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roofman

06:08PM | 09/27/05
Member Since: 09/26/05
2 lifetime posts
Bvbasement
I had a new roof installed on my house. I went with a ridge/soffit vent combination instead of the standard roof vent. Is it true that I now need to block off the gable vents? Also, can I use my whole house fan with a ridge vent and blocked gable vents?

bravey

04:15PM | 09/29/05
Member Since: 06/23/04
161 lifetime posts
No, don't block off the gable vents, they are still needed to exhaust air from the whole house fan. The ridge and soffit vents alone can't handle the volume.

All residential ridge vents are now made of a plastic honey comb or irregular plastic spaghetti that has between 40% and 60% free area. I can no longer find metal ridge vents with hardware cloth. The actual open area for air transfer in the best plastic ridge vent is approximately 18 square inches per foot (9 s.i. per side). Some are as low as 10 s.i. per foot. An average 2000 sf house has about 75 to 100 lf of ridge (some have more, some less). This means that total free area is about 100 x 18 / 144 = 12.5 sf of vent area. Compair that to a standard 36 inch high gable vent on a 6:12 roof which has about 13.5 sf of free area (75% effective free area). A house usually has two gable vents, one at each end, therefore 13.5 x 2 = 27 sf of vent area. The ridge vents offer less than half the free area of gable vents.

The best combination is to leave both ridge and gable vents open for summer ventilation to reduce heat and moisture. In winter, you could close off the gable vents and still have enough ventilation to extract moisture from the attic. The soffit vents however, should be left open year round as they won't allow any more air to enter than the ridge vents will allow out.

Regards

roofman

08:45AM | 10/02/05
Member Since: 09/26/05
2 lifetime posts
I was told that the ridge/soffit vent combo required that the gable vents to be blocked. With the gable vents open, will there be total air circulation in the attic? Or will the air circulate only from the gable vent area into the ridge vent?

bravey

10:59AM | 10/02/05
Member Since: 06/23/04
161 lifetime posts
Roofman:

With both the gable vents and the ridge vents open, air will exit from both in a greater quantity than from either alone.

With gable vents closed, the ridge vents discharge slightly more air than their contribution in combination with the gable vent simply because there is no other place for the rising convection air to exit thus placing a slight amount of increased pressure to aid the discharge.

The objective here is to discharge as much hot air fron the attic as possible. Neither the gable vent nor the ridge vent is "the best solution for all situations". Ridge vent vendors tend to phoo-phoo the competition simply because they are interested in promoting only their product. It's sort of like a Ford owner puts down Chevy products and vice versa, when in fact, they both have good points and bad.

In my view, the ridge vent industry needs to manufacture a vent with twice the free air area (30 sq. in. per foot) than is now available. In commercial applications we use custom-made metal ridge vents with 48 to 72 sq.in. of free area. Don't be confused by total area and free area. With the plastic grid/mesh openings, only about 40% to 60% of the grille face is actually open or "free" for air to pass through. Additionally, those openings are narrow and deep which causes a lot of surface drag or friction. This friction further restricts the air flow.

Try to see through the hype. Good common sense is usually more accurate than someone trying to sell something.

Regards
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