Construction Concerns: Cavity Walls

Article and photos by Greg Havel

A masonry wall with space left between the inner (structural) part of the wall and the outer (decorative) part is sometimes called a cavity wall. The cavity can extend from bottom to top of the wall, and is included as “dead air” space for insulation. Wall cavities can also be used to reduce the possibility of moisture (rain or snow) gaining access to the interior and spoiling the finishes and furnishings.

RELATED: Construction Concerns: Reinforced Masonry | Construction Concerns: Foamed-in-Place Insulation  

Sometimes foam insulation board or sprayed foam insulation fills part of the cavity, for added insulation value, as shown in these photos from a school building completed in 2005.

Cavity wall under construction


Photo 1 shows a cavity wall under construction inside a heated scaffold enclosure. The steel-reinforced concrete foundation is visible at the bottom of the wall. The structural part of the wall (concrete block wall at left) is complete; has a vertical steel reinforcing bar grouted into the core of the concrete block every four feet horizontally and extending vertically from the footing to the roof; and shows the tabs from the horizontal “ladders” of reinforcing wire between the courses of block projecting from it. This wall will have two-inch extruded polystyrene board glued to it for insulation, which has already been installed on the lower part. The synthetic fabric cords that the masons are pushing through the foam board are “weeps” that will extend out through the mortar joints to the exterior of the building, to allow the cavity to breathe and for moisture control. Part of the first course of decorative concrete (“split-face”) block has been laid at the bottom of the photo, with a cavity between it and the foam board.

Void space between interior and decorative walls


Photo 2 shows an edge view of the completed wall. The horizontal “ladders” of reinforcing wire in the decorative face of the wall are interlocked with those from the structural concrete block. This photo was taken in a doorway, where the opening to the cavity will be covered by a hollow metal door frame.

Cavity walls in building construciton


Photo 3 shows another cavity wall under construction. The structural wall has been insulated with a sprayed polyurethane foam with a di-isocyanate-base. The section of the wall at the right has had its brick veneer completed, while at the left the brick veneer has not yet been laid.

Occasionally, a fire will involve the foam insulation in a cavity wall. If the plastic foam insulation is a thermoplastic, the foam will melt, liquefy, and run down into the bottom of the cavity, and burn as a flammable liquid. If the plastic foam is a thermoset, it will burn rather than melt, producing toxic smoke containing acids, cyanides, isocyanates, chlorine compounds, and other hazardous materials depending on the composition of the plastic; in addition to the expected carbon dioxide, carbon monoxide, and water vapor. Removing a door jamb or a window frame would provide the easiest access to the cavity. Ventilating the cavity would involve removing the metal flashing and roofing material from the top of the wall, and cutting and removing the continuous wood blocking to which the flashing and roofing are attached. Either of these tasks will be heavy work even if power tools are used.

Before concluding that there is a fire inside a masonry cavity wall, use a thermal imaging camera to check the surface of the wall for hot spots, to locate the seat of the fire. Be aware that even this high-tech approach is not fool-proof, since the masonry materials may have enough mass to mask a hot spot inside the cavity.

If it is necessary to remove a door or window frame for access to a wall cavity, it is best to do so from an interior floor, or from ground level at the same elevation. There is less risk of a fall to a firefighter from a solid working surface than there is when working from a ladder.

Download this article as a PDF HERE (517 KB).

Greg HavelGregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.



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