Attic Fan in Winter?


Old 12-05-02, 07:11 AM
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Attic Fan in Winter?

Should my attic fan be covered from the inside in winter? Should it be running?
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Old 12-05-02, 03:22 PM
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Attic fans

Many attic fans are equipped with both thermostats and humidistats that turn the fan off and on as needed for temperature and humidity levels.

Q. What is a humidistat and why would I need one?

A. During winter months, an attic can become very humid. Water vapor from your humidifier, cooking, and showers tends to seep up into your attic. Water vapor is actually lighter than air. When it gets into the attic it condenses on electrical pipes and wood framing causing leaks or dampening the wood possibly causing wood to rot over time. Over 70% humidity, the humidistat turns the attic fan on to vent out the humidity and draw in dryer outside air no matter what the temperature.

Some are equipped with firestats.

What is a firestat and why do I need one?

A. A firestat senses when there is a house fire by shutting the fan off when the attic temperature reaches 180 degrees F. This stops the attic fan from running and keeps it from drawing in air and feeding oxygen to the flames. Only the higher quality attic fans (such as a Jet Fan) have this feature. For safety's sake, make sure your attic fan has this feature.


Some folks do cover attic fans to save energy and keep out cold air.

Attic Fan Cover
Where can I find a cover to fit over my attic fan in the ceiling? Size is 32"x35". Someone should make such. .

I made one too using 1" styrofoam and furring strips. Just four sides with a hinged top which can be raised by a rope (guided by pulleys) going through the ceiling of a clothes closet.

I made a cover out of 1x2 furring strips frame and covered the frame with 3/4-foam sheet insulation. It was four sides and a top. I made mine in panels to get it through the access hole then assembled the panels with sheetrock screws. When determining the height, make it tall enough to cover the housing, but not so tall that it cannot be removed by lifting it off. I also open the breaker to the fan when the cover is on during the winter so someone wouldn't turn the fan on with the cover in place.

Attic Q's and A's. handymanUSA. Retrieved 05 December 2002.

A consideration as to whether or not you should turn off the attic fan may be to take a look at how much attic ventilation you have. While the type and number of vents will vary depending on the specific roof design, house location, and amount of direct sun, a minimum venting system should have 1 sq. ft. of roof vent area for every 300 sq. ft. of attic area.

Popular Mechanics: Home Improvement
Installing an Attic Fan
By Merle Henkenius
Retrieved 05 December 2002
Old 12-05-02, 03:26 PM
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Humidity in attic in winter

Assess attic ventilation before covering attic fan in winter!

Moisture Control in Winter

The most important aspect of attic ventilation is for drying internal moisture which develops during the heating season. Internal moisture refers to the condensation of more humid interior air when it escapes to cold wall and attic cavities. Warm air can hold more moisture in vapor form than cold air. When that warm air cools, it releases the vapor in the form of condensation. When the ventilation in the attic is not sufficient to dry this condensation, a house can experience mildew buildup, wood rot, delamination of subroofs,. peeling exterior paint, and other problems.

A cold, drafty attic during the winter is the best condition for the health maintenence of a house. Insulation is used to protect the interior house from this cold area.

Without prior knowledge of the consequences, it is a normal instinct to want to close vents in the winter. At a brick-sided home with wood roof sofffits in Aurora, the diligent homeowner blocked the vents every winter and every two years scraped and repainted the soffits and roof fascia area. Another Glen Ellyn couple had a walk-up attic and every winter blocked off the large end gable attic vents. The internal moisture buildup one winter got so heavy on the subroof that they replaced the roof because they thought it was leaking.

Sources of Internal Moisture

There are many sources of increased humidity in the house air during the heating season. Humidifiers, either portable or attached to furnaces, have the specific purpose of adding moisture to the air for health, comfort, and other considerations. The human body can give off over a pound of water per day to the surrounding air. Cooking, showers, and baths, add water to the interior air.

By far, the largest supplier of humidity to the interior air can be uncovered ground areas, such as in a crawl space. When open ground in a crawl space is not covered by a vapor barrier, such as a polyethylene sheet, there is an unlimited and continuous supply of vapors from ground water. The ground surface does not have to be wet or damp to supply moisture to the air because vapors from the underlying moisture will pass. The cost to correct this problem is low and relatively easy to execute. Just cover the ground with a 4 or 6 mil Visquene (polyethylene) sheet and try to cover all of the ground.

Ventilation Methods

Generally accepted guidelines call for a minimum 1 square foot of free ventilation area for each 300 square feet of attic ceiling area when there is a vapor barrier at the ceiling. When there is no vapor barrier over the attic ceiling the ratio changes to 1 to 150. Free vent area refers to the unrestricted space of an opening . The screen or louver covering reduces the opening, so the ventilation area has to be increased accordingly. For example, one eighth inch screening if clean restricts the opening by 25%.

Ideal design has placement of both high and low level vents. Good ventilation is provided by the arrangement shown in Figure 1 with soffits or eaves vents and roof vents. Ridge vents as found in many newer homes in place of roof vents will improve the ventilation. Less ideal but acceptable ventilation can be obtained with end gable vents and eaves vents as shown in Figure 2.

For homes without an overhang or eaves for the placement of eaves vents, it is recommended that low side roof vents and gable vents be added to complement the high side roof vents. The goal should be to minimize the dead air space in the attic.

Air moves naturally through the attic space because of two forces:

1. Induction or wind force

2. The chimney effect

Induction forces are in effect in attics with gable vents where winds can pass. The chimney effect is by far the biggest factor in air movement in attics. The movement of light warm air up thru a space and out a vent creates a vacuum which forces air up from low level eaves vents. This is the chimney effect. The greater the difference in height between the high and low vents the more movement of air will take place. Soffit vents alone or roof vents alone are inadequate but of course better than nothing.

For commonly found Cape Cod homes with 2nd stories defined by knee walls which create triangular cavities on the sides, it is still ideal to have high and low vents with an air passage over the insulation on the sloped ceiling. For homes with vaulted ceilings, there is still a need for ventilation except that all the air movement should be between the insulation and the subroof.

If you ever want to test the validity of the need for attic ventilation, the next time you see a house with a peeling paint problem check the ventilation. It is very likely that the house has inadequate ventilation. Many home owners with peeling paint problems have decided to cover the symptoms of poor ventilation with new siding such as aluminum. This aluminum covering on soffits will often give the illusion of vents with screen holes in the aluminum with no hole or vent in the wood under the aluminum.

Over time vents can become blocked with dirt, paint or birds nests. To be effective, these vents should be cleaned. The benefit to cost ratio for improving attic ventilation is so large that no conscientious homeowner should overlook the problem.

Robert V. Gallo P.E.
Technical Letter
Retrieved 05 December 2002

Controls Many House Problems
Old 12-05-02, 03:35 PM
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Humidity in the attic in winter

Motorized attic vent fans are usually activated by a thermostat, which should be set at about 105 to110 degrees F. When the temperature reaches the setting on the thermostat, the fan automatically activates and continues running until the temperature drops below the setting. This type of fan can also be activated by a humidistat. Ideally, attic fans would be controlled by a thermostat at the highest area of the roof and a humidistat at the lowest area of the roof, on the north side.

Moisture generated in a home will only cause condensation during the winter months. This condition can be aggravated if a homeowner seals the attic vents during the winter. Vents in an attic should never be closed during cold weather. With proper insulation at the attic floor/ living space ceiling, the ventilation will have little, if any, effect on heat loss.

Clues To Determine the Extent of Condensation Problems or Concerns

Rust on the nail points coming through the roof. The nails are in direct contact with the exterior and condensation will always form on the coldest, most dense material in the space.
Small, blackish stains on the plywood or similar sheathing, at the nails. As water forms on the nails, some of the moisture can be absorbed into the sheathing, causing the stains.

Water stains on the floor or deck in the attic, or in the insulation below the nail points. This is caused when enough water forms on the nails to start to drop off of the nails.

Mold starts to form on the north side of the roof sheathing, at the lowest point in the roof system. This is the first clue that may require action. Mold is a problem and proper ventilation is the solution.

The mold grows up the north side. When it is covering about 50% of the north side, it begins to grow up the south side. This is a serious condition and should be addressed immediately.
If the mold continues, it will become dense and start to delaminate the plywood or OSB sheathing, or deteriorate the wood sheathing. The sheathing may become black and wet, and actually look like it is raining in the attic. This condition usually occurs only when there is 10 or more inches of insulation and someone intentionally closes or covers all of the vents in the roof system.

If the condensation moves beyond the level outlined above, it will probably impact the roofing materials. This level will typically require replacement of the sheathing and the roofing.

Attics: The Importance of Proper Ventilation
From U.S. Inspect
Yahoo! Real Estate
Retrieved 05 December 2002
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