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Shawner 10-22-2012 03:11 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.

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.

Found a site about calculating proper attic ventilation (, 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!

nealtw 10-22-2012 03:37 PM

If the white soffet was part of the original construction the cedar would not be there. If you are taking it down to drill more holes, I would just remove one peice of cedar all the way across
The lower roof is vented below the windows with 3 holes for bay, I think I would want more there and maybe better protection for that venting.

Shawner 10-22-2012 03:51 PM

Don't think the aluminum was original. Think it used to have cedar shake roof, they probably put the soffits on when they sheeted the roof and put asphalt shingles on.

Good plan on removing one chunk, lot easier than cutting lots of holes.

nealtw 10-22-2012 04:13 PM

I would take a close look at the lower one too. Those little hole give you min. venting at best but the top board looks level so you may have water sitting way to close to those windows. I'm sure something better could be done.

oldognewtrick 10-22-2012 05:30 PM

There is a exhaust vent that is applied from above. It's like half of a shingle over ridge vent, then you put a metal "L" flashing over the top of it. One brand is called Smart vent.

AndyGump 10-22-2012 05:36 PM

I think you would be better off making the high end (exhaust) the 60%.

If fact, I know it would be.


nealtw 10-22-2012 06:20 PM

Andy: You lost me, a little more detail?

Wuzzat? 10-22-2012 07:16 PM


Originally Posted by Shawner (Post 78812)
Intake = 60% = 331 sq in

sqrt(331) = ~18" on a side for a square vent. You may want to make two vents that add to this area.

I'd redo for the 300 ratio just to see what kind of latitude you have on these dims.


Originally Posted by AndyGump (Post 78823)
I think you would be better off making the high end (exhaust) the 60%.

If you're thinking stack effect that may already be in the 60/40 split.

AndyGump 10-22-2012 07:49 PM


Intake = 60% = 331 sq in
Exhaust = 40% = 221 sq in
This is what he stated.

Should reverse those percentages.


oldognewtrick 10-23-2012 04:36 AM

First thing to check is the ability to free flow air from the eave to the peak, because if it's stuffed with insulation all the venting in the world will not help.

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