Cavity Wall Insulation
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All exterior walls of your home or building should be insulated. Insulation is a key component of the building envelope and will have a major impact on both energy efficiency and the comfort level inside. Proper cavity wall insulation will reduce heat losses to the outside in winter and prevent radiant heat from penetrating the building in summer. Insulating walls will also provide air sealing, reducing both drafts and the water vapor carried inside with drafts.
Building materials vary widely in their ability to resist the transfer of heat. Metal, for example, is a very good conductor of heat. Materials that do not conduct heat well are used for insulation. Air is an excellent insulator. Houses are built with air cavities in the walls and the attic to resist the flow of heat. Most houses built in the 1930s or later have cavity walls (prior construction had solid walls). Cavity wall insulation reduces heat loss through the walls by up to 40%. The best time for cavity wall insulation is during new construction or during major renovations to either interior drywall or exterior siding.
Cavity Wall Insulation – Heat Transfer
Heat flows from a warm medium to a cold medium in three ways:
- By radiation from a warm surface to a cooler surface through air or a vacuum using infra-red heat rays. Most radiant energy striking a building comes from solar radiation.
- By conduction through solid or liquid materials resulting from direct contact.
- By convection, which involves the physical movement of air – warm air rises. As air warms it will expand creating pressure to move outward through the walls.
Heat moves through wall cavities by a combination of radiation, conduction, and convection with radiation being the dominant method of heat transfer. Research shows that control of radiant heat transfer is the core of heating/cooling climate control. Radiation accounts for 65-85 % of all heat transfer through walls.
Traditional insulation materials use air spaces between fibers or in plastic foam bubbles to trap expanding warm air and prevent it from escaping the building. Reflective insulation uses fibers, foam or bubbles sandwiched between two layers of reflective aluminum foil sheeting to effectively block both radiation and convective heat transfer. Reflective insulation installed as a radiant barrier is essential in warmer climates to reduce air conditioning requirements.
Insulation is rated by its ability to resist both conductive and convective heat flow in units called R-value. R-value gives the insulation resistance per inch of material. Construction materials with higher R-value ratings are more effective insulators than materials with lower ratings for the same thickness. For example, heat will flow through an R-12 insulated wall only half as fast as through an R-6 wall.
All new buildings should incorporate a radiant barrier as part of the insulation system. The concept is simple: each unit of radiant heat energy that is reflected away from your building in summer, and each unit reflected back in during winter, means less operation of your heating and air conditioning systems, less wear and tear on your equipment, and less money you pay in utility costs. Use either a foil radiant barrier or reflective insulation. Radiant barriers (aluminum foil with no center layer) become reflective insulation because they face an air space like an attic, wall cavity or crawlspace. Radiant barriers primarily impede the downward flow of radiation. The largest benefit of using a radiant barrier is reduced air-conditioning costs in warm climates.
A vapor barrier (or more accurately, vapor diffusion retarder) is an essential part of the moisture control strategy for a home. The primary purpose of a vapor barrier is to keep moisture from getting inside your walls. Condensation within the walls can cause wood rot, mold and fungus growth. A vapor barrier acts as a physical shield to repel moisture. In addition to its properties as a radiant barrier, reflective insulation can be used as a vapor barrier. When properly installed, waterproof reflective insulation can reduce or eliminate condensation. As reflective insulation is non-absorbent, it will not mildew or promote fungus growth.
In colder climates, a vapor barrier should be placed on the interior or warm side of the wall. The US Department of Energy map below shows that in some southern climates, the vapor barrier should be omitted, while in hot and humid climates, such as along the Gulf coast and in Florida, the vapor barrier should be placed on the exterior of the wall.
Air Infiltration Barrier
Air sealing and moisture control make a building more comfortable inside and save energy by making insulation more effective. A significant amount of moisture enters walls through air leaks and capillary action from the ground below. Installation of house wrap will prevent moisture from penetrating from the outside.
When properly installed, reflective insulation will act as an air infiltration barrier thereby sealing the wall to reduce heat loss from expanding warm air and prevent water vapor in the air from entering the wall cavity.