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Traveling across the country, I see many homes with a PAV installed on the roof. In fact, many times I see two or three. Recently I actually saw a brand-new home still under construction with six of them - and the home appeared to be only about 2,500 square feet.
Why all of these PAVs on roof tops? The most common reasons I hear are that they remove the heat from the attic, lower the temperature of the roof shingles, lower the load on the second floor, and save money. This may be true. However, know that a PAV can - and I did say c-a-n - add to oversizing of HVAC equipment.
AIR IS STUPIDLet me clarify something. Passive attic ventilation does lower the temperature of air in the attic, and a PAV lowers it even more. Unfortunately, in order to do so, it may also pull some conditioned air from the home.
The problem is simply that air is stupid. The replacement air doesn’t know it is all supposed to come in through the soffits and gable vents.
Know that a PAV may also exert suction on the attic floor leaks. After all, many - if not most - homes have a significant number of holes in the attic floor: recessed can lights; attic hatches or pull-down stairs; open chases around chimneys, plumbing, and ducts; open wall cavities around dropped ceilings, duct leaks, and wiring holes; and gaps along the top plates.
Insulation covering these leaks doesn’t necessarily stop airflow. It may just filter it. Even with significant amounts of intake ventilation, house air still gets sucked up in many homes. This means the house, in turn, can go to a negative pressure to bring in hot and humid outside air to replace what the PAV sucked up into the attic.
Let’s take a look at an example where the attic has plenty of passive intake ventilation, yet still has a problem. A 1,200-square-foot attic should have four square feet of free area intake ventilation in the gables and soffits to meet the normal 1:3 ratio required by codes. Let’s assume, instead, that it has 9 square feet of vents and other leaks in the soffits, more than double the normal recommendation. And, let’s assume that all the air leaks in the attic floor leading down into the house add up to just 1 square foot. (In truth, this is quite conservative for a typical 25-year-old house.) And, finally, let’s assume a relatively small PAV is mounted in the roof that sucks 1,000 cubic feet per minute (cfm) out of the attic.
Many mistakenly think that air “follows the path of least resistance” and the replacement air will all come in through the visible passive vents. In fact, the incoming air is proportional to the total holes. In this case, the PAV is an “equal opportunity sucker,” and doesn’t discriminate between good leaks and bad.
In this case, 90 percent of the replacement air may come in through the soffits and gables, and 10 percent may be sucked up from the air conditioned house below through the 1 square foot of attic floor leaks. Now, 10 percent doesn’t sound like much but, in this case, it is 100 cfm of conditioned air. If the house had two big PAVs, up to 300 cfm could possibly get sucked out of the house below.
RESEARCH SAYS...In an eight-home study done in North Carolina, researchers found that when running one PAV, which typically has the ability to move 1,200 cfm, on average 200 cfm was coming from the home (Tooley & Davis, 1994). This doesn’t seem like much, but, again, in an example home from Nashville, Tenn., 200 cfm equates to 4,104 Btu sensible load and 4,080 latent load, for a total of almost three-fourths of a ton of air conditioning being lost.
Therefore, instead of reducing the load, the PAV actually may create more load, especially of the latent (humidity) variety. The higher humidity causes discomfort, which may lead to the home-owner mistakenly thinking that an even bigger replacement air conditioner is needed.
In addition, PAVs may also cause other IAQ related issues, such as back drafting of gas water heaters. They can also bring air into the living space from areas that we do not want air from, including crawlspaces or attached garages.
In short, PAVs may not help an HVAC system and they may actually add to poor IAQ in the home. Again, let me clarify. I am not saying attic ventilation is bad. It’s just that when a PAV is used to “super-charge” the ventilation, problems may start. While a PAV may seem like a silver bullet, adding one may cause problems. A PAV is a classic example of how a house works as an interactive system.
MINIMIZE NEGATIVE EFFECTSIf your client insists on installing a PAV or keeping the one(s) he/she has, make sure the PAV does not make a negative impact on the home or HVAC system; completely seal the attic floor using materials such as two-part spray foam. Also, add lots of passive intake ventilation. (Given the nature of the ratios involved, it is usually more important to seal the holes in the attic floor than to add more intake vents.)
Or, one can install an additional ventilator fan to blow in the same amount of air that is being exhausted out by the PAV to create a neutral pressure in the attic. These measures may help. However, unless testing the home afterwards with digital manometers, you are simply guessing.
Other measures may be taken to reduce the load associated with the hot attic. Where there is no ductwork or equipment in the attic, just add more insulation. If equipment and ductwork are in the attic, add insulation to the attic floor and also install either radiant barrier aluminum foil or radiant barrier spray. Both should lower the temperature in the attic by reducing the emissivity on the plywood roof decking. In fact, when measuring with an infrared camera, you can easily see up to 25 to 30 percent reduction. In Southern climates, radiant barrier can also help with summertime sweating ducts by keeping the temperature in the attic warmer at night.
SIDEBAR: THE BOTTOM LINETom Heidel, product manager, Indoor Air Quality & Controls, Broan-NuTone, does not agree totally with Ken Summers’ assessment regarding powered attic ventilators (PAVs). He gave The NEWS the following assessment:
“For starters, many times whole-house ventilators are also called attic ventilators and can be confused with a roof-mounted or gable-mounted attic ventilator. A whole-house ventilator is mounted in the uppermost ceiling in the home and removes air at a high rate (1,200 to 3,600 cfm) and usually in a very noisy manner. By doing so, it can potentially draw in a large amount of dust, allergens, and other particles that contribute to poor IAQ.
“In an air conditioned home, there would also be a rapid loss of conditioned air, which causes concerns about the lack of energy efficiency. This was typically used in non-air conditioned homes. With central air being more common, the market has moved away from these types of products. Broan no longer offers whole-house ventilators.
“In many climates and conditions, the heat in the summer is too much for natural ventilation to properly ventilate an attic. In those situations, contractors and homeowners have had great success using roof- or gable-mounted powered attic ventilators (PAVs).
“Attics can reach temperatures of 150° to 160°F during a summer day, although outside air temperatures are only 95° to 97°. The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155° to 105° will result in a significant reduction in cooling load. Heat movement through ceilings may account for 30 percent or more of the total cooling cost.
“With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. A ventilation rate of one air change per minute for a typical attic using 95° air will lower the peak attic temperature to about 101°.
“A properly installed PAV is mounted on the roof or the attic gable and is designed to remove air at a rate of 1,000 cfm to 1,600 cfm. The contractor needs to provide adequate inlet space, such as soffit or gable vents to the outside, to provide air movement through the attic. When the ventilator is operated, it will pull cooler air through the inlets and evacuate the extreme air to the outside. When inadequate inlet space is provided, the pressure in the attic can go negative and begin to draw air into the attic from the interior of the home. This scenario can cause an attic ventilator to appear to be less efficient in reducing the cooling load on the home.”
Heidel can be reached at email@example.com.
Publication Date: 06/25/2007