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NORA Advanced Efficiency

National Oilheat Research Alliance 33 nects the two aforementioned barriers: the Thermal Barrier and the Air Barrier. The Thermal Barrier is composed of good insulating materials that resist conductive heat loss. Air is a heat sponge. If it is prevented from moving, it will hold the heat in the building. Insulation has many air pockets that trap air and prevent the flow of heat. The building also needs a pressure or air barrier, otherwise the pressure differences from wind, and other factors changing the air pressure will cause the air in those loose pockets in the insulation to flow, carrying the warm air out of the building. The Pressure (Air) Barrier is the shell of the house. It should not have air leaks and should stop air under pressure from pushing through insulation, cracks and holes in the building. If there is a big pressure difference between inside and outside, a tiny hole can have a huge impact. Think how quickly a small nail hole in a tire can create a flat. The Home Performance goal is to have the thermal barrier always in contact with the air barrier and form a continuous, leak-proof wrap around the building. In the northern parts of the country, the air barrier is installed inside the thermal barrier. Inside, Outside, or In-Between? It is important to determine what parts of the building are inside and outside the air and thermal barriers. Generally, the unheated parts are outside: attic, garage, porches, and crawlspaces. All heated parts should be inside the barriers. Buildings also have rooms that are semi-conditioned. They are not heated deliberately, but heated by waste heat from appliances and passive solar heating. This would include basements, crawlspaces, sunrooms, and porches. Note that barriers can be installed to place crawl spaces, basements, and attics inside or outside the heated space. The Air Barrier The air barrier can be sheetrock (drywall), plastic, wood—in fact, any material impenetrable to air. It may even be the insulation itself (dense-pack cellulose and closed-cell foam are both a thermal and an air barrier). Doors and windows—the glass, weather-stripping, and the framework and trim—are also air barriers. Air barriers should provide a continuous impenetrable surface or membrane surrounding the entire conditioned space of the house, in direct contact with the insulation. The air barrier is supposed to be a complete wrap with no leaks; however, in building the house, the air barrier is cut many times. Wires, pipes, chimneys, windows, bathroom vents, attic hatchways, electric outlets, and ceiling lights all create potential gaps in the air barrier. Figure 7 on following page. Finding and sealing these leaks is the most import energy conservation job home performance contractors do. Barriers The facings on fiberglass insulation are air barriers. They protect insulation, provide air and or vapor barrier, facilitate fastening, and hold it together. The effectiveness of a barrier depends on flawless installation and continuous, sealed seams. The facings can be made of Kraft paper, aluminum foil, or woven polyethylene (weather resistant— lets vapor escape). Air barriers can also be vapor barriers. Water vapor moves from inside to outside in the winter, and outside to inside in the summer. A vapor barrier is only suggested in rooms with lots of water vapor, such as the room with a shower. The vapor barrier should be on interior surfaces in the north, and exterior in the south. A vapor barrier is not strongly recommended because if it leaks it can allow water to flow into the walls or ceilings and trap it there, causing mold growth and the walls and ceilings to rot. Air Leakage Air leaks need air, a hole, and a pressure difference (driving force) between each side of a wall or ceiling. The bigger the hole or


NORA Advanced Efficiency
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