Waterproofing

Author: Sheryll Poe | January 9, 2025

Even with the most cautious application, bubbles and blisters, including osmotic blistering, can happen during the waterproofing process.

While bubbles and blisters are unsightly, they can also compromise the integrity of the waterproof coating or seal. Professional contractors need to know what causes these unsightly blemishes and how to prevent them before coating application.

What Causes Blisters?

Blisters, or raised bubble-like areas, occur when moisture and/or gas accumulates under a non-permeable waterproof coating or film and expands, creating pressure beneath the membrane. There are two types of blistering: non-osmotic and osmotic.

Non-Osmotic Blistering

Non-osmotic bubbles can be caused by the condition of the substrate the waterproofing is applied to. For example, applying a moisture-sensitive coating on a damp surface can result in blisters or bubbles as the coating material reacts with the moisture creating bubbles or blisters in the film. Another example is when a coating is applied over a porous concrete surface that is exhibiting "outgassing" due to fluctuating temperatures. This can result in air pockets forming as this moving air is sealed by the applied coating.

Similarly, applying a waterproof coating too thick can result in issues. When many coatings are installed too thick, they can run the risk of blisters and microbubbles. One-component materials that cure from the top down can skin over and prevent the materials below from curing properly, trapping solvents and gases within the film, resulting in blisters and microbubbling. Two-component materials can also exhibit issues including microbubbling as well as material that is expanded during curing, resulting in a soft film with lessoned physical properties that can lead to early wear.

Osmotic Blistering

Osmotic blistering is more of a chemical phenomenon that occasionally occurs in cold-applied polyurethane waterproofing membranes.

In biology, osmosis is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell's partially permeable membrane.

Similarly, in waterproof coatings or membranes, the moisture on one side of the semi-permeable coating (say, moisture within a concrete substrate) tries to move through to the other side of the coating, which builds pressure and creates osmotic bubbling.

It's important to note that not all waterproofing coatings or membranes are at risk, and the issue of osmotic blistering (as well as solutions for the phenomenon) is being studied extensively. For example, RDH Building Science has even developed a proprietary osmotic testing program to determine what types of waterproofing materials are likely to develop blisters.

How to Prevent Non-Osmotic Blistering

Preventing these types of bubbles and blisters is typically related to application. Ensuring the substrate the waterproof coating will be installed on is suitably dry is one of the best ways of preventing these issues. Manufacturer data guides will typically contain information on moisture levels and testing for the coating under their surface preparation recommendations.

When installing materials, it is also important to take into account ambient conditions and surface temperatures. A concrete surface can seem dry to the touch in the early morning hours, but may actually contain surface moisture that can cause issues in the coating. An industry-accepted practice is to ensure the substrate surface temperature is at least 5°F above the dew point to prevent moisture from condensing on the substrate surface.

A concrete deck exposed to early morning sun is heating up. As this occurs, outgassing can cause blisters, bubbles or pinholes in certain coatings. In these cases, it may be most prudent to install the coating later in the day. Again, consulting the product data guide for acceptable tests that can measure this surface moisture may prevent blistering issues.

If surfaces are visibly wet or if there's a chance of precipitation, the application of a waterproofing system should be rescheduled for a better weather window.

How to Prevent Osmotic Blistering

While osmotic blistering is still being understood, there are several common recommendations on how contractors can counteract osmosis of waterproofing systems.

The first step is to conduct moisture testing. For example, with the use of a moisture meter, a contractor can identify potential areas of surface moisture in the substrate and make adjustments relative to the surface conditions. Another industry-accepted method is ASTM D4263, "Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method." Also referred to as the plastic sheet test, it involves laying a plastic sheet over the prepared surface for at least 16 hours, then checking the underside of the sheet for visible signs of moisture.

Other tests that can help determine if moisture within a concrete surface may lead to this type of blistering are ASTM F1869 "Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride" also known as the calcium chloride test, and ASTM F2170, "Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes" also referred to as the relative humidity test. These tests, while more involved than the plastic sheet method, can determine the actual moisture levels deeper in the substrate which could lead to osmotic-type blistering. It is important to consult with the manufacturer of the coating to determine which moisture test is recommended for a particular project, and what is an acceptable level of moisture to proceed with application.

Moisture prevention should always be at the forefront of any contractor's efforts. A lot of blistering prevention comes down to ensuring the proper adhesion of a waterproof membrane or coating. The key to this is proper surface preparation. Follow all manufacturers' recommendations for surface profiles, bond tests and moisture testing to ensure these issues will not be part of your project.

Waterproofing system manufacturers can also recommend certain ancillary products, such as moisture-tolerant primers, that can ensure good adhesion.

Preventing Blistering with Proactive Planning

By taking these preventive steps, a contractor can add longevity to a waterproof coating or membrane. In the meantime, waterproofing system manufacturers are continuing to innovate and seek out moisture-prevention materials and products to help mitigate these issues.