Ventilation During Warm Weather | |
Why, on a hot day, are the upper rooms of a home always warmer? Part of the answer, of course, is simple physics: hot (lighter) air rises while cooler (denser) air falls. But in most homes, the vast majority of homes without adequate attic ventilation, a far more important factor comes into play: the downward migration of heat. Consider what happens in such a home on a typical summer day (Figure 1). Radiant heat from the sun hits the roof.The roof temperature increases and heat travels (technically, it conducts through the roof sheathing) into the attic. As heat builds up in the attic, it radiates to the attic floor, then into adjacent living areas, raising temperatures there. You appreciate the effects of that process when you look at the temperatures involved. These are typical temperatures for a home with no attic ventilation, on a sunny day, with an outdoor temperature of 90 degrees F (32 degrees C). Temperature at roof sheath: as high as 170 degrees F (77 degrees C). Temperature at attic floor: up to 140 degrees F (60 degrees C). Temperature in rooms directly beneath attic: uncomfortable. Of course, the longer these hot, sunny conditions last, the more uncomfortable it becomes in the home. That's because an unventilated - or inadequately ventilated - attic seldom loses enough heat overnight to compensate for the heat gained during the day. Ironically, the effect is magnified in modern homes with heavier insulation (see the insulation/ventilation connection below). eventually, this accumulation of heat begins to have more practical - and costly consequences. The most obvious are the actions taken by homeowners to cool themselves. To reduce the effect of the heat - not only the daytime heat gain but also the excess heat being stored in the attic - they turn on fans, window air conditioners, or central air conditioning systems. As the hot weather continues, these appliances run longer and longer, a fact well documented by utility companies across the country. Homeowners pay for all this added energy consumption in higher utility bills. A less obvious-but equally costly-consequence can be found on the roof itself. Homeowners can't see it happening, but over time excess attic heat can cause some shingles to distort and deteriorate. The result is premature failure of roofing materials, and perhaps a leaky roof. Once that happens, the cost of a new roof is the least homeowners can expect to pay. More than likely, they also may face added costs for structural and interior repairs related to water infiltration. Ventilation can't eliminate the transfer of heat from roof to attic, but it can minimize its effect. To do that, a well-designed system must provide a uniform flow of cool air along the underside of the roof sheathing. That steady flow of air carries heat out of the attic before it can radiate to the attic floor. It's critical that this air flow is uniform. That means intake and exhaust vents must be balanced for both position and air flow capacities. Otherwise, "hot spots" can develop under roof sheathing, drastically reducing the efficiency and effectiveness of whatever ventilation is installed. Insulation/Ventilation Connection Efficient insulation increases the need for effective ventilation. Why? Because heavier insulation absorbs and holds more heat. That means it's less likely overnight cooling can remove heat that builds up in an attic during a prolonged period of hot, sunny weather. The solution to this dilemma isn't to reduce the insulation in on attic. That would only create problems at other times of the year. Instead, the goal is to design an attic ventilation system that effectively compensates for the additional heat gain produced by the high levels of insulation. In short, effective attic ventilation also helps cool attic insulation. Source: Air Vent Inc. A Certainteed Company Reprinted with permission. |