adding attic ventilation

DIYGST · 04-14-2010, 01:06 AM

Hi. I have a 1000 sq foot half of a duplex (divided attic)
So, I only have one gable end. For vents there is a standard 20x14 approx vent at the top of the gable end... with a gable fan thermostat controlled. one eye brow vent (maybe 1 sq ft) and 4 soffit vent (3"x22"/144) each. If I calculate correctly every vents added up to total around 4 sq ft. I want to install a 5000-8000 cfm whole house fan that requires 6-10 sq feet vents.

I was wondering if I could add another vent on the gable side to increase the surface area. I have redwood siding outside of attic and installation should be easy.

When the WHF is run the vent should open, but at day time when the gable fan is run in at the original 20x14 opening... it shouldn't draw air in the new vent since the shutters should be closed.

So anyway my question is if this is the easiest way to add a vent and does it cause problems to have 2 vents (1 being gravity shutter) on one gable side.

thanks in advance.

Phantom1910 · 04-15-2010, 07:28 PM

If what you want is a whole house fan with powered dampers, you should check out the airscape series of whole house fans.

**Gary in WA** · 04-15-2010, 08:24 PM

4- 3"x22" = 66 sq.inches OR the same as a 4x16" = 25 sq. inches x 4 = 100 sq.in. total soffit (intake) vents. You should have 1000/150 = 6.66 total 1/2 at soffits (3.33 x 144 = 480 sq.in.) and 1/2 in ridge or top of gable, 480 sq.in. You have a 20x14 (exhaust) opening with about 109 sq.in. NFVA. The gable and soffit vents are about 1/4 of the minimum required venting.
http://www.lomanco.com/intakemain.html

http://www.airvent.com/homeowner/pro...it-specs.shtml

http://www.buildingscience.com/docum...on?full_view=1

Be safe, Gary

DIYGST · 04-15-2010, 08:38 PM

Thanks for your calculation and assessment, Gary
I am not sure why the NFVA discount the openings so much, is it to factor in louvers and grills.

Anyway I think I am looking for whole house fan that can be ducted outside as an option. I only know of one product and that is quite expensive since it is an inline fan.

The airscape maybe positioned correctly aiming at the siding maybe I can cut open a vent so most of the wind will be escaping through it.

**Gary in WA** · 04-16-2010, 11:33 PM

"I am not sure why the NFVA discount the openings so much, is it to factor in louvers and grills." ------ Yes!

The eyebrow vent is short-circuiting the gable from drawing air from the soffits.

The WHF should not be a problem as it is an exhaust. BUT, sometimes gable vents act as intake and exhaust vents. The very best would be a continuous ridge vent with baffles at 18sq.in. per ft. and continuous soffit vent at 1/2 that number per ft. (times 2 sides)
Air sealing an attic is very important: http://www.rd.com/how-to-seal-attic-...icle18158.html

Be safe, Gary