A cavity wall is comprised of two separate masonry walls, parallel to each other but a few inches apart. These walls, constructed from bricks, stones, concrete blocks or poured concrete, make excellent sound barriers, but they are not energy-efficient. Today, when a new building installs cavity walls, insulation is part of the construction process. However, there are ways to insulate existing cavity walls.
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The ancient Greeks were well versed in cavity wall construction and ruins of partial buildings testify to the popularity of this building method. In addition to creating a sound barrier, archaeologists suspect the double-wall structure inhibited moisture penetration. Enjoying a resurgence in the late 1800s, exterior cavity walls were common in areas where natural stone was abundant. Today, cavity wall construction is still popular in commercial buildings, often coupled with cellulose sheet insulation.
A cavity wall consists of two walls, complete from floor to roof. Each wall is narrow, usually less than 5 inches wide. A large concrete footing or stem wall serves as a solid foundation to support the double wall. The air space between the two walls ranges from 2 inches to 4 inches, and wall ties may extend through the entire width of the wall.
If you have cavity wall construction, you can save money and increase energy efficiency by insulating the air space between the walls. If you're not sure whether your house has cavity walls, open your front door and look at the wall from the side. A cavity wall is usually at least 10 inches thick.
There are two main types of insulation commonly used in cavity walls. Because there is no way to access the cavity to install batt insulation, the material must be blown in or injected in from small holes drilled in the wall and later patched. It's common to install cavity wall insulation from the exterior of the home. Dry cellulose fibres are blown in through holes drilled in the upper portion of the exterior wall, and they settle downwards, filling the void. Expandable cellulose foam is installed only by certified professionals, and it enters the air space as a liquid but quickly expands into a solid foam core.
Once the cavity walls are insulated, the homeowner can expect to see a substantial reduction in the amount of energy needed to heat or cool the home. Cavity wall insulation should be one part of an overall energy plan for your home. Expandable foam has the highest insulating R-value but it should not be applied in a wall with moisture problems.
Unfortunately, cavity wall insulation isn't without problems. Wiring and plumbing within the walls may block the free fall of cellulose fibres or expandable foam, leaving a void at the bottom of the wall. Check references of expandable foam installers and resist do-it-yourself foam kits. Once set, expandable foam is a permanent fixture.
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