0 Helpful?

Proper ratio for attic venting?

We have a very simple gable roof with flat ceiling that requires venting. the code says the unobstructed vent area shall not be less than 1/300 of the insulated ceiling area. The vents must also be evenly distributed on both sides of the house with not less than 25% of the required openings located at the top of the space and not less than 25% at the bottom.

Our plan is to use 2" continuous PVC soffit vent with a net free area of 6.9 square inches per lineal foot and to vent the top portion of the roof via gable vents. Given that the insulated ceiling area is 603 square feet and the length of the eave is 31.5 feet, what size do i make gable vent?

We want the attic space to be slightly pressurized, i.e., more NFA at the soffit location. My other concern is that the soffit vent will provide 3sq' of NFA whereas we only need 2 sq' total both eave and gable combined.

I would like to keep the gable vent as discreet as possible. By having more NFA at the eave than is required am i needlessly encouraged windwashing of the blown cellulose attic insulation?

The project is in the mountains with strong winds. ODD IS -31c and 5660 HDD. We have problems with blown snow accumulating on the top of attic insulation in this area.

Thanks again...

Asked by erik olofsson
Posted Oct 29, 2012 5:15 PM ET
Edited Oct 30, 2012 4:38 AM ET

Tags:

1 Answer

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.

Erik,
You'll get lots of different opinions when it comes to attic ventilation.

First of all, most experts recommend balancing soffit vents with ridge vents, not gable vents. Ventilation will be more dependable that way.

Second, have you read this useful article? Lstiburek’s Rules for Venting Roofs

If you are concerned about wind-washing near the soffit vents -- and you should be -- the best solution is to do a conscientious job building airtight insulation dams at the perimeter of your attic.

Answered by Martin Holladay, GBA Advisor
Posted Oct 30, 2012 5:09 AM ET

Other Questions in General questions

CC Spray Foam in Conditioned Crawl, Sill Gaskets, Windows, and Air Sealing

In General questions | Asked by Michael Geoghegan | Jun 30, 15

Trapped moisture on SIP roof

In GBA Pro help | Asked by Edward Mallett | Jun 29, 15

Attic hot after closed cell spray

In GBA Pro help | Asked by Mark hippchen | Jun 27, 15

New Unvented Attic and High Humidity

In General questions | Asked by Nick Schuler | Jun 30, 15

Foundation capillary break & waterproofing

In General questions | Asked by Martin Frank | Jun 30, 15
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!