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 burnt03 10-22-2012 04:09 PM

Cathedral Ceiling - How much venting do I need?

Guys, meet my roof. Well, part of it anyways. The raised part is the cathedral section.

http://imageshack.us/a/img688/4563/img3500x.jpg

Here's the exhaust side of the venting. Aluminum soffits are covering up cedar with holes drilled into it. Haven't pulled them down yet to see exactly how much there is.

http://imageshack.us/a/img21/5564/img3501bi.jpg

Found a site about calculating proper attic ventilation (http://www.trinityexteriorsinc.com/1...n-5-easy-steps), wondering if you could check my calcs below and see if I'm correct.

Area = 25' x 23' = 575 sq ft

TFNVA = 575 sq ft / 150 = 3.83 sq ft

Convert to inches = 3.83 sq ft * 144 = 552 sq in

Intake = 60% = 331 sq in
Exhaust = 40% = 221 sq in

So if I pull down the soffits, count the number of joist bays I have, figure out the size of hole I need to cut in each to total 331 sq in on the intake side, I should be good, right? If that's ok, should I staple in some screen material over the holes to keep the critters out or do the soffits do that already?

I'm not positive but I don't think I have to worry about baffles (don't think the insulation is up that high in the "attic")

Thanks for any help!

 shazapple 10-22-2012 07:39 PM

Any reason you are concerned? There would have to be 12 1" holes per foot in order to have adequate ventilation. I'm going to guess that isn't the case. If you cut a 1" wide slot the entire length you should meet your vent area.

 burnt03 10-22-2012 08:03 PM

I had pulled a piece of soffit down on the exhaust side to check out where a bat had been living and noticed that there looked to be 1 - 1 1/2" hole every foot or so, so like you say, nowhere near the amount of venting needed.

Cutting a strip out is a way better idea than a bunch of random holes, thanks :)

 Grumpy 10-22-2012 08:12 PM

Ditch the soffit vent. Look into the Air Vent, Inc. Peak Filter Vent. I commonly use it on projects like yours. It is continuous so will give you maximum exhaust. You should also look into some intake.

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