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Old 12-16-2011, 02:40 PM   #1
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Cathedral Nightmares


Good afternoon all!
I'm having issues and am running out of ideas. I have an older 2 story house and the top floor is a cathedral ceiling; 6/12 pitch, 30" knee walls, 4x8 rafters on 32" centers with center ridge beam. Floor area is ~450 sq. ft. When we bought the house, it had finished exposed beams/rafters with 2" of what I believe to be stone wool insulation w/white textured surface between and applied directly to the bottom of the roof sheathing. The roof was unvented and had condensation problems galore.
This fall I had to replace the sheathing and shingles due to rot. The plan was to vent the new roof, insulate between and enclose the beams. The roof already had 14.5" soffits full length on the eaves, so I opened up the top of the walls between the rafters and installed a GAF Cobra ridge vent according to their instructions. I've run Duro-vent baffles full length with the recommended 1" space for vapor escape.
My intention was install r-19 insulation between the beams and cover the whole thing with 1/2" foil backed rigid foam. However, I'm having problems with condensation dripping off of the nail tips where they protrude through the sheathing. I've already stripped off the insulation/baffles once and enlarged the eave openings to try to get more airflow, made sure the soffits were clear of obstructions, and cleaned the mold that was developing on the underside of the sheathing. It helped considerably, but the problem hasn't stopped. If anyone has any thoughts or suggestions I would greatly appreciate it cause I'm running out of hair to pull out.

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Old 12-16-2011, 03:53 PM   #2
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Post up a picture of the interior of the room.

How many penetrations are there? What is the climate zone?

When you say that your intention was...are you planning on doing a re-roof?

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Old 12-16-2011, 07:14 PM   #3
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The first mistake was to try and use that style product.
All my roofing supplyers have stopped selling it, for one thing it's a pain to install and not have it end up looking like a snake eating a rat.
It also does not allow enough air flow.
This style works far better. http://roofing.owenscorning.com/home...protector.aspx
Any ridge vent I've seen or use calls for 2" on each side of the roof peak to be removed not 1".
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Old 12-17-2011, 01:36 AM   #4
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Hmm... So the Cobra isn't recommended. That would certainly explain a lot, like why the mold only started a little over half way up the roof. The exhaust restriction is creating green house zone due to lack of flow? Nice. I wondered about the ridge opening when I was cutting it as it seems pretty small IMO. Figured they knew better than I.

Sorry, not able to post pics. I'm in MN, so that puts me in zone 5-6, which means I should be looking for R30-R38 insulation, and possibly more. However, due to the placement of windows, I'm not able to build down the rafters to accommodate the thickness, so I've gotta make due with what I've got. And yes, it has already been re-roofed and I'm working from the gutted interior. Right now I'm kicking myself for not adding an extra rafter between the existing ones to reduce the wide spans.
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Old 12-17-2011, 09:55 AM   #5
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I looked a home this week that had similar issues as a result of the cobra vent.
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Old 12-27-2011, 09:29 PM   #6
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Joe, if you check the GAF steep slope installation guide, you will see a cut back of either 3/4" or 1-1/2" depending on whether or not there is a ridge board.

Even so, you might check out the Cobra Snow Country, the regular Cobra rolled ridge is not my favorite. It is similar to the rigid vent joe gave the link to.

Finally, did you say you had soffit venting? Is the 14.5" vented? I couldn't tell.

Last edited by Gary in WA; 12-27-2011 at 11:08 PM. Reason: removed web link- belongs under sign. line.
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Old 12-28-2011, 10:04 AM   #7
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I don't mind Snow Country, but prefer Air Vent's Shingle Vent II which has some pretty compelling smoke test demonstrations comparing it to what you have on there now:

Or you could forget about the ventilation and spray foam the underside of the decking, which would have the best chance of eliminating the condensation problem long term IMO.
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Old 12-28-2011, 11:04 AM   #8
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Quote:
Originally Posted by jmiller View Post
I don't mind Snow Country, but prefer Air Vent's Shingle Vent II which has some pretty compelling smoke test demonstrations comparing it to what you have on there now:

Or you could forget about the ventilation and spray foam the underside of the decking, which would have the best chance of eliminating the condensation problem long term IMO.
Careful, Jim. The "hot roof" application you suggest (spray foam the underside of the decking) can be a disaster if you don't do it right. It takes a lot more than spray foam to do it right.

I looked at a job done by a DIYer that was leaking/condensing/utterly failing that was done with just spray foam. The "fix" was about 10K to undo what he had done. Not pretty.

Check out http://www.buildingscience.com/ and search on "hot roof".

Otherwise, I agree that AirVent makes a nice product, though I think the snow country is easier to find at the home store.
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Old 12-28-2011, 11:57 AM   #9
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Quote:
Originally Posted by jmiller View Post
I don't mind Snow Country, but prefer Air Vent's Shingle Vent II which has some pretty compelling smoke test demonstrations comparing it to what you have on there now:

Or you could forget about the ventilation and spray foam the underside of the decking, which would have the best chance of eliminating the condensation problem long term IMO.
I can't stand the cobra vent roll out stuff.

Have you guys used the lomanco stuff of the OC ridge vent?
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Old 12-28-2011, 04:08 PM   #10
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Careful, Jim. The "hot roof" application you suggest (spray foam the underside of the decking) can be a disaster if you don't do it right. It takes a lot more than spray foam to do it right.
Yeah, I forgot to mention it would be best to call a pro.
Quote:
I looked at a job done by a DIYer that was leaking/condensing/utterly failing that was done with just spray foam. The "fix" was about 10K to undo what he had done. Not pretty.

Check out http://www.buildingscience.com/ and search on "hot roof".
What exactly was the problem? Not using closed cell?

I love the building science site, but haven't seen the unvented assembly caveat you're alluding to. I'm also curious how you'd insulate it without being able to fur the rafters or insulate above the decking.

@WoW, No I haven't tried either of those. Well, maybe the Lomanco metal ridge vent
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Old 12-28-2011, 05:18 PM   #11
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What exactly was the problem? Not using closed cell?
He didn't use enough, and he didn't have any other plan to create a thermal break to the roof.

"I saw this builder do it on TV" was his comment.

It's not a situation that lends itself well to trial and error ... at least if your initial goal was to save money.
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Old 12-29-2011, 07:40 AM   #12
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Anytime you are doing a hot roof deck, it is all about dewpoint and moisture control.

Inadequate levels of open cell foam and no moisture control/air tight ceiling = rot.

We have done several roofs without above deck insulation or furring out the interior. The most recent one was 2x12 construction so you could get goo insulation depth in there to start with.
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Old 01-02-2012, 12:43 AM   #13
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I apologize for the delay. I'd really like to stay away from the whole "hot roof" and spray foam option if at all possible. I think when spring gets here I'm going to swap out the cobra for one of the better designed vents mentioned, as well as explore the possibility of enlarging the ridge opening just a hair more.

I do have a question regarding insulation options. I read on inspectapedia about increasing the air space in the rafter bays to 3" instead of the usual 1.5" with baffles. Was also wondering about ditching the fiberglass R-19 and replacing it with stacked 2" non-foil backed rigid foam, such as the Owens Corning R-10, and positioning it with furring strips attached to the inside of the rafters? I'm including the link I found about the "High Labor" method, and am curious if one can eliminate fiberglass and baffles altogether. http://inspectapedia.com/interiors/I...l_Ceilings.php

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