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Soffit Venting

July 19, 2014


Soffit Venting.

Last week I talked about the need to properly vent your roof; and how to determine how many vents you need in the roof to do it properly. That was posted on the Mighty House Blog page for you.

This week I want to talk about soffit venting. Why is it important? Because you can have the correct number of vents on your roof but, without the soffit vents they will not perform as designed.

Simple rule of thermal dynamics, for every cubic foot of air you want out you have to have a cubic foot of air enter.

So based on the rule 1 square foot of vent for every 300 square foot of attic, you need the same square foot of soffit vent.  What is the best vent to use? Like your roof there are a number of different styles to choose from. And like roof vents they are rated in “net free area”, NFA.

If you have aluminum soffits you should have perforated panels. The panels are typically come in two main types double 5 and triple 4 wide. So depending on the depth of your soffit the NFA will vary. Let’s keep it simple and say it’s a 12” deep soffit. The NFA of double 5 slotted soffit panels is 6.20 sq INCHES per foot. You need 144 square inches to make 1 square foot you need 23 linear feet of soffit at 12” deep to make 1 square foot of venting. Our example last week needed 5 square feet of roof vent. This means you need 115 linear feet of slotted soffit vent 12” deep to properly vent your house. Triple 4 full vent is rated around 14.34 square inches per foot. So you would need less than half.

If there is plywood soffit above the aluminum none of this means anything. It must come down.

If you have plywood soffits you would want to cut in vents. The best out there are 8” x 16” vents. They cost just over $2 each. They are a whopping 65 square inches, NFA. You would need to install 11 soffit vents by cutting a hole in the plywood, clearing any insulation that may block airflow in the soffit and over the wall plates and nail up a vent cover.

It has always amazed me how simple all this really is when you do it correctly but, rarely is it. For additional information or help call into the Mighty House Radio Show.


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