I am building a cold climate home (northern minnesota) that will have an unvented vaulted ceiling. The trusses vary in depth along their pitch, allowing for between 11 and 24 inches of insulation space. To fill the space and superinsulate the roof without breaking the bank, I am considering putting in Icynene isulation against the roof underlayment, then filling the remaining space with blown-in cellulose. Has anyone done this type of arrangement? How deep should the foam be vs. the cellulose? Vapor retarders?
Any thoughts appreciated,
Shawn
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Put the foam on th einside of the trusses.
First, have the space above blown with the cellulose. They will staple a mesh up that will hold it there.
Then nail up Thermax to the bottom chords using enough plasticap nails tpo hold it there. tape the seams and nails.
Then run strapping ( furring) over tht perp to the trusses at 16" OC using screws to go through and into the trusses bottom chords. Now you have something to attach the finish surface to, you have stopped thermal bridging, and your VB is on the right side of the assembly for your climate
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Put the foam on th einside of the trusses.
Just to clarify, are you saying to spray the foam up against the roofing underlayment first, then add the cellulose towards the inside of the living space? Or the other way around??
Any suggestions on how thick to go with the foam vs. the cellulose?
I am also concerned with using vapor barriers. The Icynene installer I have talked to tells me not to use a vapor barrer as you need to allow the foam to dry out in case water vapor gets inside the wall. Thus, he tells me to use a vapor retarder, which would be a specific type of paint that can be applied directly to the inside surface of the Icynene. However, if i am using cellulose next to the icynene, then it seems improper to add this vapor barrier in between the different types of insulation....
I like your suggestion to use furring strips on the interior to block thermal bridging. This was another concern i had and you have solved it!
thanks for the feedback,
shawn
I'm afraid that I mis-understood. I was thinking in terms of polyisoanurate panels instead of sprayed in place foam.
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Your misunderstanding raises a good question, though. I was originally planning to put the spray-foam to the outside, backfill with cellulose, and apply a vapor retarder to the interior. But would it be better (cheaper and just as thermally effective) to fill the truss cavity with cellulose first, then add rigid foamboard on the interior side, which would double as the vapor barrier???
shawn
probably would be cheaper.
And I prefer the VB there for this type application.Would you be using some baffles? I am no expert on teh blown cells, but it seems like that depth and volumn and elevation would tend to see some settling.Nice design BTW
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Shawn
Could you clarify please, Are these timber trusses that you want exposed or fabricated trusses that you'll hide behind sheetrock?
These are manufactured trusses (parallel chord) that will eventually be covered with drywall.
shawncal
OK, I understand what you are attempting to do now, What I would do is staple up some of that plastic or carboard roof venting. Then I would nail the foam insulation up on the rafters remembering to tape up the seams with a good Tyvek type tape (not a duct tape) I then would go ahead and sheet rock right up to just short of the ridge. Then pump the bays as full of celluliose as you can get, retracting the hose as you fill..
Insulation on the ceiling has three rules.. somes good, mores better, and too much is just starting to be fun!
You'll have to do a little dance to seal off the ridge but you should be able to force enough in to achieve some good..
Myself I'd just call several foam sprayers in and have one of them spray the ceiling. whole lot less work for a whole lot better insolation.. (but probably a bit more expensive)
I'm curious why you would have him use the vent channels when he said this will be an unvented assembly. Certainly he could add venting at soffit and ridge and then the channels would do something, but with no vents they'd be a waste of time and money.Also, your method of putting up all the foam and rock first makes it a lot harder for both the guy blowing the cellulose and the guys hanging the sheetrock. And it gets pretty tricky trying to hit a 1-1/2" chord face with a SR screw after running through the sheetrock and the foam.
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Piffin,
Sorry I missed that. Foolish me. I assumed everybody would vent to prevent ice dam issues here in Minnesota.. They get really serious here because of the climate.. He didn't mention the pitch of the roof but shallow pitches (less than 6/12 have major ice dam issues while extremely steep roofs are not totally immune from problems either..
As for my method of blowing insulation it's backwards from what I would do if I were to ever stick frame a roof with trusses.. I'd put on the foam then pour in the insulation before the plywood goes on. (I assumed that he's already dried in)
Sheet rock would be last after the shingles are on.. Yeh, I'd be careful checking weather to ensure that I wouldn't get rained on insulation..
I'm not sure what sort of problem a sheetrocker would have hitting the "stud" all studs are an 1 1/2 wide aren't they? So you make sure that the screw is going in straight isn't that a normal requirement for sheetrockers? Granted I'm spoiled since anyplace you put a screw in a SIP you hit home, <big 'ol GRIN> But I've done my share of sheetrocking and having to hit the stud.. It's not really a lot differant from those in the south who put foam on the outside of their house and have to hit the stud is it?
Yes, it is.
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It's not really a lot differant from those in the south who put foam on the outside of their house and have to hit the stud is it?
Not fundamentally, true enough; but, in practicality, yes, to make it cats to puppies.
That's because the rockers are never quite at their best up under that cathedral ceiling. It's either just at the limit of reach, or doubled over. Then, there's the "fatigue factor" all celings have with rock, combined with that "low end" of the sheet getting heavier and heavier with every sheet.
After a couple hours, that "along the side" feels as tight as a dead-center framing hit.
Which just means somebody has to get the framing lines well laid out on the foam sheets if they are inboard of the trusses. But, that's needful in any event.Occupational hazard of my occupation not being around (sorry Bubba)
Attached is a photo to help show what I am dealing with here. this is a barn-home with an arched roof that I made by rounding over the parallel chord trusses. The trusses are about 11 deep at the bottom and more like 22 inches at the peak, so there is plenty of room for insulation of any type. I sheeted it with 1/2 CDX, then tarpaper, then finished with DECRA metal shingles.
To clarify, yes- it is going to be unvented. With the steep pitch of the roof, the tight seal of Icynene SPF insulation, and elimination of thermal bridging, I feel comfortable going this route (but would appreciate any comments either way)
thanks,
shawn
Is that an observatory on top of the "silo"? ;)
BruceT
Edited 11/26/2007 9:39 pm by brucet9
Semi-related.... an insulation contractor I am using is starting to push a new system where a 1/2" layer of SPF is applied to all wall and roof sheathing, and then the balance of the cavities filled with FG. The goal is to air-seal the building with the foam, which will allow the FG to perform more to its rating. He's coming over tomorrow to look at the job we're doing, and I'll get a quote on it. I'm more likely to want a 100% foam job but it's the owner's call. Cellulose doesn't seem to be a material that insulators here use, maybe due to where it's manufactured... not sure about that. Typical blown in is chopped FG, which I have used several times.
Sounds like you and I are on a similar path. I have researched insulation types extensively over the past year, considering both the environmental aspect as well as thermal efficiency. I really have nothing good to say about fiberglass- it is the poorest performer in both of these areas. Your contractor's method of a skim coat of SPF on the outside plus a backfill of fiberglass sounds like a great combo, and this is what I am wanting to do for my roof- except I would chose dense-pack cellulose in place of the fiberglass.
I don't know what climate you are in, but vapor movement is an issue that weighs heavily on my mind, and I think it is a very important consideration for any builder. The skim coat of foam is going to be a vapor retarder on the outside, which may or may not be acceptable depending on the other factors of climate conditions, interior finish, etc. I'd be interested in your installer's opinion on the subject.
Definitely lots of questions. First would be is the foam open or closed (I use the acronym SPF but I don't actually know what the product is yet). Second would be, does the FG have a kraft face or not. Our local BI is actually very knowledgeable about insulation and VB issues, I will be chatting with him about this also.
The most informative, expert advice I have found on the subject can be found here: http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
I think this article will answer most of your questions, as it did mine! Let me know what your installer recommends.
shawn
That chopped FG is the worst insulation I have ever had the displeasure of repalcing...If your going to use foam use foam. If not find an insulation contractor that uses cellulose. Garett
"That chopped FG is the worst insulation I have ever had the displeasure of repalcing.."Why?.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.