Replies

1. 
   Steve_Zerby | Dec 10, 2001 06:39pm | #1

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   bobbilly,

   What is the finish ceiling going to be? You want an airtight plane between the interior and the rafters.

   Steve

1. 
   Mike_Beuche | Dec 11, 2001 05:21pm | #2

   *BB,What you need is R38 Cathedral Ceiling insulation. This is 10 inches thick by your width. Mine is 24" This is in lieu of the 12 inch R38 you indirectly described. I've built a 16/12 pitch with 1-1/2 by 12" I joist rafters 24"OC. The air gap between the insulation and the OSB is at least 1-1/2 inches in the worst case. I used standard plastic fiber type ridge vent and have pierced-vented aluminum soffit.Home Depot or those other joints won't have the Cathedral type in stock. Find a wholesale or contractor grade supplier. If you have to describe it more than once just tell them you left your car running an leave the store. You will probably have to buy it in master bundles. Masters are 5 of the usual bales you find in the stores. They will be wrapped in a fiber bag that makes you feel like a WWF wrestler by the time you finish moving them around the jobsite. Expect about a buck a square foot for this stuff. I my buddy got it for me at about 80 cents a square foot.As for the vapor barrier I chose Poly rather than the foil. I have the foil for a sauna but can't believe that it really makes a s-bit of difference. Poly will seal the nail holes better and I believe last longer against moisture and abuse. But who is to say.I'd make sure the collar ties (if you have them) are sealed to the interior of the house. Mine are exposed and moisture leakage wouldn't be pretty on a nice cedar T&G ceiling.Don't use the 12 inch insulation. You are making a mountain out of a minimal increased expense. Get the right material and save your labor for the cabinets.Have fun scratchin' & itchin'Mike

1. 
   bobbilly | Dec 13, 2001 05:38pm | #3

   *the finish ceiling will be t&g.in my first post i mentioned that I'm putting up foam board as blocking from the sill plate to the vent channel. That leaves a small space open on either side of the vent channel. I was going to plug those but up top leave them open, or would you plug up both?

1. 
   Steve_Zerby | Dec 13, 2001 11:31pm | #4

   *bobbilly,With a t& g ceiling it is imperitive that you have a really good air barrier as the next layer, singe the t & g will leak air like a sieve. I would recommend running foil-faced polyiso board right over the rafters and taping the seams with foil HVAC tape. Better yet, space the sheets 1/4 inch apart and foam the joints, scrape off the overflow, and then tape. Then strap, then t & g. If it were me, I would forget the venting at all, and dense pack the cavity with cellulose. But that raises the venting argument again...(search the archives if you feel inclined to hear the arguments for and against the unvented approach). Either way, the foam would be a great start for whatever you decide to p[ut behind it.If you go with vent chutes and FG, don't worry about plugging anything. What's behind the foam board is essentially outside the air barrier anyway. BTW, the plate that the rafters sit on is called the rafter plate. The sill plate is the first board that goes on top of the foundation. Steve

1. 
   David_Thomas | Dec 14, 2001 02:47pm | #5

   *Sounds like my own house (12:12, asphalt shingles, radiant floors) in a less extreme climate. I'm a venting guy myself and have continous air flow through all the bays, including the gable (which took drilling some holes). Has worked great even with large snowpacks on it. I used 2" poly iso (blueboard) to create air channels before having urethane foam sprayed onto it. Went with urethane only, no FG, so the R-value is high but so is the cost. 1 or 2" or urethane would have given the same tightness and filling the rest with FG or cellulose would be a better return on your money in your milder climate.Any of the approaches mentioned will address conductive heat losses quite well. Focus on air infiltration to further reduce your energy costs and minimize moisture problems. -David

2. 
   Rob_Susz_ | Dec 19, 2001 06:00am | #6

   *Mike,what did you do to stop the air from rifling past the batt in the channetls created by the TJI's.I have looked at a few of these roofs with the IR camera and they ain't been pretty.-Rob

3. 
   Mongo_ | Dec 21, 2001 05:18pm | #7

   *I prefer Steve's approach...2" foil-faced polyiso, sheets gapped, foamed, and taped. Furr, then rock.Foam and seal well both at the peak and at the plate.

2. 
   bobbilly | Dec 21, 2001 05:18pm | #8

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   first off I live in the mountains of western MD at about 2500 feet

   I have lots of cathedral ceiling and am trying to figure out best way to ventilate and insulate. 12 pitch, 2x12 rafters, continuous soffit and ridge vent, asphalt shingles on plywood sheathing.

   This is a log home that will have radiant floor heat with some form of duct system for air exchange/emergency heat.

   Here's what I'm thinking of doing.

   1. raf t r vent
   2. unfaced r38 c insulation
   3. reinforced foil barrier
   4. 1x3 furring strips
   5. finish ceiling

   Questions?

   1. Most contractors around here just sort of roll the insulation on top of the sill plate. I don't think that is a good air barrier. What about putting a piece of foam board between vent and sill plate? You could then also seal the two openings that would be created on either side of the vent channel. You could do the same thing up top but I'm not sure about closing up the two small openings on either side of the vent channel.

   2. in lieu of foil would I be better off just using 1.5" foam and then firring that out?

   thanx in advance.