For most of its long history, "cavity wall" referred strictly to masonry, and involved no insulation other than the mass of the wall material and the air space itself. According to the Masonry Advisory Council, the main reason for the space in cavity wall construction was to prevent moisture penetration. Insulation to retard the flow of heat is a relatively modern innovation.
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Greek and Roman Cavity Walls
Hundreds of years ago, the ancient Greeks and Romans built masonry walls made of multiple vertical layers, called wythes. In some, a vertical space, or cavity, was left between the inner and outer wythes with regularly spaced stones turned crosswise to tie the layers together. Wind-driven rain that penetrated the mortar joints in the outer wythe flowed down inside the air space and out through weep holes at the bottom, rather than making it all the way into the building interior.
Modern calculations assign an R-value of just 0.44 per inch to the air space in cavity walls. A dry room probably felt warmer than a damp one, but the air space did little to impede the transmission of heat through the stone.
In the early 1800s, builders in Great Britain began to use cavity construction for the exterior walls of brick buildings, with brick headers across as much as a 6-inch gap between wythes. By mid-century, metal ties had come into use, but any thermal insulation was installed on the interior walls, not between wythes.
Builders in the United States began using similar cavity wall construction in the 1930s. Energy for heating and cooling was cheap and moisture control was still the primary reason to build cavities into exterior walls. Typical masonry cavity walls were made of two or three wythes of brick, or a single exterior wythe of brick tied to a backup wall of 8-inch concrete blocks.
The oil embargo of the 1970s brought a suddenly sharp increase in energy costs, and the incentive to design more effective thermal barriers into exterior walls. Rigid panels of foam insulation, applied to the inner face of the cavity, are able to withstand exposure to moisture and increase the effective R-value of the masonry wall composite.
Stud Wall Cavities
Modern stud-frame construction creates another cavity wall--inside the moisture envelope of the building--making it suitable for insulation materials that must stay dry to be effective. Fibrous insulation materials, such as fibreglass, mineral wool and cellulose, are relatively inexpensive products to improve both thermal and sound transmission through walls. Exterior grade foam insulation can be used in stud wall cavities but requires a gypsum board or plaster fire-barrier covering.
Modern construction incorporates both interior and exterior cavity insulation, even when the exterior wall finish is something other than brick or masonry. Effective use of cavity insulation helps reduce energy used to heat and cool buildings. Organizations such as the National Association of Home Builders Research Center are working to find ways to use recycled materials and to develop insulation products that use less energy to produce.
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