Catherdal ceiling ventilation, insulatio
fredth
| Posted in Energy, Heating & Insulation on
I am about to build a home with a cathedral ceiling in northern Maine. Right now I am thinking of going with 2×12 roof rafters and insulating in between with glass and then using 2″ rigid on top, then having a 2†air space with a ridge vent for ventilation, sheathing the roof and shingling. My concerns are moisture control and ventilation. Is this the right configuration for this type of roof? Any knowledgeable info would be appreciated.
Replies
Add soffit vents and you are fine. Make sure all the seams in the foam panels are sealed tight - no penetrations for moisture to leak up
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Fred, a year and a half of lurking has provided you with quite a few threads on this subject.
Is this your own house?
If so, bite the bullet and go with spray foam.
If you can't afford that, dense pack cells.
Forget the FG, forget the vents.
Joe H
Please, NO. Blow it full of foam, if you have th $$$$$$. Just did my own home. 16' cathedral. Got drips of water coming down from ceiling. Did alot of reserch. PPack it full to the deck. No vent, ridge or sofit. I did it wrong. Hadn't done before. Trust me , pack the cavity full with whatever!!!!!
Nailer01, Mark
Fredth
Checking for understanding here, as I am about to do the same project on Cape Cod.
From top down:
Shingles
Deck
2"air gap vented w/soffit and ridge vents
2"rigid foam
FG batts
Drywall
My questions:
Kraft faced batts for warm side vapor barrier?
What type of 2" foam? It seems like it should be vapor permeable to let insulation dry to outside?
Cheers
Chris
...around, around the Mulberry bush...
'Nemo me impune lacesset'No one will provoke me with impunity
I agree, it seems like this discussion is frequent when you search for it, but having exactly your situation addressed is is the beauty of posting to this forum.My bldg inspector requires a vented roof-and I'm looking to save a little money with the FG batts. Also planning can lights, so the ides of foam on the outside to stop air infiltration with FG around the cans is appealing.Thanks for your thoughts
Yes, i posted on an insulation question 68747.1 and realize it will die a solo death in that such contention arises without absolute clarity on the issue, folks just get gunshy or are sick about dealing with it.
The whole issue is tied to the vent/ don't vent controversy which has war stories here that would fill volumes.
I'm thinking the book hasn't been written yet.
Cheers
'Nemo me impune lacesset'No one will provoke me with impunity
I'm finishing mine up at Moosehead. Plaster going on now. I learned vent the roof or throw out the shingle warranty. 2X12 roof joists. 2" air space. Dense pack f/g then covered with 1" foil faced Thermax with taped joints and nails. God I hope it's right with the money I've spent for everything below the roof. Spent nearly 3000.00 in propane (vented) and kero drying the place prior to insulation. It's been a good year for building, lousy for snowmobiling.
The spray foam was my first plan. Not at double and almost tripple the cost. I even called the IKO rep. His reply was no vent, no warranty with spray foam.
Fred,
Unlike you, I took the time to research the voluminous archives on this subject. One gem that I kinda figured out for myself is that the answer to the question has a lot to do with what climate you are in. Since you didn't fill out a profile, I can't tell.
I'm in the SF Bay Area. I figured out that my needs were tremendously different from a guy in the Northeast.
Advocate
Thanks for the info; to clarify a few things Maine is in the Northeast. I am on a budget so I going with FG batts and rigid insulation. I am definitely going to vent the roof. But I am still not sure If I should go with rigid on the inside (behind the sheetrock) or outside (below the 2" air space) I also do not know If I should use kraft glass or not. I have just started my "research" and it seems that there are a lot of contradictory theories out there even when you are talking about the same climate. Thanks
Edited 1/27/2006 10:02 am ET by fredth
My bad; I must have had a brain fart at the same time I was reading your post; I didn't notice that you had said Maine. :-(
Yes, there is some contradiction in the threads, but, hey, what do you want for free?
You might also check out the JLC site.
Advocate
Why not use the rigid to create your vent channels? Two birds, one stone, doa.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
I did that in mine- 1" EPS panels with spacers to form a vent channel between rafters. Venting was required- no way the inspector would go with a non-vented roof.
Unanticipated problem though: because I didn't get the soffits on before winter, the addition is still exposed to "wildlife". There are no holes big enough for racoons, and the squirrels can't get into the main house, and the addition isn't heated, and there's no food in there of any kind, so I figured no problem. WRONG! The squirrels have discovered the vent space and are tearing the sh*t out of the foam panels to make nests. I get my Icynene installed Monday, so I'm on the squirrel warpath at the moment!
What was it that the Sex in the City character said? "Squirrels are just rats in nicer outfits?!"
Rats in nicer outfits! I like it. I think of squirrels and strawberry-eating chipmunks more as moving targets, myself, and then as disposal problems.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Andy: Mr. Nice Guy left the building a long time ago. The original plan was to live trap them and take them to work and release them, but my neighbour's live trap was a complete failure. Food doesn't seem to be in short supply- they're stashing walnuts in the addition, so why would they go into a cage looking for peanut butter! The spring trap was also a failure- guess the black squirrels are too big for it, unlike the red one that took up residence in my out-building a few years ago. The ultrasonic gizmo didn't seem to do much, the glue traps haven't caught anything yet, and the chemical warfare has just started so we won't know the results for a few days. The Icynene guys come tomorrow- wish me luck!
The good news is that so far they've been leaving my workshop alone. The bad news is that they're focusing on the ceiling above my wife's future studio. Guess they know which one of us really hates squirrels- she was trapped in an outhouse with one when she was about three and has had a morbid fear of them since!
Maybe you should be talking to Gunner about trip wires and rodent-sized Claymores?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
The ultrasound thingy seems to be working today (I moved it much closer to the problem area), so the electronic warfare seems to be working. Or maybe it's the fact that it's not raining today. Either way, no squirrels last night or this morning. Let's hope it stays that way until spring!
Icynene truck's on its way right now- wish me luck!
For those of us in the Northeast-
If you use rigid foam to make the vent channel are you creating a cold side vapor barrier, or can the insulation still dry to the outside thru certain types of foam?
Cheers
Excellent question. Yes, you can be. But it's done all the time with foam wall sheathing. The thinking is that the foam should stay warm enough on the inside not to reach the dew point. That said, I'd advocate 1 in. EPS - Styrofoam beadboard - which allows some vapor migration over either the pink or blue xps or polyisocyanurate (Ultra - R).Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
I've got to ask you what you think about the organization that recommends exterior foamboard with fiberglass batts between the studs as a 'best practice'?.
The video in question is under 'insulation' foam sheathing on exterior walls.
http://www.hgtvpro.com/hpro/pac_ctnt_ihdr/text/0,,HPRO_20976_28891,00.html?cat=4&vid=832
'Nemo me impune lacesset'No one will provoke me with impunity
Edited 1/30/2006 12:27 pm ET by razzman
I don't think it's possible for any insulation system that relies on fg to be considered best practice. I think spray foam is best practice. I think cellulose is the best compromise between cost and return. I think foam sheathing on the outside improves the overall wall performance of any insulation by short circuiting the thermal bridging of the studs. It also creates detailing issues for the siders, and shear issues for the engineers. Foam might best be used inside, under the drywall, to both eliminate thermal bridging and perform as an air and vapor barrier.
TANSTAAFL.
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Edited 1/30/2006 2:50 pm ET by Andy_Engel
So, no concerns of creating a double vaporbarrier with that exterior foamboard over fiberglass batts between the studs?
be TANSTAAFL That's All Now Stand Tall And Always Fart Loud?
'Nemo me impune lacesset'No one will provoke me with impunity
No, because the theory is that foam will never get cold enough to reach a dewpoint. That said, I wouldn't advocate an iterior vapor barrier in any but the coldest climates. Although that new interior housewrap stuff by DuPont is kind of interesting.
There Ain't No Such Thing As A Free Lunch. R. Heinlein.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Fred-
To repeat from previous message. I just built one in Michigan.
Year one.
According to my builder/framer, I vented (sofit and ridge with baffles).
r-38 craft faced and then t&g wp4 pine.
Year two. Got rain coming down from ceiling during winter (15 to 25dg).
Year three. Plugged up sofit vent with the theory to stop air flow. Theory of mine was can lights (as with you) creating heat in the cavity. Freezing and then melting. Creating frost, melting and dripping.
This winter..........no dripping, no nothing.
Spring.....I will a) remove ridge vent and, or permantly remove sofit vent.
CATHEDERAL= NO VENT.
From Expierience, Mark, nailer01
I dont think the "experts " have figured it out. Too many opinions and too many micro climates. My neighbor did his cathredral without vents as the inspector OK'ed, later he noticed carpenter ants coming from the eaves. The inspector said" well you built it too tight". Its vented now, but after the fact its hard to achieve the desired results.This is on the Oregon coast. My father in law put up unfaced batts (R 30) as recommended by the lumber yard..."na we dont use faced stuff anymore" with no vapor barrier added and I wasnt surprised when he said "we got ice up on top of that insulation" . The attic wasnt vented very well also.This is in central Washington with hot summers,fairly cold winters and about a foot of precipitation a year. Let me know what you decide to do.
fredth,
Use SIPs!
Structural insulated panels
Great efficent and cheap!
would you like details?