Attic Venting for Whole House Fans

Whole house fans depend on adequate attic venting to function properly. As they pull cool air in through open windows, they force hot air into the attic, where it must have a means of escape. Without enough venting, there will be too much resistance and pressure will build up in the attic which can dramatically reduce your airflow. In extreme cases, you can get attic air forced back into the house through ceiling fixtures and other small air passages.

So proper attic ventilation is critical. The good news is, most modern building codes specify more than enough. Still, we recommend confirming your attic venting capacity before installing your AirScape. And if you have already installed your AirScape and it doesn't seem to be effective for you, this is something you definitely want to check.

AirScape Roof Venting Requirements
AirScape model	1.0 WHF	1.7 WHF	2.5 WHF	4.5 WHF
Net free area required	2 sq ft	4 sq ft	5 sq ft	9 sq ft

How to calculate if you have enough roof vents

First, understand net free venting area

Most roof vents have screens on them to prevent insects and debris from entering the attic. These screens typically block up to 50% of the airflow capacity for that vent. So if you require 5 square feet of "net free area", this means that you will to have 10 square feet of total venting area in order to achieve your goal.

Identify and measure your roof vents

You need to identify and measure all the roof vents in your home. This can be trickier than it sounds as there are various types of vents and not all of them are in view or accessible. Where you can't measure, just estimate for now.

flat roof vents: these are the most common in newer homes, placed on the roof slope near the top. These are typically 12 inches square, with a screen, so each one counts for 0.5 square feet (or 72 square inches).
gable vents: these are often found in older homes on the side of the house, and vary greatly in size. Measure or estimate the size, calculate square feet and then, as these are usually louvered and screened, divide in two.
ridge vents: these usually run the entire length of the roof peak, and can be hard to spot. Look for a slightly raised peak and small vents at the ends. typically, ridge vents offer about 1 sq ft of net free area for each eight linear feet of roof peak. So if your ridge vent is 40 ft long, count five square feet.
under-eaves vents: these are found on the under-side of your eaves, where the cladding will have many small holes or slats to provide ventilation. Typically, you'll get about 50% of total gross area as usable ventilation -- same as screened vents.
whirlybird vents: these are the round bulbous vents that catch the wind to provide a measure of active venting, which can increase their effectiveness. But for our purposes, to be conservative, just count the square footage of roof space that the unit occupies.

Doing the calculations

Once you have measured all your vents, it's time for a little math. Simply calculate gross area for each one, adjust for screening and then total up your net free area. A few tips:

if you measure or estimate in feet, be sure you are multiplying your fractions correctly! Remember, 1/2 times 1/2 equals 1/4 ! (e.g. 0.5 x 0.5 = 0.25)
if you measure or estimate in inches, just multiply out the area of each vent, add it all up and divide by 144
remember to account for vent screens and louvers by cutting the gross area in half to calculate your "net free area"

If you are having trouble determining your venting capacity, we may be able to help: call us!

TIP: Keep your attic cool and get even better natural cooling results

When it comes to providing venting for a whole house fan, placement of your attic vents doesn't really matter. However, a well-designed passive attic ventilation strategy can minimize heat buildup during the day when the sun is heating up your attic and your fan is not running. The idea is to create a convection current that lets super-heated air out the top and draws cooler air in from below. This is best achieved with a combination of ridge venting at the peak and under-eaves vents on the bottom. This combination can lower your daytime attic temperature significantly, which can greatly impact the heat buildup inside the home. Talk to your roofing contractor!