How to spray Cat 5 air barrier on CMU

The top 5 things you need to know to get an airtight envelope with this common building material.


CMU (concrete masonry units), also called architectural block, is one of the most common building materials used in construction due to its versatility and durability. A sprayable, fluid-applied air barrier coating like R-Guard Cat 5 is a great option for weatherproofing CMU and creating an airtight, watertight building envelope, but there a few things to keep in mind for the unique characteristics of this building material, and some tips that can make your process go more smoothly. Here are the top 5 things you need to know.

Tip #1: The mixing step is important

This step of preparing the material for application seems simple, but there’s a wrong way and a right way. First, you want to use a ½-inch, low-speed drill with a Jiffy mixer. For this part, leave your mud paddle in the truck – it will introduce too much air into the product. For 3-5 minutes, move the paddle up and down and around the pail of Cat 5 to make sure you have evenly blended material. Now you’re ready to spray it on.

Tip #2: Pay attention to the pump

For spraying Cat 5, you need the right equipment. We recommend a direct-immersion pump that will provide 3500-4000 psi at the spray tip. The demonstration in the video uses a Graco 833 direct-immersion pump, but there are other acceptable pumps for this application that are outlined in our product data sheet. One equivalent is the PowrTwin 1200 di (direct immersion) pump. Either of those will work, but the important thing is that the pump gets you 3500-4000 psi at the tip.

Another important thing to note regarding the equipment is the filter needs to be removed from the pump to ensure a steady flow of the product.

When cleaning the pump, be sure to only use mineral spirits to flush the hoses and equipment before you put it away for the day. This is important because Cat 5 is a water-cured product, so water should not be used to flush the equipment.

Tip #3: Apply a thin coat first to save time

Because CMU is a highly porous material, it needs more than one application of a fluid-applied air barrier coating. Here’s the most efficient way to tackle this: Apply a light coat first, sometimes called a “kiss coat,” which causes the Cat 5 material to shrink the pores of the CMU and cure quickly on the surface. This better prepares the surface for a heavier second coat, which fills out the surface and gives you a smooth, pinhole-free wall.

This is a better method than what we see many times when people attempt to do it all in one coat. They add so much material that it takes longer to cure, and the weight of the Cat 5 material gets pulled into the CMU, and the installer has to come back out to the wall anyway to fill in pinholes. It defeats the purpose of trying to get it finished in one coat, it’s frustrating, and it costs more in labor and material.

Tip #4: Your application pattern matters - how to do a cross-hatch

In the video below, our Senior R&D Specialist Greg Reinke demonstrates how to apply Cat 5 in a cross-hatch pattern. For the first thin coat, Greg applies the Cat 5 in a light horizontal pattern from a little bit further away from the wall. This follows the preferred method of applying a thin first coat that will allow the product to cure quickly and begin to shrink the pores of the CMU.

Note the way Greg maintains what we call a “wet-on-wet” style of applying the product. He comes across with the first pass, and then the second pass partly covers the material from the last pass and partly covers new substrate. So it’s continually wet as you install the product.

For the second coating, he cross-hatches the pattern in a heavier vertical application, which helps provide that smooth, pinhole-free surface you need for total coverage.

Tip #5: Back-rolling: When you need it, when you don't

For the purpose of the demonstration in the video, we wanted to include a situation you might encounter in the field when you have a little bit too much material, and show you how to handle that. When too much material ends up on the substrate, you may have some orange peeling that starts to sag or run or create pinholes. When this happens, you will want to come back and back-roll the substrate with a roller. Get plenty of the product on your roller and then roll it over the substrate to fill in any pinholes and push around any orange peeling you may have on your job.

On CMU, you’ll get about 50 square feet of coverage per gallon – and that includes both coats. With OSB you’ll get that same coverage rate of around 50 square feet per gallon. For gypsum board, you’ll get more like 80-100 square feet covered per gallon, and in one pass or one application.

All of this information, including coverage rates, pump equipment specifications, cleaning instructions, and more, is included in our product data sheet. As always, you can contact us with any questions at 1-800-255-4255.