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Ventilation Explained

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Go to Ventilation Calculator to determine how much attic ventilation you currently have.

Understanding attic ventilation

A whole house fan works by pulling in large amounts of air into the house through the open doors and windows and blowing all that air into the attic. The air increases the pressure in the attic and then finds its way outdoors through the holes or vents that lead outdoors. (Go to our Ventilation Calculator page to see the different ways to vent an attic.)The fan will work under any circumstance, that is, the blade will spin and some air will move through the system. Without enough venting, the fan just will not work as well as it can.

Imagine a bus with two entry doors in the front and only one exit door in the rear. More passengers can get onto the bus than can leave. The result is  congestion and delay. That's exactly what happens to the air molecules trying to get into the attic. 

Without enough venting from the attic to the outdoors the fan cannot work properly. Some air will be able to flow out the vents but the rest will be compressed. The fan acts partly as a ventilator and party as an air compressor. This creates back pressure on the fan, preventing it from working effectively, causing noisy operation and reduced fan life. Larger fans need more ventilation than smaller fans, so that proper ventilation is important for quiet operation, long fan life, and effective operation.

How venting is described There is an exact phrase that describes attic ventilation. That phrase is: Net Free Area (NFA). Net free area simply means the measurement of the real opening of the vent, after all obstructions have been deducted. 

Even bug screening will reduce the net free area of a vent by 25%! Look at the screen covering a window. Now imagine pushing all those thin wires together. You will see that a substantial amount of the screen opening is reduced by those small wires. When we speak of the ventilation needs of a whole house fan, we always refer to the net free area requirements, usually in square feet.

Why do attics need ventilation?

A. The attic, being an enclosed area, can be prone to moisture buildup. If there is no way for this moisture to evaporate, the roof and attic structure can be affected and long term damage created. It would also be possible for mold to grow in the attic and possibly adversely affecting the occupants.

Why don't I have enough ventilation?

A. It's usually just not that important, so most attics are under ventilated and suffer no problem. Ventilation is not installed to cool the attic, only to prevent moisture problems. Often attic ventilation does not even meet minimum building code requirements, but this rarely creates difficulties. However, when adding a whole house fan the lack of ventilation is often the cause of excessive noise, reduced air flow and shortened fan life. It is often necessary to add more ventilation for the fan to function at full capacity. 

A whole house fan pulls a large amount of air into the house through the open doors or windows and ultimately all that air is blown into the attic, through the shutter in the ceiling. It is the size of the shutter that indicates the minimum amount of window opening as well as the minimum amount of attic exhaust venting required. Air in should equal air out.

If there is not enough venting, the fan will still function, but not as well as it would with adequate ventilation. Imagine driving a car in first or second gear: it would take you everywhere you wanted, but without the performance built into it and with more stress and wear and tear on the engine. The same is true for an undervented whole house fan. With enough venting, a whole house fan will run effectively for many years. Without it, it will never perform as it should, be more noisy and prone to premature breakdown.

Roof Ventilation is the type of ventilation we prefer to use: it is relatively attractive, inexpensive and as many vents can be installed as necessary.  Leakage is not an issue. The vents are completely guaranteed not to leak for 20 years!

The best roof vent that we have found is the Pop Vent™. It is a passive, engineered exhaust ventilator, not just a cover for a roof opening. It prevents wind from entering the attic, and allows hot air and moisture to leave. It helps to protect and extend roof and shingle life.  

   * Pop Vent may be painted to match existing roof extrusions

    *Each vent provides 1 square foot of net area exhaust venting

    *Seamless spun cap

    * Stamped vent louvers to prevent bug entry and provide maximum ventilation

    * Screws hold cap on for easy attic entrance

    * Durable all aluminum construction to last the life of the roof, paintable

    * Lifetime guarantee 

    * Compact; unobtrusive, blends in well with other components on roof

Why we don't like other venting solutions

Gable Vents can be installed in the triangular ends of the attic. Usually there are only two gables. After the hole is cut and the vent unit installed, half or more of the original hole is eliminated by the frame, slats and screening. This makes it difficult to achieve enough venting using gable vents. Some companies put several vents into each gable, but most homeowners find this unappealing. In addition, gable vents added after the house is built are fairly unattractive unless they are in the rear or hidden from view.

On some homes, those with metal or concrete tile roofs, for example, we sometimes are able to install a large, motorized gable vent. An electric motor automatically opens the shutter when the fan runs and and closes it when the fan is turned off. This system makes the entire hole available for air flow from the attic and a single shutter is enough to provide sufficient venting for the fan. If you have such a roof and want more information, please contact us. (Photo on this page).

Soffit (overhang) Vents would be great, except that with the higher levels of insulation in attics, most soffits are now filled with insulation, blocking the existing vents and making it difficult to add new ones that actually allow the air to exhaust. Cleaning them is labor intensive and often not very effective. Also, the soffit vents are small and 3 to 12 of them are necessary to achieve 1 square foot of venting.

Ridge Vents are becoming more popular, but only provide 10 to 15 cubic inches of air per linear foot. A 30 foot long ridge vent provides a little more than 2 square feet of net area opening; 45 linear feet will provide 3 sq, ft., and so on. Without a sufficiently long roof line, ridge vents alone do not provide sufficient fan ventilation.  

If you want to determine how much ventilation your house now has, go to our Ventilation Calculator Page.



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