrane.

While permeability and absorption values can play a role in the osmotic blistering process, another measurable property plays a critical role in predicting waterproofing performance yet is often overlooked: Adhesion.

Let’s explain why!

How to Prevent Osmotic Blistering: Adhesion

The adhesive bond, or the adhesive strength of a coating, measures the force required to remove a coating from the substrate or surface to which it has been applied. The stronger the adhesive bond the coating has to the surface, the better the coating holds up against the common symptoms of blistering: premature coating failure, delamination, peeling, etc. Waterproofing membranes with adhesion values (90° peel) perform the best in preventing osmotic blistering.

Surface preparation is critical to achieving long-term adhesion, as with all coating applications. Primers can also be used to increase the bond strength for some applications.

Related Article: Concrete Surface Preparation: Preventing Moisture Contamination

While there have been some failures for cold fluid-applied membranes with osmotic blistering, it’s ill-advised to discount all products without first understanding the failure factors.

The likelihood of osmotic blistering correlates to permeability and absorption values; however, we mustn’t forget that proper adhesion to our substrates is an essential factor in preventing osmotic blistering in waterproofing membranes.