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Attic ventilation is very important. Ridge vents and attic ventilation fans help to control the high temperatures that can build up in your attic in turn heating your home in the summer heat. Ventilation also helps to protect crawl spaces from damage caused by excessive heat and moisture.

attic ventilation

Continuous ventilation is extremely important to avoid roof rot, reduce heating and cooling costs, and to maintain warranties of some asphalt shingles. Why was attic ventilation never an issue in older homes? Building practices have changed. If you are building new or updating an older home, give your home “lungs” to breathe through air exchange in roof or ridge vents and soffit vents. To keep your attic cool, your attic ventilation requires moving massive amounts of air. More than natural ventilation will do. Attic ventilation fans or turbine roof vents will assist in moving the air. But, powered attic fans can draw your air-conditioned air out of the house.

Turbine vents can replace the hot air in your attic in minutes. Mother Nature blows across the fins in the roof turbine vents as they exhaust the heat build up.

Attic ventilation is an important part of roofing. Proper attic ventilation extends the life of a roof and reduces problems because it minimizes the temperature differential between the attic and air outside. Proper ventilation will remove moisture and heat from the attic.

A balance circulation system will have one square foot of venting for every 300 cubic feet of free air in attic. There is a ratio used to find how many vents you need. You should have an equal distribution of intake (soffit) and out-take (roof) vents for circulation to be effective. Intake vents which are located at the underside edge of roof, known as eaves; allow fresh air flow in the attic. Exhaust vents which are located near the ridge (top crest) or directly on the ridge line of the roof, depending on the vent type; allow the air to leave the attic. Both intake and exhaust vents must be in place, and unobstructed in order for system to work.

How much attic ventilation do I need for my attic?

Air Vent recommends 1 square foot of attic ventilation for every 150 square feet of attic floor space divided evenly between intake vents at the roof’s edge or in the overhang or undereave and exhaust vents at or near the peak of the roof. Here’s what the formula looks like for a 1200 square foot attic space: 1200 divided by 150 = 8 square feet of attic ventilation. Then divide that number by 2 to provide half the ventilation for intake and half for exhaust. Thus, 8 divided by 2 = 4 square feet of attic ventilation for intake and 4 square feet of attic ventilation for exhaust.

The final step is to figure out how many vents would be needed to provide 4 square feet of attic ventilation. To do this let’s start by converting the number to square inches by multiplying by 144. Thus, 4 x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust. Air Vent ridge vents provide 18 square inches of Net Free Area per linear foot. To determine how many feet of ridge vent would be needed the formula looks like this: 576 divided by 18 = 32 feet of ridge vent. A typical 8″ x 16″ undereave vent provides 56 square inches of Net Free Area per vent. To determine how many undereave vents would be needed, the formula looks like this: 576 divided by 56 = 10.2 (which can be rounded up to 11).

If using Air Vent’s The Edge™ Vent, ProFlow Vented Drip Edge or continuous soffit vents simply double the amount of needed ridge vent linear footage because all of those intake vents provide 9 square inches of Net Free Area per linear foot.

I have attic ventilation but why doesn’t my attic seem to be any cooler?

With any attic ventilation system, the attic could be about 20 degrees hotter than it is outside. Attic ventilation is meant to protect the roof sheathing, insulation and shingles from temperature and moisture extremes. However, many variables will affect the temperature that the attic can reach. For example, the color of the shingles will have a large impact on the temperature. Black shingles will cause the attic to be much hotter than if white shingles were used. Other factors are the geographical location, intensity of the sun, orientation of the primary roof plane, and amount of total ventilation NFA.

What products do you have that will work on a flat roof?

All Air Vent products are designed for sloped roofs with a minimum 3/12 pitch. Unfortunately, Air Vent products are not designed for flat roofs.

What size nails should I use?

Taking in to account the size of the felt paper, shingles and roofing deck, the nails should always penetrate the roof deck. If the nails do not penetrate the roof deck, the vent can start to pull up the roofing nails.

Do I need to oil the motor on my fan?

No, it is not necessary to oil the motor. The bearings are pre-lubricated for life.

Ridge Vents

What size slot do I cut for your ridge vent?

For ridgepole construction, cut a 3/4″ slot on each side of the ridgepole. For truss construction, cut a 1.5″ slot at the peak of the roof.

What is the purpose of the filter in the ridge vent?

Air Vent’s internal weather filter is designed to be a secondary layer of defense against weather, dust and insect infiltration in the attic.

Do I have to close off my gable vents when I use a ridge vent?

Yes, the gable vents (a type of exhaust vent) should be closed off whenever a ridge vent (which is also a type of exhaust vent) is installed because vents installed at the roof’s edge or in the overhang should supply the intake air needed by the ridge vents. Air should flow in through the intake vents evenly along the roofline and exhaust out the peak. Any vents in place between the ridge vents and the intake vents may interrupt or short-circuit that flow of air along the roofline. The gable vents will end up becoming intake for the ridge vent — an undesirable situation that could lead to weather infiltration through the gable vents and also could prevent the attic from being properly ventilated. The same is true with mixing wind turbines or roof louvers with ridge vents.

How does the external baffle work?

As wind hits the external baffle it is forced over the ridge vent creating a low air pressure above the vent openings on the ridge vent. This low pressure above the vent openings allows air to be pulled from the attic through the exhaust vent — enhancing the vent’s performance. There’s a special name for this phenomenon. It’s called the Bernoulli Effect and it’s the same principal that gives lift to an airplane.

Can I install a power fan if I have a ridge vent already on my roof?

Mixing a power vent with a ridge vent can short-circuit the attic ventilation system just as a gable vent can as discussed above. This happens because air follows the path of least resistance. When the power vent turns on, it can pull air from the ridge vent, which could lead to weather infiltration and unbalanced airflow along the underside of the roof deck. When the power vent turns off, it acts like a roof louver — an opening on the roof without a motor. In this scenario, the ridge vent pulls its intake air from the power fan leading to possible weather infiltration and less than optimal ventilation along the underside of the roof deck.

Basically, mixing two different types of exhaust vents on the same roof that has a common attic is not recommended because it can lead to short-circuiting of the attic ventilation system.

Can I use your ridge vent on the hips?

No. In fact, Air Vent does not recommend using ridge vents on the hips because water may flow down into the ridge vent.

Intake Vents

How important is intake ventilation for ridge vent?

Intake ventilation is very important for all types of exhaust vents, including ridge vents. Each attic should have an equal amount of intake ventilation low at the roof’s edge or in the overhang or undereave and exhaust ventilation at or near the peak of the roof. This is known as a balanced ventilation system.

If there isn’t enough intake ventilation, for example, the ridge vent can pull air from one side of itself right out the other side. Picture a wave going into the vent and right out. The problems with ridge vents pulling air in one side and out the other is that it will only be ventilating the very peak of the roof instead of the entire underside of the roof deck. Furthermore, it could bring into the attic precipitation from outside.

I don’t have any overhang. Can I use roof louvers or roof pots near the bottom of the roofline?

A roof louver (they are also called roof pots or can vents) placed low on the roof can lead to weather infiltration because the roof louver is not designed to be an intake vent. As water flows down the roofline air is coming into the roof louver. This can result in water entering the vent. In situations where there isn’t an overhang, Air Vent recommends using The Edge™ Vent or ProFlow Vented Drip Edge—both of which have been designed for homes with little or zero overhang.

I have a knee wall. What should I do?

In a knee wall situation the goal is to get the intake airflow past the wall. There are two ways to do this.

  1. If the roof deck is being replaced, install insulation baffles in every rafter from the overhang or undereave level to the peak of the roof. This will allow an even flow of air along the roof deck that will flow past the knee wall.
  2. Slide wood slats up the underside of the roof deck (for short spans only) and then flip them on edge to force down the insulation. Pull down the fascia to allow access to the underside of the roof deck from outside the house. Someone can feed the wood slats from outside while someone in the knee wall area guides the slats toward the ridge.

Power Vents

My power attic ventilator has burned out completely. What do I need to check to make sure this doesn’t happen again?

Check the intake ventilation. Make sure that the attic insulation has not covered the intake vents preventing air from feeding the power vent. It might be necessary to either insert insulation baffles or to pull the attic insulation back to ensure the intake vents are not blocked.

Also check to make sure that the plywood the intake vents are installed in have actually been cut or opened to allow for air to flow into the vent.

Finally, make sure that the power vent has enough intake vents. To determine how many intake vents the power vent requires, take the CFM (cubic feet of air moved per minute) of the vent and divide it by 300. This will determine the square feet of intake ventilation needed. Take that number and convert it to square inches by multiplying by 144. For example, 1500 CFM divided by 300 = 5 square feet of intake ventilation needed. Converting to square inches looks like this: 5 x 144 = 720 square inches of intake ventilation needed for the power vent.

If you don’t know the CFM of the power vent or the net free area of the intake vents, check with the manufacturer.

My power attic ventilator runs all the time. Is there anything I can do?

Set the humidistat higher. The numbers on the humidistat control run from 1 to 9, which is 10% to 90%. If it’s set too low, the power vent will run all the time. Set it around 8.

Can I use Air Vent power attic ventilators around chemicals?

The motor on Air Vent power vents is not closed, which means chemicals could affect the motor. Air Vent recommends not using power vents around chemicals.

Can I use a gable-mount power attic ventilator to vent a workshop, restaurant, dry cleaners, etc?

Air Vent gable-mount power attic ventilators are meant for attic spaces in residential houses. The reason they will not usually work in an open space is the power vents need an outside flow of air. The power vents pull air from outside through intake vents at the roof’s edge or in the overhang or undereave. If the power vents don’t have that outside flow of air, the motor will burn out.

Why do I need a humidistat with my power attic ventilator?

The thermostat control on a power attic ventilator monitors the attic solely for temperature or heat buildup. Thus, the power vent will only run in the hot summer months. During the winter, moisture can build up causing a reduction in the effectiveness of your insulation. The humidistat allows the power vent to run in the winter, which will keep the moisture from building up.

My turbines aren’t spinning. Why?

Generally, if standing outside while the wind is blowing the turbines should be spinning. If the turbines spin by hand but not in the wind then the bearings are probably too tight. Using an internally braced wind turbine, be sure that the upper bearing is straight and tightly fastened to the crown plate (top plate) of the turbine. With externally braced wind turbines, make sure that there is about 1/8″ of the shaft showing between the top of the upper bearing and the underside of the brace plate (plate that the 3 external braces rivet to at the top of the turbine). In both cases be sure that the retaining cap on the bottom of the lower bearing assembly is snapped securely in place and not crooked.

Whole-House Fans

Can I mount the whole-house fan vertically?

Air Vent whole-house fan cannot be mounted vertically. There are several reasons for this. First, the shutter will not open or close in a vertical position. Second, the fan blade will eventually crack over time. Third, if mounted on a vertical wall facing the outside the fan will pull the air from outside into the attic rather than attic air being vented to the outside. Unfortunately, the fan blades and motors are not reversible. If the motor is reversed, it will burn out. If the fan blades are reversed they will crack. The warranty on Air Vent whole-house fans requires installation in a horizontal position.

Why is the shutter for my whole-house fan a different size from the whole-house fan?

The rough opening that is cut for the shutter is a different size so the whole-house fan can be pushed through the opening from inside the hallway rather than try to get it through a smaller attic opening.

Why do I have to cut a joist for the belt-drive fan?

The louvers on the shutters all open up towards the fan. If the joist is not cut, the louvers will hit the joist and get bent.

The shutter on my whole-house fan rattles and is very noisy. What can I do?

Open more windows. Sometimes there is not enough air moving towards the shutters and the louvers cannot open all the way. Try is putting felt strips or rubber stripping on each louver to give the louvers a little weight and catch the vibration from the whole-house fan. Check to make sure there is enough clearance above the fan. A minimum of 30″ of clearance above the whole-house fan blades is needed. Without this clearance, the air will be forced back through the shutter. Check to make sure there is enough “exhaust” ventilation, including both intake vents at the roof’s edge, overhang, undereave, exhaust vents high on the roof at or near the ridge. Without enough “exhaust” ventilation, air will also be forced back through the shutter. It might be necessary to add a 2″ x 4″ to the housing of the fan for more clearance between fan and shutter.

Can I use a rheostat with your fan?

If a rheostat is used on Air Vent fans, the motor will burn out.

My fan is vibrating a lot. Anything I can do?

Remove the fan blade and see if the motor still vibrates. If the vibration is gone, it means the fan blade is out of balance. A bearing could have been knocked loose if it still vibrates.

Can my control on my fan be mounted upside down?

As long as the controller is in the flow of the fan, it can be mounted upside down.

My whole-house fan will only run on low. What is wrong with it?

Check to make sure theres enough “exhaust” ventilation, including intake vents at the roof’s edge or in the overhang or undereave as well as exhaust vents high on the roof at or near the ridge. Possibly there isn’t enough “exhaust” ventilation for the whole-house fan to run at the higher setting.

Call the manufacturer to find out the net free area of all the vents in your attic (intake vents and exhaust vents). Most net free areas are less than half the size of the vent. A 12″ x 12″ gable vent’s unobstructed airflow is 56 square inches even though the opening measures 144 inches. Calculate the unobstructed airflow of all the vents in your attic.  If they equal the minimum requirements for the whole-house fan, the fan should run on high.