I need help with attic ventilation design. My attic has two triangular gable vents and two 12" whirlybirds along the ridge on the longest part of the house running north and south. The gable vents of course are at the ends, the north side does not have a soffit to install a vent. There are no soffit vents anywhere else. I am going to add soffit and ridge vents on the areas with ridges running east to west, which are the garage and kids bedrooms. Should I install soffit vents on the south side below the gable and on the northwest side for the master bed? If I should install soffit vents, should I cover the gable vents or leave them as they are?

Drawing of house
http://farm1.static.flickr.com/145/429465585_f745ca79a3_b.jpg

 

The best ventilation will be achieved by providing the lowest possible inlet for air and the highest possible exit. With that said, of course installing soffit vents will aid in getting the lower air moving. I would not block off any gable vents. I am not a big fan of the whirlybird type vents. IMO, a properly installed ridge vent is far superior for moving the air.

 

I called for an estimate on ridge vent installation. The guy said since I only plan on staying here another 5 years that I should just add 2 more 12" turbines, maybe add a ridge vent over the garage area, and 14-19 soffit vents for cost effectiveness and do it myself. They can't come out for less than $500. Also that I shouldn't have ridge vent where the whirlybirds are located since the whirlybirds would become intakes. I have 1500 sf of attic space over the living areas and 2000 sf including the garage. I wish I knew how much any additional attic ventilation is going to save or how much less heat the house will gain making the summers more comfortable in the house. Does anyone know of any savings calculators for ventilation like this one for radiant barrier - http://www.ornl.gov/sci/roofs+walls/radiant/rb_05.html ?
NFA
I checked the NFA of the gable vents and the 2 turbines. Each gable vent has 367 sq. in. NFA. Each turbine has about 90 NFA. Currently with the Gable vents and 2 turbines I have 914 NFA exhaust. With 2 more turbines I have 1194 NFA exhaust. I read somewhere that 8"x16" soffit vents have about 64 NFA. So that would mean I need 18 soffit vents for the intake to match the exhaust. For 2000 sf at 1/150 the NFA needed is 1920. 960 for intake and 960 for exhaust. For 960 intake I need 15 soffit vents.
COST
Each turbine = $40. 2 turbines = $80. Each soffit vent = $1.98. 15 soffit vents = $30. Ridge vent = $2.50/foot. Ridges are 110 feet. 110 x $2.5 = $275. I could use a combination of ridge vent (east/west=60 ftx2.5=$150) and 1 whirlybird ($40) for a total of $190. nah. 2 turbines + soffit vents = $110 is already enough spent. Besides I have to replace the turbine heads I already have. Those are $20 a piece. Making subtotal attic costs $150. I have also bought radiant barrier at about $230 for 1500 sf. Plus an electric stapler and jigsaw. Total = $456. ouch. At least I got that much from my tax return.