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Your Energy Wise Guy…..Willie
What is Wind Washing
Wind washing is the movement of unconditioned air around or through buildings in such a way as to diminish or nullify the intended thermal performance of the home. Wind washing can occur in attics that are vented at the soffits. It can also occur in areas of a home where conditioned spaces abut unconditioned space, for example in areas that are cantilevered (i.e., sections that bump out above the foundation) if those areas are not adequately air sealed with complete air barriers to prevent air from flowing through the floor joist cavities connecting the cantilevered area to the rest of the home. Symptoms of wind washing including cold or hot spots along exterior walls, along second-story walls that border attic spaces, over cantilevered floors, and over or along other areas of the home that border unconditioned spaces.
Air can enter a house by means of natural infiltration (driven by wind and temperature differential forces) and by means of mechanical infiltration (driven by the heating, cooling, and ventilation mechanical systems). Natural infiltration comes into the house from outside through a variety of pathways. Mechanically induced infiltration is driven by exhaust fans, dryers and duct leakage, which can create a negative pressure inside the home, drawing unconditioned air inside.
A common example of wind washing is when wind blows through soffit vents and pushes the attic insulation back away from the edges of the ceiling deck, leaving patches of ceiling deck bare along the outer edges of the attic. Where wind washing occurs, rooms will feel hot in the summer and cold in the winter, especially along exterior walls. This kind of wind washing can be prevented by installing baffles, which are cardboard or plastic panels installed in each roof rafter bay with a soffit vent to direct air up past the insulation and along the underside of the roof deck toward the ridge vents. The sides are stapled to the attic ceiling or rafters; the baffles provide a 1-2-inch gap between the ceiling and the insulation to serve as a chute for ventilation air to flow from the soffit vents to the attic ridge vents. The lower edge of baffle typically angles down and is sealed to the attic floor just inside of the soffit vent; thus, it provides both a dam to keep insulation from falling through the vent and a solid barrier to keep wind from flowing laterally through the insulation.
Wind can also enter the attic or cavities between floors from cantilevered floors that aren’t properly air sealed and from porches if the wall between the house and the porch attic is not properly air sealed. Wind that is allowed to flow laterally through the attic can get into interstitial floor spaces, such as under attic kneewalls.
In cooling-dominated periods or climates, wind blowing into attic vents can push hot attic air into the inter-story floor or knee wall cavity, bypassing the typical thermal boundaries of the building and introducing considerable heat into the house. Even in the absence of wind, thermal buoyancy can create air transfer between a hot attic and the interstitial cavities. Wind washing can also occur when little or no attic is involved, where outdoor air can infiltrate directly into the floor cavities or knee walls. These breaches of the air and thermal boundaries of the house can lead to significant entry of sensible heat and, in humid climates, latent heat (water vapor) into the structure. Wind washing has been identified as a significant issue impacting energy demand, comfort, and humidity in two-story homes. Wind washing can result in substantially increased second-story cooling or heating loads. When the outside air is hotter than the inside conditioned air, first-floor cooling loads can increase due to warmer ceilings and air penetration from the floor cavity through recessed lights and other ceiling pathways. Factors contributing to increased energy losses due to wind washing include the size and amount of openings between unconditioned spaces and interstitial spaces, the temperature of the air pushed into the floor cavities from the attic space (attics with tile or light-colored roofs and taller roofs will tend to be cooler in summer than dark, asphalt-shingled, and low roofs); the presence of complimentary holes on the opposite side of the building cavity from the prevailing winds, and the prevalence and strength of the wind.
Windwashing can be prevented at initial construction by ensuring that exterior walls, cantilevered floors, and attic kneewalls are properly air sealed; and by specifying unvented attics, or by properly designing the attic eave area and installing insulation dams and baffles. The level of effort needed to repair areas impacted by wind washing can vary greatly depending on the difficulty of accessing the repair areas and the total area needing to be sealed.
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