Cathedral ceiling - how much venting do I need?

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Shawner

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Guys, meet my roof. Well, part of it anyways. The raised part is the cathedral section.
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.
img3501bi.jpg



Found a site about calculating proper attic ventilation (http://www.trinityexteriorsinc.com/1284/calculate-proper-attic-ventilation-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!
 
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.
 
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.
 
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.
 
I think you would be better off making the high end (exhaust) the 60%.

If fact, I know it would be.

Andy.
 
Shawner said:
Intake = 60% = 331 sq in
Click to expand...
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.

AndyGump said:
I think you would be better off making the high end (exhaust) the 60%.
Andy.
Click to expand...
If you're thinking stack effect that may already be in the 60/40 split.
 
Last edited:
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.
 
AndyGump said:
This is what he stated.

Should reverse those percentages.

Andy.
Click to expand...

The link stated the following: "Calculate the intake and exhaust venting percentage based on 60% intake vents (soffits and eaves) to 40% exhaust vents (near the roof ridge). You could make simpler calculation of 50/50, but most experts recommend a 60/40 split."

That's incorrect?
 
I think if you dig into the science of venting and calculating and percentages, you would find that it's all about a regular attic space and how you want to move the air thru the attic. Keep in mind that a shed roof has a very limited air flow path in cubic inches. It also has two heat sorces in the winter so I think your best is just to figure as much as possible in and out.
 
oldog/newtrick said:
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.
Click to expand...

Opened it up on the intake side and saw this:

img3509o.jpg

Great! They cut ventilation in when they did the soffits, guess I was worrying for nothing!!


Then I cut it out further so I could stick my head in:

img3510un.jpg

Pulled that piece of insulation down and it looks like it's insulated to the sheeting, I assume over the length of the roof.

Olddog, like a prophet.

Not planning on doing anything about it until I re-roof (roof has been on for 10 years or so, no major issues as far as I can tell). What can I do? Pull off the sheeting, shim it and re-sheet?
 
You can put a vented ISO board over the top of the existing when you re-roof. It consists of a ISO board, an air space and decking on top.
 
By your last picture it would appear that the rafters are 2x6s I suspect that they are 2x10s and the bottom has a flat section sitting on the wall and the section in the eaves have been cut down to 2x6 just for the look. I think if you could reach further in you would find some space between the insulation and sheeting and you might find that air shoots were installed in every other or every third bay.
It doesn't make sense that the same people who put down new insulation and sheeting and then took time to cut holes in old soffet if they new air could not get thru.
 
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