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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 Mon, 10/29/2012 - 16:15
Edited Tue, 10/30/2012 - 03:38

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1 Answer

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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 Tue, 10/30/2012 - 04:09

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