How many attic fans should i have

Comfort Cool. You can get your square footage by your floor length x width. Multiply by wall height and you have CFM. If you take that number and multiply by 1. As a general rule, your roof needs 1 square foot of vent area for every square feet of attic space. Thus, if your attic is square feet, you need roof vents equaling 3 square feet. A whole house fan is effective whenever the outside temperature is lower than the indoor temperature - typically in the evenings and at night.

A whole house fan is mounted in the attic drawing cool outside air in through open doors and windows while forcing the hottest indoor air out the existing roof vents. Attic Fan Ventilation However, if your attic has blocked soffit vents and is not well-sealed from the rest of the house , attic fans will suck cool conditioned air up out of the house and into the attic. This will use more energy and make your air conditioner work harder, which will increase your summer utility bill.

Often the property's HVAC system duct-work is routed through attic spaces, which, even though they are insulated to protect the house below, will still get very hot when in the direct sun or the heat of high summer. There aren't a whole lot of downsides to installation.

Essentially, you can use your AC unit and your whole house fan together. However, if the air outdoors is cooler than the air indoors, it doesn't make a whole lot of sense to turn your air conditioning system on.

A whole-house fan is usually installed on the attic floor near the center of your house. In the late evening or early morning, the fan is turned on to exhaust hot air from the house. Cooler outdoor air enters through open windows, lowering the indoor temperature. Only use your fan when the outdoor air is cooler than your indoor air. We recommend running the whole house fan all night.

Install electric ventilators and attic fans, which remove hot air from an attic. They have thermostats that turn the fan on at a recommended preset temperature of degrees.

Alternatively, install passive vents such as gable, soffit and ridge vents, which are openings in the roof that allow hot air to escape. More Attic Ventilation is Good Insufficient ventilation can lead to moisture problems during the winter and decreased energy efficiency during the summer but too much ventilation can be just as bad, if not worse.

Most attic fans operate with a thermostat that automatically turns the fan on and off based on a temperature the homeowner sets. Most manufacturers and contractors recommend a temperature setting between and degrees Fahrenheit. The formula above gives an approximate value of the CFM needed for an attic fan.

Some professionals multiply the square feet value with 0. So for example, if you have an attic area of sq. For an area of sq. This is often used as a quick reference point. However, calculating the exact CFM for the attic fan takes a few more steps than this. An attic fan is used to ventilate the attic. That means the air inside the attic needs to be exhausted to reduce attic temperature.

The reduction in this temperature depends on how many times the air is changed inside the attic which is termed as air changes per hour ach. The volume of the attic space can be easily calculated using length x breadth x height. If it is a sloped roof the calculations differ 0. The number of air changes depends on how hot your attic gets. But generally, it is 6 to 8 times per hour. As mentioned before it gives an approximate size for the attic fan.

Practically no attic is 10 feet high. If you replace the height in the above formula with 7 ft you get Formula 1 in effect. Depending on the number of air changes and the height of the attic the first formula makes sense. If your house has a steep roof then you will need a more powerful fan. The minimum attic fan size for a steep roof is CFM per sq. For example, if you have a sq. For an attic with a dark roof, the minimum size required for the attic fan is CFM for every sq.

So if you have an attic of sq. So far the calculations are for determining the size of the attic fan. Very quickly the entire mass of the ceiling structure also becomes a giant heat radiator. This heat moves back up towards the roof again and will continue to radiate heat well after the sun goes down. If your ceiling is not sealed airtight and extremely well-insulated, much of this heat is going to radiate downward into your home too. Air in the attic that comes in contact with the surfaces of the hot framing, will become heated through conduction.

This creates a kind of hot air sandwich formed between the roof and ceiling structures. The warming of all these surfaces is why the attic interior gets so much hotter than the exterior temperature.

Help diagnose hard-to-find heat-loss areas, and pinpoint water leaks behind walls with new Thermal imaging cameras. Read More…. Trapped air and heat in the attic would cause problems for some roofing materials. It could also cause condensation leading to Mold A deep penetrating fungus that grows under moist humid conditions. This is why attic spaces are required to have ventilation built into the structure. Air openings are placed along the lower eave areas and the upper roof ridge or gable end.

These openings allow fresher and cooler outside air to circulate naturally via convection up from the eaves and out at the gable or Ridge The uppermost horizontal line of the roof. This is where an attic fan can help. A mechanical fan working in an attic space equipped with adequate ventilation openings can move hot air out and increase the flow of air through the attic.

This will indeed help reduce the temperature in the attic, but only marginally unless the incoming air from outside is substantially colder not likely in the summer till nightfall You also need to exchange the air quickly. For instance, if the attic is degrees and you circulate air through it that is coming in at degrees from the outside, then you can expect that the cooling effect may bring the attic temperature down to perhaps degrees.

The faster the air changes, the closer you can get to the incoming air temperature. But you cannot get lower than There are formulas for this that engineers use in determining heat transfer and the proper amounts of air flow needed.

The problem with attic fans is in how they are marketed. Homeowners need to know that fans can only be effective if sized properly and mated with the proper amounts and placements of ventilation openings. Fans will only lower temperatures a bit in the attic, and will require some power to do so. Still, a little improvement may be better than nothing. If you consider using an attic fan, you will need to also decide as with any solution suggested here if the benefits are worth the cost in installation and operation.

This will be a topic for another article. A hot attic space is bad enough. But if you have equipment, ducting or piping up there, working in elevated temperatures, you can expect a stressful and likely shortened life for those items.

If you have ductwork, furnaces, air conditioners, water heaters, water piping etc. The point here is that if you have a hot attic space, it is going to shorten the life of any equipment residing there. Even though an attic exhaust fan can incrementally lower the temperature of a very hot attic, using a fan does not stop the source-radiant heat.

During the day, any cooler air brought in by the fan will be heated up immediately by the surrounding structure. Most fans cannot keep up. Any cooler air brought in from outside will eventually lower the the attic structure temperature, but that will happen VERY slowly. As soon as the sun rises in the morning, the radiant heating process will start again. It was only rated at CFM meaning it would take forty five minutes to replace all the hot attic air with outside air just once.

It could not keep up with the radiant heat gain. Steps you can take to cool down your attic — and your whole house! Seal off any air leaks at the ceiling. This will also prevent excessive moisture from migrating in and out of your attic.