roof venting cathedral ceiling

Pete H · 08-14-2006, 08:29 PM

Hi everyone, This is my first post here. I am glad I found this forum.
I believe you guys (and gals) can help me and I think I will be able to help some of you.
I am insulating my garage. I am leaving the attic space open to the garage. This is where the unit heater hangs to conserve space.
I want to insulate the sloped part of the truss. I have 2in. foam insulation (4x8) board which is meant for commercial roofs under the
rubber roof membrane. I attached 4 sheets to the trusses and soon
realized that I am not venting the roof. I have created these cavitys
between the roof truss which open to the soffet at the bottom but no
opening at the top. I could Have a ridge vent installed for hundreds of dollars which would work...Right? But my idea is (and my question)
Why cant I cut the foam board to fit tight between the trusses and
against the roof sheathing??? Would moisture still get to the wood.
Would condensation accumulation on the insul. board be a issue?
The heater would only be used on occasion when its very cold

Bonus · 08-14-2006, 10:48 PM

The condensation issue is one thing, you will see any condensation as (if) it forms, and the foam is a vapour barrier. The other issue is the temperature of the roof in summer. Venting keeps the roof cooler. What is on the roof? Asphalt shingles? Shingles will last longer if you have venting.

Pete H · 08-15-2006, 07:57 PM

Thanks bonus, So if the foam was adhered to the wood then heat is
a issue. It reduces shingle life? I did a search and found this article
saying you could pack a cathedral ceiling tight with insulation to stop any air movement and cover it with gypsum board and have had no problems. Im thinking the ridge vent might be safer bet though.

AaronB · 08-15-2006, 09:08 PM

wELL, THE CURRENT SPECULATION IS THAT FOAM DIRECTLY UNDER THE ROOF DECK FOR SHINGLE INSTALLATIONS MAY NOT BE SO BAD, AND IS EVEN ACCEPTED BY ONE SHINGLE MFG THAT i KNOW OF.

Bonus · 08-16-2006, 12:09 AM

That is my accumulated wisdom, for what it's worth, which I suspect is not much. This stuff has not, IMO, been turned into a science yet.

I love the expression "THE CURRENT SPECULATION IS THAT ... MAY NOT BE SO BAD" Doesn't that make you feel all warm and cozy? If they'll warranty the shingles (for what that's worth) then ok.

Sorry if this isn't helping, Pete.

Pete H · 08-17-2006, 08:19 PM

www.nuwool.com/products/technical/cathedral.html

Thanks for the replys, would you guys mind reading this and telling me what you think. It seems my idea is not much different.
I appreciate your proffesional opinions.

Bonus · 08-18-2006, 12:36 AM

If they are gonna warranty the thing then...

It's not what I was taught, but Aaron knows more about it than I do I'm sure, and knowledge evolves.

AaronB · 08-20-2006, 09:03 PM

Ill come back and read that when I have more time.

AaronB · 08-21-2006, 09:39 PM

You could have a few turtle vents installed. Thats pretty simple, low cost, and functional.

Pete H · 08-25-2006, 08:26 PM

So I decided to have a ridge vent installed for this cathedral
ceiling type of application. There is a 2x6 at the peak where the trusses meet and my insulation board. So each cavity between the trusses have soffet at bottom and 15.5in of ridge vent at top. The guy has about 1.5 in. cut back from the 2x6 on one side but less than 1/2 in.
on other side. What size is adequate opening at top? He has less than a
1/2 in in many places and is comming back to increase.Is 1.5 in enough?

Pete H · 08-25-2006, 08:33 PM

By the way, I asked a general contractor about packing a cathedral
ceiling w/insul and not venting it...He said thats the way they did the
house he bought and the roof failed after 2 yrs. He had to tear it all
off and reinstall WITH venting

AaronB · 08-26-2006, 12:54 AM

Well , all I know is what I read. One of the shingle mfg's is saying that closed cell SPF under the deck is just fine...no moisture gets at the roof deck to delaminate the plywood.

Im probably wrong. Will have to wait until the NRF opr NRCA issues a bulletin.

Log · 09-24-2009, 09:30 PM

http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf
This link will explain some of the SCIENCE study history behind roof venting.

AaronB · 09-24-2009, 10:49 PM

Did he say what the cause of the failure was? I bet he used either open cell foam or fibrous insulation.

Closed cell foam will not allow the condensation issues at the deck cause it is a vapor barrier at 2", and creates a thermal break, cold on the cold side, warm on the warm side, and no moisture present at the dew point, which is located within the depth of the foam.

Different isssues, better solutions. Rose Roofing & Sheet Metal.

AaronB · 09-24-2009, 10:53 PM

Also, closed cell foam does not act as a heat sink applied to the deck because it does not hold heat... it merely stops it.

This has been one of possible issues that the anti-direct-to-deckers have proposed in the past.