# Cathedral ceiling - how much venting do I need?

Discussion in 'Roofing and Siding' started by Shawner, Oct 22, 2012.

1. Oct 22, 2012

### Shawner

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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 (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!

2. Oct 22, 2012

### nealtw

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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.

3. Oct 22, 2012

### Shawner

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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.

4. Oct 22, 2012

### nealtw

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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.

5. Oct 23, 2012

### oldognewtrick

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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.

http://www.dciproducts.com/html/smartvent.htm

6. Oct 23, 2012

### AndyGump

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

If fact, I know it would be.

Andy.

7. Oct 23, 2012

### nealtw

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Andy: You lost me, a little more detail?

8. Oct 23, 2012

### Wuzzat?

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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.

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

Last edited: Oct 23, 2012
9. Oct 23, 2012

### AndyGump

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This is what he stated.

Should reverse those percentages.

Andy.

10. Oct 23, 2012

### oldognewtrick

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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.

nealtw likes this.
11. Oct 23, 2012

### Shawner

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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?

12. Oct 23, 2012

### nealtw

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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.

13. Oct 24, 2012

### Shawner

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Opened it up on the intake side and saw this:

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:

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?

14. Oct 24, 2012

### oldognewtrick

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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.

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15. Oct 24, 2012

### nealtw

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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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