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September 24, 2003 9:01 AM CDT

Cavity Wall Issue


Cavity walls are often regarded as the premier masonry wall system. Long common in Europe, they were first built in the United States as early as 1850. Although the first building code acceptance of this form of construction was not accepted until 1937, interest in and use of cavity wall systems has increased rapidly. They primarily help resist rain penetration, fire, sound transmission and extreme thermal capabilities. Extensive research and testing has been conducted to determine properties and measure performance.

There have been a number of issues following masonry contractors for quite some time. Cavity wall contractors have been trying to decide which form of insulation to use in their structures. Awareness of the different insulation benefits seems to hold many contractors back from the most efficient answer. There are those contractors who continue to use the same form of insulation because of loyalty, lack of knowledge and preference. Whatever the case, here is the truth about what goes on between those walls.

Several main variables must be addressed before masonry contractors can achieve a successfully insulated wall system. The first one is water within the cavity. There are two forms of insulation to solve this problem. Contractors can use the traditional extruded expanded polystyrene or use a polyisocyanurate insulation to line the inside of the cavity. The two are completely opposite, from insulation thickness to true R-value.

Although almost 90 percent of the business is polystyrene, that number is drastically changing. Several insulation manufacturers, including Dow and Owens Corning, have long cornered the masonry contractor for this type of structure. The question is, does the end consumer really benefit from this type of insulation? Depends upon what you base your decision.

Polystyrene insulations typically get an R-10 insulation value for a 2-inch thickness. Inside a typical concrete masonry cavity wall, there is an 8-inch block. Outside, there is a 3-inch face brick, which leaves roughly 2 3/4 to 3 inches of air space. The type of insulation that lines the air space can make all the difference in the world. In fact, different types of insulation can dramatically alter the R-value for the wall system.

For example, if a contractor fills the cavity with two inches of polystyrene insulation, that leaves just one inch of air space. When the exterior brick or masonry is laid, it fills the cavity full of mortar. When rain is driven against the wall, water fills the cavity and has no place to escape or drain.

Mason contractors quickly realized polystyrene was keeping the insulation value at a minimum. "When you put a two-inch piece of polystyrene insulation inside the cavity, there isn't enough room for the water to exit the system," said Russell Kenny, President of R.J. Kenney Associates, Inc. "The best way to stop the water from penetrating the wall is to have a good two-inch cavity of space."

An additional and costly option is to waterproof the substrate before placing the insulation into the cavity. This will allow far less water to penetrate the wall, however costs more because of the waterproofing process.

"We also don't like to use expanded polystyrene boards," added Kenney. "They really have such a high absorption to them that it keeps the water in and loses all its insulation R-value."

Polyisocyanurate foam core insulation boards provide a simple, yet cost effective solution to the cavity wall issue. The R-value for a 1 1/4 inch rigid foam core insulation is roughly an R-10, approximately the same R-value as a two-inch polystyrene insulation. The polyisocyanurate insulation almost doubles the air space in the cavity from one to two inches.

One product, Celotex Tuff-R Commercial, at 1 1/4 inches thick has the same R-value as its two-inch thick extruded polystyrene competitor. However, when the reflective airspace R-value due to aluminum foil facers is added, the Tuff-R Commercial insulation provides more thermal protection. Reflective airspace R-value based on ASHRAE and ASTM C236 tests is 2.1 with polyisocyanurate, thus raising the R-value to 12.1 compared to a reflective airspace R-value of 0.6 with polystyrene, which places its R-value at just 10.6.

"We placed ads in various magazines like Architectural Record that discussed the cavity wall issue and focused our message on the benefits of the polyisocyanurate insulations," said Randy Zulager, Senior Product Manager for Celotex rigid foam insulations. "Those ads ranked number one in that magazine for three months as we received some very positive feedback from our customers. We see more and more masonry contractors who are using our insulations in their structures because they now see the benefits."

Jason Thompson, Structural Engineer for the National Concrete Masonry Association agrees. He says that masonry contractors look to rigid foam insulations when trying to achieve the highest possible R-value in their wall systems. "If you want to increase the R-value, then you want a good rigid foam insulation," said Thompson.

Masonry contractors aren't the only people who have fought with the issue over the years. Many architects and designers have tried to find a way to get the most amount of air space between the walls. They realize the importance of a proper drainage system, thus limiting the water and moisture penetration through the wall.

A cavity wall is designed as a drainage wall system, so any moisture that does penetrate the exterior brick will run down the back of the brick to the bottom of the cavity where it is diverted to the outside. Manufacturers of polystyrene have incorrectly alleged that the polyisocyanurate foam core insulation boards absorb water resulting in less R-value for the thermal protection. Although a heated argument, most architects and masonry contractors have found that the consumer receives a better R-value when using the polyisocyanurate insulation boards.

The drainage portion of the wall system is equally important to the insulation value. There are codes designed to monitor the amount of air space within the system. The ACI-530, for example, is designed to enforce the amount of air space. "The ACI-530 dictates a number of recommendations for cavity wall construction and air space," added Thompson. "That code is designed to monitor the issue of maintaining some type of water path between the exterior and interior masonry. There is a benefit if you use the rigid board insulation as you can still get an increased insulation value while maintaining an air space within the cavity to keep the water away from the interior portion of the system," said Thompson.

Though an expanding industry, masonry contractors continuously find ways to provide better insulation values to concrete and brick structures. The cavity wall system has been successful for quite some time now. Issues such as moisture absorption, fire protection and thermal R-value will be satisfied through the expanded use of polyisocyanurate insulations.

About the Author

he Mason Contractors Association of America (MCAA) is the national trade association representing mason contractors. The MCAA is committed to preserving and promoting the masonry industry by providing continuing education, advocating fair codes and standards, fostering a safe work environment, recruiting future manpower, and marketing the benefits of masonry materials. Visit to learn more.


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