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Old 09-22-2010, 01:42 AM   #1
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Insulating a cathedral ceiling...please help


Hi -

I live in NH and have a house built in 1949. I am in the middle of a living room renovation and discovered there was no insulation. It is a hot roof design...there are no eave or ridge vents and installation of eave vents is not possible due to the design of the roof/wall connection.

I have installed framing members 16 inches on center to the ceiling and walls in preparation for insulation. Now, what to use? The ceiling will be tongue and groove pine.

I am leaning towards Roxul insulation, and I am also thinking of adding rigid foam board between the rafters and the tongue and groove pine.

My question is, would I still need a vapor barrier if I used Dow Super Tuff-R which seems to have vapor barrier properties already? So, from the inside looking up, I guess I'm thinking of pine, foam board, roxul insulation. Will this work? Should I have a plastic vapor barrier in there somewhere?

Thanks!
Mike

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Old 09-22-2010, 02:03 AM   #2
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Insulating a cathedral ceiling...please help


These are the instructions directly from Dow. Makes sense to me. Thoughts? Only difference is that I will not have the inch of ventilated space.

Cathedral/Vaulted Ceilings – Installing STYROFOAM Square Edge, RS, Tongue & Groove, DURAMATE™ Plus, TUFF-R™, Super TUFF-R, or THERMAX
(Gypsum board exposed on inside of ceiling with rigid insulation between gypsum and wood framing)

1. Install batts between rafters in the conventional fashion, leaving at least an inch of ventilated air space below the plywood/OSB roof deck.

2. Install vapor retarder on the underside of the rafters. This could be 4 mil poly or kraft facer on batt insulation.

3. Nail insulation to the underside of the rafters over vapor retarder.

4. Install drywall finish over the foam insulation. Drywall screws will need to be long enough to compensate for the thickness of the foam insulation.

5. Avoid penetrating the vapor retarder when installing light fixtures.

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Old 09-22-2010, 07:40 AM   #3
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Insulating a cathedral ceiling...please help


If you have a vaulted ceiling with no soffit vent and no ridge vent you are asking for severe problems. That roof/ceiling has got to breath or you will be tearing it all off and starting again as it will sweat. I have seen this too many times personally. Each rafter chase should have one of the styrofoam deals that will allow the air to move upward and out of the roof system. There must be some way to install vents at the low end and upper end of the roof.
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Old 09-22-2010, 10:55 AM   #4
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Insulating a cathedral ceiling...please help


i know venting is the correct method, but there are several websites to include buildingscience.com that discuss hot roof designs.

my roof has never been vented and there are no signs of moisture problems. all of the sheathing looks new after 50 years.

it is what it is at this point. i have no way to install soffit/ridge vents without drilling a bunch of holes and both compromising the structural integrity of my roof and leaving the door open for water intrusion.

so, like i said, i have no vents and am looking for ideas on insulating the roof. buildingscience.com recommends spray foam followed by rigid foam on the underside of the decking if you are going with a hot roof design. due to the cost of spray foam, i was thinking about using Roxul insulation, which does not store or transfer moisture and is completely resistant to mold growth, between the rafters and topped with rigid foam insulation as recommended by Dow.

any thoughts (besides telling me to add vents)?
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Old 09-22-2010, 11:37 AM   #5
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Insulating a cathedral ceiling...please help


Quote:
Originally Posted by mklang View Post
i know venting is the correct method, but there are several websites to include buildingscience.com that discuss hot roof designs.

my roof has never been vented and there are no signs of moisture problems. all of the sheathing looks new after 50 years.

it is what it is at this point. i have no way to install soffit/ridge vents without drilling a bunch of holes and both compromising the structural integrity of my roof and leaving the door open for water intrusion.

so, like i said, i have no vents and am looking for ideas on insulating the roof. buildingscience.com recommends spray foam followed by rigid foam on the underside of the decking if you are going with a hot roof design. due to the cost of spray foam, i was thinking about using Roxul insulation, which does not store or transfer moisture and is completely resistant to mold growth, between the rafters and topped with rigid foam insulation as recommended by Dow.

any thoughts (besides telling me to add vents)?
I am from the old school and new and better materials and ways are coming forth everyday that I am just up on. I am sure some of the younger fellows will be here soon to help you. I am just not in the loop to know how to tell you about something I just haven't heard of. I will for sure do some studying up on that method.

Let me tell you about an experience I personally had on my own house. I built a vaulted ceiling in my house (which was early in my career) and the soffit was vented but the ridge wasn't. In the winter it got cold and being nice and warn inside, it looked like it was rain running down the walls. My sheet rock was ruined so back to square one after tear out. I had a friend who did the same thing except his floor was ruined also from the water from the ceiling and he lost his decking also. When we tore into the ceiling there was frost completely covering the underside of the decking.

I have been doing this kind of work for over 40 years but I am just not up on the new stuff. I am sorry I can't help you, I wish I could.
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Old 09-22-2010, 12:26 PM   #6
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Insulating a cathedral ceiling...please help


i do appreciate the insight and your time. from what i can tell, the key to a successful hot roof design is that the roof is completely sealed from any air/vapor intrusion.

like you said, i think folks have trouble when air can get in but not get out...like having a ridge vent with no soffit vent, or vice versa. everything i have read indicates that a poorly ventilated roof will do much more harm than good and, because of my funky roof design, i know that i will not be able to ventilate it properly.

because my roof looks great and has never been vented, there must be some truth to this. the one thing that does concern me is that it wasn't really insulated before (just a 1/2 layer of homosote between the roof sheathing and the pine). as i am adding insulation, it will make it hotter, but as long as it is air tight i think i will be okay.

thanks again!
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Old 09-22-2010, 02:22 PM   #7
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Insulating a cathedral ceiling...please help


If absolutely impossible to ventilate it, follow the Building Science as you read. Air seal the cathedral on the inside, use Roxul and foam board (stops the moisture from condensing on the cold roof sheathing). http://www.buildingscience.com/docum...on?full_view=1
Also read BSD 135, and BSD 102 at the bottom of the pages under "related" articles.
You may find this interesting as well, an "Oldie, but goodie": http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf

Gary
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Old 09-22-2010, 03:20 PM   #8
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Insulating a cathedral ceiling...please help


Quote:
Originally Posted by GBR in WA View Post
If absolutely impossible to ventilate it, follow the Building Science as you read. Air seal the cathedral on the inside, use Roxul and foam board (stops the moisture from condensing on the cold roof sheathing). http://www.buildingscience.com/docum...on?full_view=1
Also read BSD 135, and BSD 102 at the bottom of the pages under "related" articles.
You may find this interesting as well, an "Oldie, but goodie": http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf

Gary
Well I will be, that is interesting reading. That has been around a long time, I must have been living under a rock all my career or something. Thanks for the links Gary.
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Old 09-23-2010, 02:28 AM   #9
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Insulating a cathedral ceiling...please help


thanks for the help everyone!

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