Log Home cathedral ceiling insulation
user-121395
| Posted in Construction Techniques on
Hi all. New to the forum and need some advice. Renovating a log home. Changing the cathedral ceiling detail from existing, which is purlins, t & g pine, rigid foam, firring strips, sheathing, felt, asphalt shingles. Replacing with typical roof system consisting of 2 x 12’s over the t & g, high density R-38, baffles, sheathing, felt, shingles. There will be no typical sheetrock as a thermal barrier. Is this a potential moisture problem and if so, any suggested techniques as an alternative? Thanks much.
Replies
I can't say I think of sheetrock as a thermal barrier. I would ventilate the roof, continuous soffit vents and a continuous ridge vent with Propa vents running the length of the rafters on top of the insulation.
Beat it to fit / Paint it to match
Thanks, Hammer. Planned on those details, also. Just want to make sure I'm on solid ground with the T & G alone under the kraft-faced. Don
If your just going over the t&g wy not use stress skin panels. It woul save a lot of work.When we use them we put 1x3 or 1x4 straping and a layer of 1/2 adavntech.
Over the panels to create a air space never had any problems. Alot faster than re framing the whole thing.
Using airtight recessed lighting and didn't think I could efficiently detail the skins around the cans and wiring to remain energy-wise. Am I wrong?
use high density spray foam and you can skip the air space. Have done it a number of times without any problems.
Edited 3/9/2006 10:29 am ET by watersea66
Edited 3/9/2006 10:30 am ET by watersea66
Considered the spray foam. Is there any special consideration(s) to take into account, given that it would have to be sprayed on the top of the t & g vs. the typical underside of the roof deck?
If i understand you, you're leaving the purlins and the tounge and groove and you're using 2X12 so you can get enough insulation. If i was doing it i'd make sure there were no gaps or holes for the foam to expand through or put down felt and than use 2X8 rafters
Welcome, Captian Don!
First I'd ask what's wrong with the existing system? Sounds pretty good unless I'm missing something.
If you do replace as you suggest - you ought to have a proper air barrier between the conditioned space and your unconditioned vent channel. T&G isn't really a good air barrier the the FG is definitely not an air barrier.
I think this article from Paul Fisette is similar to the setup you are proposing and illustrates the problem with T&G as the air barrier:
http://www.umass.edu/bmatwt/search/faq_articles/faq_insulation.html
Best of luck.
high density spray foam acts as a vapor barrier, as does dense pack cellulose
Watersea66,High density spray foam is a vapor retarder. But dense pack cellulose is not.Take a look at :
http://www.buildingscience.com/designsthatwork/buildingmaterials.htm
Thanks, Bob - You're onto my dilemma. Changing to typical framing for increased insulation, recessed lighting, and for transitioning into new addition area. Aside from the standard sheetrock/tape application, what do you suggest for an air barrier? Would poly, housewrap, or felt paper applied over the T & G work in conjunction with the kraft-faced insulation? And forgive my ignorance, but what is FG that you refer to? Thank you once again.
There are a lot of variables here, Don. You should fill out the part of your profile indicating your location - where are you? Location really affects these decisions greatly.How much and what kind of foam is ontop of the T&G now? What is the total R value?FG is fiberglass. A lot of people are down on fiberglass here from a performance point of view.You currently have no rafters - correct? So by adding rafters and putting insulation in between you will introduce thermal bridging from your conditioned space to the roof sheathing via the rafters (wood is a lousy insulator). You should also take a look at the buildingscience.com website. They have some excellent papers on roof design.Also is this house occupied year round? Just in the winter, just in the summer?
Edited 3/9/2006 12:16 pm ET by BobS
Thanks again, Bob. We live in NW New Jersey. Existing insulation was 3.5" of 1978-vintage styrofoam. Upon disassembling the roof structure, it was a nightmare. It had obviously not been detailed correctly and/or underinsulated. There was clear evidence of condensation, mold, insects chowing down on the foam, etc. The house will be occupied year-'round. I want to install recessed lighting, so replacing the existing foam insulation didn't seem feasible to me. I'll research the building science website. Thanks again. Don
Recessed lighting can create lots of problems because its a slight heat source in your attic and can allow air to get to the unconditioned space if not detailed well.
The system you had can be reworked and perform better than most others. If you do decide to revamp and go the traditional attic route - you'll need an air barrier and not a vapor barrier (like poly sheeting). Vapor barriers are only for much colder climates than yours. A vapor retarder like the kraft facing on FG insulation would be more appropriate.
Best of luck. There's a lot of info in the archives too on this topic.
Edited 3/9/2006 9:58 pm ET by BobS
Thanks Bob. You seem very knowlegeable. I checked out the building science papers. Not sure what direction to take now. Going to research some more, but have to pull the trigger soon. Thanks for your insights.
Thanks for starting a great discussion. My roof is similar, also 1978, and I've been contemplating increasing the insulation. I like the spray foam idea, but in my house I think it would find its way through some knotholes and the occasional gap between the boards. Felt over the T&G before foaming would solve that, though. I"d love to see some photos as you progress and hear how you've solved your problems.