The following info to chew on was compiled by Martin Holladay, former associate editor at the Journal of Light Construction, former editor of Energy Design Update, currently senior editor for Green Building Advisor. GreenSpec recently delisted powered attic ventilation fans, to which the powered attic fan manufacturers have taken great exception.

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Effective Attic Ventilation

Increased attic ventilation has long been promoted by roofing material and attic ventilator manufacturers as a way to increase shingle life, decrease attic temperatures and lower cooling costs. Unfortunately, there is no scientific data to validate any of these points and mounting evidence and research to the contrary.

Attic ventilation was added to the nationwide building codes to prevent/eliminate roof damage caused by trapped, moisture-laden air migrating into the attic during winter. This code change followed the widespread introduction of indoor plumbing and central heating systems. As the use of attic insulation increased, ventilation proved valuable in controlling another problem, ice damming. Ice damming occurs when certain areas of the roof become warmer than others, causing a thaw- freeze cycle that allows accumulated snow to melt, then re-freeze on colder portions of the roof. This re-freezing creates an ice dam that works its way under shingles only to later melt and create roofing leaks. Ventilation ensures the roof deck temperature remains consistently cold so ice damming cannot start.

At some point, the original purpose for attic ventilation was forgotten and/or replaced, in cooling climates, with the belief that it was to reduce roof and attic temperatures, thus lowering cooling expenses and increasing shingle life. This was further compounded by the leap of faith that increased or powered ventilation would be even better. While sounding logical, there is simply no research to validate it. In fact, scientific testing has shown that attic ventilation has almost no effect on roof surface/ shingle temperatures and very little effect on attic temperatures.

There is however, a growing list of research, computer modeling and field data that indicates powered attic ventilation can be a detriment to health and safety and actually increase cooling costs. The latest and most compelling findings by Natural Florida Retrofit and the AEC Applied Building Science Center found that in all the homes they studied, powered attic ventilators (PAVs) offered no benefits and sometimes caused serious health and safety problems. Their research found three reasons why PAVs are not a good energy investment:

1. PAVs can create negative pressures in combustion appliances causing the backdrafting of flue gases such as carbon monoxide in the living space. These same negative pressures can also draw increased levels of moisture and soil gases, such as radon, from the crawlspace.

2. PAVs can draw conditioned air out of the house and into the attic, causing the air conditioning to run more. Conditioned air is then replaced with moisture laden outside air, creating increased humidity levels inside the living space.

3. PAVs can increase utility costs substantially because of the increased energy necessary to run the fan and cool/dehumidify the outside air being drawn into the home.

The most effective strategy for minimizing the effects of summer attic temperatures and eliminating the potential for winter ice damming is as follows:

1. Eliminate air leakage between the attic and living space. Common leakage points are penetrations made by plumbing and electrical fixtures, top plates of interior walls, attic pull downs/ doors and plumbing chases. Use expanding foam or caulk for plumbing and electrical penetrations and weatherstripping for doors or pull downs.

2. Seal ductwork located in the attic using a commercial grade duct sealer or mastic and insulate with a minimum 2″ insulated duct-wrap with vapor barrier.

3. Insulate the attic floor to a minimum R-30.

4. Use light colored shingles to reduce roof surface temperatures.

5. Provide ridge and soffit or gable and soffit passive ventilation to building code specifications to remove moisture and prevent ice damming.

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Solar Powered Attic Ventilation by Arnie Katz

Q: My sister Louise cut an article out of the newspaper about solar powered ventilation fans for attics. Do these things really work? I could install it myself, since no wiring is required, and the article said it would save money by cooling off the attic.

A: Ask Louise what she’s doing hanging out in the attic. And if she’s not spending time up there, why does she want to make the squirrels and bats more comfortable? If I wanted a powered attic vent fan, a solar powered one would be a good choice. But in my opinion, powered attic ventilators are generally not a good idea, whether they’re powered by nuclear electricity, burning water buffalo dung, landfill-generated methane gas, or directly by the sun.

I happen to think that using photovoltaic (solar) cells to create electricity to power our houses and businesses is an excellent technology that is proving itself both practical and economical in lots of different ways. But using the sun to power a marginal and even potentially dangerous item like a powered attic vent fan doesn’t make the fan more appealing.

Powered attic ventilators are promoted as doing three things: reducing summer air conditioning bills, removing moisture from the attic, and extending shingle life. Let’s look at each of these.

Theoretically, these fans reduce attic temperatures by pulling outside air into the attic. Unfortunately, somebody forgot to tell that to the air. In most of the houses we’ve tested, the attic fans were drawing some of their air from the house, rather than from the outside. In other words, they are cooling the attic by drawing air-conditioned air out of your house and into the attic. Air conditioning the attic is not recommended by anyone I know as an effective strategy for reducing your bills. Effective strategies include sealing the air leaks between the house and the attic, and making sure there is enough attic insulation and it is installed properly.

Another problem is that a large fan in the attic that is pulling air out of the house can create a negative pressure in the house. This negative pressure can suck the flue gasses out of a water heater or other combustion appliance. In one house we tested, we measured substantial levels of carbon monoxide (CO) in the daughter’s bedroom in the basement. The CO was coming from the water heater next to the bedroom, which was backdrafting. The daughter had been suffering from flu-like symptoms for some time. The backdrafting was caused by the powered attic vent fan.

Vent fans are also promoted to remove moisture from the attic. In our part of the country, the humidity is typically high in the summer, when we’re advised to run the fans to “cool” the attic. To the extent that the fan is pulling outside air into the attic, that air will tend to have a high relative humidity, so it’s unlikely that it will reduce the moisture level in the attic.

If there’s an air conditioning unit with a leaky cabinet or leaky ductwork in that attic, there’s a chance that the moisture in the air will condense on the cold spots and cause moisture damage. Rather than solve moisture problems, the powered attic vent fan may, in fact, cause it.

Finally, powered attic vent fans are promoted as a strategy to extend shingle life by reducing the attic temperatures. Shingles are heated up by radiant heat from the sun. It’s possible that ventilating the attic can reduce the temperature of the air in the attic, which could reduce the temperature of the roof decking, which could reduce the temperature of the back of the shingles. I’ve seen no research, however, that supports the idea that powered attic fans actually increase shingle life.

If you are in a climate where you can be comfortable in your house without air conditioning, an attic fan won’t have the drawback of pulling costly air conditioned air out of the house. Also, if there is no air conditioning, there is little likelihood of having cold condensing surfaces in the attic, so moisture problems are unlikely. In these circumstances, a powered attic fan may be helpful. But be sure to check that the water heater is not back-drafting.

If you’re in a warm, moist climate where you need air conditioning, I would tell Louise that you’d much rather get a solar-powered outdoor lighting system or a solar hot water system. But a solar-powered attic fan? It’s like smoking cigarettes made with vitamin C.

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Ask the Builder

DEAR TIM: I am trying to make my house more comfortable and lower my air conditioning bills at the same time. It seems to me that a powered attic ventilation fan will help. Will such a fan exhaust enough air to lower my attic temperature significantly? If not, what is the best way to exhaust hot air from an attic and lower cooling costs? Amy V., Magnolia Park, FL

DEAR AMY: This is a classic example of where common sense does not always prevail. I used to think like you that powered attic ventilators (PAVs) are a great way to get hot air out of attic spaces. It turns out that these rooftop and sidewall mounted attics fans can actually suck air conditioned air from your house into your attic space. What’s more – in extreme cases – they can actually create serious life safety problems by back-drafting combustion gases into a house living space. I used to recommend PAVs for houses, but I don’t anymore.

This is a powered attic ventilation fan. If it doesn’t have excellent intake air around the soffits, it can actually vacuum air from inside your home.

When a PAV is turned on to exhaust hot attic air, it begins to move massive quantities of air. Smaller fans can easily move 1,000 cubic feet of air per minute (cfm). More powerful or multiple fans can extract thousands of cfms of air.

This movement of air creates a natural low pressure inside your attic space. To equalize this pressure difference, air begins to rush into the attic space from multiple points. If you have many passive roof vents, large side gable vents, and/or excellent unobstructed soffit vents, then there is a good chance that most of the makeup air will originate from outside your home. If you have few passive attic ventilation inlets, then most of the replacement air will come from the inside of your home.

If you have your air conditioner running at the same time your PAV is operating, you are sending expensive cool air from your living space into your attic. This air seeps into your attic through the attic access panel and hidden holes in the interior wall top plates where plumbing vent pipes and electrical wires enter the attic. Air also can easily slip between the space between bowed wall plates and drywall. To complete the circle, hot and humid exterior air enters your living space. Your air conditioner now has to work harder and longer to keep you cool while that PAV is merrily spinning away up in the attic!

Recently completed studies by the Florida Solar Energy Center have proven that as the sun heats up the wood roof sheathing and framing members in your attic they quickly and invisibly send this heat directly to the top layer of your attic insulation. The insulation in turn re-radiates this heat into the attic airspace. PAVs can lower the overall attic temperature to a slight degree but the secondary effects of sucking cool air from your house and possibly combustion gases into your living spaces offset this benefit. The danger of sucking combustion gases from hot water heaters and any other combustion appliance is real. This is especially true in tighter, newer homes. To feed the voracious appetite of the PAVs, air will enter your home at the point of least resistance. This point is often a chimney or hot water heater metal B-vent pipe.

Your desire to lower energy costs and keep cool should begin by boosting the amount of attic insulation to offset high attic temperatures. Not only will the added attic insulation help you in the summer, it will also help you to lower winter heating costs. Remember that you must maintain at least 2 inches of air space between insulation and the roof sheathing. At the same time, you must be sure that air can easily enter the attic space through the lower portions of your roof or soffits and travel unimpeded into the attic space. Once there it can be gently vacuumed from your attic through continuous roof ridge ventilation systems every time Mother Nature exhales across your roof.

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Drawbacks Of Powered Attic Ventilators

Powered attic ventilators, already suspected of using more energy than they save, can also create excess moisture, structural problems, discomfort, and combustion safety problems for home occupants, according to a recent study. John Tooley of Natural Florida Retrofit, and Bruce Davis of Alternative Energy Corporation’s Applied Building Science Center in North Carolina, presented “The Unplanned Impacts on Houses by Powered Attic Ventilators” at the 1995 meeting of the Energy Efficient Building Association.

The paper describes research conducted on eight homes over a period of three months. As a result of this research, Davis said that he wouldn’t recommend the use of powered attic ventilators. He emphasized, “If someone chooses to use a powered attic ventilation strategy, they need to do additional performance tests and take responsibility to be sure that it won’t cause other problems.” The potential for hazardous conditions is particularly high in homes with combustion gas appliances, because the ventilators can create negative pressures that cause backdrafting.

Tooley and Davis took measurements from July through September 1993 at eight homes in Research Triangle Park, North Carolina. Each attic contained passive ventilation in addition to a powered ventilator. All the houses had some depressurization when the ventilator operated, with pressures ranging from -0.5 to -2.5 Pascals (Pa). The tests showed that the powered attic ventilators, on average, drew 231 cubic feet per minute (CFM) of conditioned air out of each house and caused, by themselves, 0.72 air changes per hour (ACH). This flow from the house to the attic ranged from a low of 104 CFM to a high of 646 CFM, and ACH ranged from 0.38 to 1.2.

The houses had air volumes ranging from 13,000 to 32,000 ft3. The high figures above were for the largest house in the study, which had two ventilators and noticeable air leakage paths between the house and the attic. Tooley and Davis concluded that all eight of the sample homes wasted energy due to the high leakage of conditioned air into the attic.

Two houses had combustion safety problems resulting from the ventilators operating in conjunction with other mechanical equipment. At one, the water heater, located inside the conditioned space, stayed in a complete backdraft mode when the ventilators were operated with other equipment. During the seven minutes of the testing cycle, the area containing the water heater reached 40 ppm (parts per million) of carbon monoxide, and at five minutes the water heater flue contained 700 ppm of carbon monoxide. The study also reports anecdotal cases of combustion safety problems, including a Colorado family of three who died from carbon monoxide poisoning when an attic ventilator caused the furnace to backdraft.

Another two houses had moisture problems resulting from ventilator-induced negative pressures. At one house, a chain of events started with the repair of a leaky return duct for the air conditioner. Condensation occurred on the tighter (and colder) duct, which was located in a framing cavity, and moisture began to appear on the ceiling Sheetrock of the first floor. The suggested remedy was to increase the powered attic ventilation and turn on bath fans to remove moisture from the structure. This actually exacerbated the problem by increasing the pressure difference, moving more humid outside air through the building cavity, which created more condensation on the sheet metal duct. The moisture saturated the Sheetrock, which eventually fell to the floor. Discomfort problems were found in two study houses, one of which also had safety problems, while the other had moisture problems.

Tooley and Davis concluded that if a powered attic ventilator is to be used, the installer should provide a good air barrier between the ceiling and the attic, and ensure that adequate net free vent area is provided in the attic. The amount of air the fan moves must also be considered so that it can be properly sized (a bigger fan is definitely not better in this case), and pressure measurements should always be used to confirm safe operation. Powered attic ventilators are commonly used in some regions of the United States, such as the Southeast and the Midwest, and they have a positive reputation. They are actively promoted by builders, roofers, HVAC contractors, ventilator manufacturers, do-it-yourself retailers, and even some utilities, although few utilities are sponsoring installation programs as they once did. Davis said that ventilators are sometimes used as a quick fix to meet attic vent codes in homes with an unusual roof structure or design. Most powered ventilators are thermostatically controlled, with variable settings. Davis said that some manufacturers are also promoting these or similar products as crawlspace ventilators to help remove moisture, but this use can also cause other problems if it is not properly evaluated.

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The Office of Energy Efficiency, part of the Ohio Department of Development, produced a leaflet called “Office of Energy Efficiency Debunks Myths about Energy Efficiency.” That leaflet states, “Myth #4: Powered attic fans will reduce your air conditioning costs. Under ideal circumstances, attic fans would increase the air exchange in your attic, reducing the attic temperature. … However, in most homes, attic fans do not work this way because unsealed air leakage paths connect the attic and living spaces. When the attic fan exhausts attic air, conditioned air is actually drawn from the home and out the attic. This loss of conditioned air will increase your air conditioning costs. To add to the problem, as air is drawn from the house, warm and humid air from outside is drawn into the living spaces. Your air conditioner will now have to work harder to cool the air and remove the moisture.”

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Energy Design Update, August 1997

“Ventilation is like a little boy who goes around the house looking for a job,” notes Bill Rose of the University of Illinois Building Research Council. “He can do some things well, but can’t do anything really well.” The point, he says, is that people expect ventilation to solve more than its share of problems.

The solution for ice dams is not to place a fan in the attic, but to use adequate insulation and to seal leaks in the building envelope.

“In terms of dealing with ice dams you have a point,” admits Stanley, “but that doesn’t address the summertime issue of heat buildup.” Unfortunately, researchers have yet to accurately document the effect of attic ventilation on cooling load. In fact, says Rose, research suggests that the energy to run the fan for a powered attic ventilator can be higher than the savings in cooling energy.

The biggest potential problem, says Rose, is that power venting can cause a negative pressure in the attic. Since you can’t guarantee the airtightness of the ceiling, this could draw humid air from the house into the attic, causing moisture problems in winter. “There’s nothing that guarantees wide open soffit vents,” he says. “One of the worst things that can happen is to draw quantities of indoor air into the attic, and powered equipment is more likely to do this.”

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“Based on the matching period analysis, estimation of annual space cooling savings are on the order of 460 kWh. These savings have a value of approximately $37 at current Florida energy prices. Given that the costs for the two units was approximately $600, or about $850 installed, the payback of the ventilators is not very favorable at over twenty years.”


Attic vent fans

Powered attic vent fans ventilate just the attic and not the interior of the house.

Frequently, attic vent fans are thermostat controlled and run only when the attic gets hot.

Attic vent fans operated by a manual switch are also available.

Data measured at FSEC and elsewhere show that attics with nominal natural ventilation and R-19 ceiling insulation do not need powered vent fans. Such fans cost more to operate than they save in reduced cooling costs, so they are not recommended.

Consider the following alternatives in place of attic vent fans:

1. In new houses, install R-19 ceiling insulation with an attic radiant barrier. In existing homes, install an attic radiant barrier. See FSEC publications DN-6 and DN-7 for tips on using and installing radiant barriers.

2. Use continuous soffit vents at eaves and ridge vents at the roof peak to increase natural attic ventilation. For gabled roofs, gable bents may also be used. In all cases, use vents near the peak of the roof (either well-installed ridge vents at roof peak or strip vents near the roof peak). Avoid wind turbines. They work when new, but often become noisy and leaky after a few years.