Chalet roof: frame over or strip it off?
I have a mid-70’s vintage ranch/chalet house that is need of a new roof. Most of the job is a straightforward re-shingle. However, the chalet portion of the house with its 12/12 pitch cathedral ceiling of exposed 2×10 decking is insulated by only 2″ of yellow foam insulation – and that had been applied at two different times. Each layer of foam apparently has a layer of OSB on top of it. Needless to say, it isn’t insulated well enough for MN winters.
One builder gave me a bid to strip off the first layer of OSB, and lay a frame of 2x8s, 24″ on center on top of this, and add 7″ of extruded foam.
I am wondering if I would be ahead of the game, instead, to strip this portion of the roof entirely down to the 2×10 decking rather than compound past errors? From here the roof could be framed to accommodate extruded foam insulation. Is 8″ of insulation enough?
What about this kind of framing with sprayed in foam? Anyone know the cost comparison of sprayed-in foam vs. blue foam or similar product? Would a ridge vent be necessary if the entire deck was sealed with sprayed in foam?
Replies
Hello, my name is Grant and although I don't know the answer to your question this will bump it up where some who does know will answer.
I invented Rez.
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8" of extruded foam should be plenty if not a tad more than necessary to make your payback worthwhile.
That subject has been addressed here on Breaktime a number of different times in the past.
In the event you fail to receive the information you desire, you might find it in the archives.
If you scroll down in the lower left corner of your screen there is a search function that will take you to previous threads dealing with whatever you type in the search bar.
If you type in 'foam insulation' or other keywords of the subject matter you'll get a supply of data from those old threads.
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Thanks, I'll check out the archives at some point. Yep, 8" of foam ought to do it. Any opinion on stripping the roof down to the decking vs. building over anything existing?
I'm not the one to ask but I can tell you the more accurate detailed description of the scenario the more complete any answers will be.
If you could post a pic of the area in question that would help.
Hopefully some experienced roofers will lend an ear here.
sleeps till noon but before it's dark...
If you strip off the sheathing, the inside finished side is at more risk of damage. At the very least I wouldn't be surprised if some drywall nails popped as the rafters twisted when they are released from the strain of being held in place by the sheathing. I may not be picturing the situation right, but would it be possible to pull up a row or or so and insert the insullation?
I thought he said he had exposed 2x10 decking - that means no sheetrock, right?
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You are probably right about the no sheetrock. I can't imagine 2X as decking, particularly 2X10, so I figured he meant the joists/rafters (I guess they are rafters in this situation?). But for a chalet type building, it does seem like the underside would be exposed. But now I don't understand where the insullation is going?
me either, so we'll have to wait for clarification and drum our fingers on the keyboard in the menatimelike thisfhl;JFDSJIFSLAJIFD;AJFIIRKNCISCIRNIRNIJVIFNVRJNVIFNIORENVINRIAONVIOANCDNIRVIROAJFIROENAVIIFIOR
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I suspect what he means is that the 2x10 decking is laid perpendicular to rafters and the rafters and underside of the 2x decking is exposed inside.Thast is exactly the configuration I have except my decking is 1x6 knoty pine. My rafters are 3x6 and my roof pitch is 2:12. I have 1/2 inch Celotex board on top of the 1x6 decking and it originally had a built up tar and gravel on top of the Celotex. Public Service loved me, when it snowed the only snow on my roof was the 2 ft overhang all around.I sweped off the gravel and sprayed urethane on the roof about 2-3 inches thick. Has worked great but after 20 years, it is shot and needs to be redone but I think I will possibly increase the roof pitch and use shingles, but not sure. I have been rasslin with that decision for a couple of years now.I like your approach....now lets see your departure
I think Dinosaur is tracking with what I'm trying to accomplish. Another clarification for others: I have exposed rafters inside the cathedral ceiling room, exposed 2x10 decking on top of the rafters, and currently 2 inches of insulation (seperated by 2 different layers of sheathing) on top of that - all layed flat fastened to the decking with no 2x nailers, 15lb felt and shingles on top of that. I want to strip the roof down to the decking and insulate it better/right before replacing the shingles.
Question: The Black Joe appears to serve as a vapor barrier between the room below and the roof above. Wouldn't extruded foam (blue/pink) do the same thing if used? Is polyiso foam permeable to water vapor? It sounds like cost is one of the main drivers for using polyiso.
The Black Joe would indeed serve as a sort of over-thick felt in your configuration. One of the reasons I suggest it is it goes on quick and easy, and once you tape it with some Glasclad ("red" tape) it'll pretty much keep water out while you finish the work. Building that crib and then laying in the polyiso, then oversheathing for shingles or skip-sheathing for shakes or metal is gonna take you some time, and it's always nice to know your roof ain't gonna leak too bad if it rains while you're gettin all that done....
Yes, XPS panels would accomplish more or less the same thing as the Black Joe...but they are much more expensive (like $40-odd bucks for 4x8 coverage vs $4-6 bucks for the black joe). I don't recommend using XPS as the insulation panel because (1) polyiso is rated as good or better on R-factor; and (2) XPS has greenhouse-gas issues connected to its manufacture. Polyiso panels are manufactured with no use of HCFC's.
The "Energy-Shield" product manufactured by Atlas comes with either a foil-facing or a felt-type facing, both of which can act as a VB. When applying thinner panels (1" max) directly over the roof deck, the panels can be taped and an effective vapour barrier achieved. Strapping is laid on top of the panels and screwed right through the insulation to the roof deck. The shingle deck is nailed to the strapping, thus achieving a ventilated (hence cold) roof.
But you can't attach 3" of insulation that way. The weight of your shingle deck and shingles or steel roofing will be too much for the long screws you would need to drive through 3" of insulation and ¾" of strapping or skip sheathing. The moment arm (leverage) acting on the short portion of the fastener that you could drive into the 2x roof deck below the insulation would be too much, and the finish roofing and whatever it's fastened to would slip downhill slowly but inevitably. Hence the need for a structural crib.
However, as soon as you separate the panels with a structural crib, you lose the vapour barrier effect of the facing on the insulation panels. Hence the Black Joe....
Dinosaur
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But it is not this day.
Thanks for the details! I see it and understand.
I've done a few of these types of jobs. There are lots of old summer chalets up here that people want to winterize, and this is a common request.
You will probably find that sprayed on urethane foam will be one of the most expensive ways to accomplish your insulation needs. There is usually a minimum charge to get the foam truck out to your site, and it's not economical unless they are going to do the entire house.
Also, 8" of that stuff would be excessive. Here in Canada, 8" of Fibreglas¯ batts in the ceiling is usually considered sufficient, and urethane foam provides a much higher R factor. You will probably be fine with 3" of polyiso foam board laid over a half-inch of Black Joe oversheathing the original roof deck.
You will need to strip to the roof deck, nail on the Black Joe, tape it, and then build a crib of 2x4s (set on edge). The 2x4's need to be lag screwed to the roof deck through the Black Joe. Pre-drill them on the ground, and use a pneumatic impact gun to drive the lags. Lay the 2x4s vertically up the roof, spaced laterally on 2' centers, and set the polyiso panels between them. Run skip sheathing (for metal or cedar) or strapping (for plywood sheathing if you want to roof with asphalt shingles). The skip sheathing or strapping allows the uppermost roof deck to stay cold, which is especially important in buildings such as yours with a cathedral ceiling.
If you want to see a job I did using this method, go to this thread: http://forums.taunton.com/tp-breaktime/messages?msg=34292.1 The photos may make some my the verbal explanations clearer. That particular job was a bitch because of the steepness and the type of steel that was used. Everything else went well.
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
Being a new-bee to this forum perhaps I left out a few details for some of the others that have responded: the 2x10 decking is exposed inside...no sheet rock on the cathedral ceiling inside.
Thanks for the info, Dinosaur. I've been told by a couple of contractors that R-40 (approx 5 inches of sprayed foam) ought to be enough at this latitude. So, as you point out, 8" isn't necessary. A question: Blackjoe = fiber board sheathing? (aka buffalo board). I haven't made any comparisons yet on the cost of spray foam vs. extruded foam sheets.
I will check out your posted pictures...and may have another question or two.
Yes, Black Joe is basically tar-impregnated pressed fibreboard. It is code-approved for exterior sheathing in some configurations, but my suggestion to use it in your application has more to do with it's impermeability (think of ½"-thick tarpaper, LOL) and a small-but-measurable insulating factor.
The roof deck in the thread to which I referred you is 2x6 T&G red pine; this is a common material used for that type of application, and has a v-joint milled into the underside to present a nice-looking ceiling. Your 2x10 deck is a new one on me but if it's just ordinary 2x material, it will be an eighth of an inch thicker than the finished size of the 2x6 in those photos.
Since you have an exposed roof deck as the inside finish ceiling in your chalet (a very common detail up here), you won't want any fasteners penetrating through from the outside. Nail on the Black Joe with 1¼" roofers on 16" centers. As to the crib, the length of the lag screws you use to screw down the 2x4s will be critical. You want to penetrate as far into the roof deck as you can but not punch through. Measure carefully, and use washers under the heads so they won't crush down into the 2x when you run them down. It's okay if the heads sit proud; you will be nailing 1x strapping or skip sheathing on across the tops of the 2x crib anyway. Pre-drill the 2x4s on the ground; one lag screw every 24" should do it.
The polyiso foam board I used on that job would be ideal for you. It's available from http://www.atlasroofing.com/residential/energy.asp They manufacture it in several stock thicknesses up to the 3" thickness we used. It's very easy to work with and the client on that job reported his winter heating bill dropped by half after we installed it. It was also only about 40% of the cost of bringing in a foam truck to spray the roof.
The 3" thickness of this product is rated R-19, which is about half the R-40 usually spec'ed for new construction in this climate. However, with an exposed roof deck in a cathedral ceiling, there are some practical and/or economic limits as to how much insulation one can put up there. It would be possible to install two layers of 3" foam board to bring you up to R-38 (plus a bit for the wood and Black-Joe), but you would need to build your crib out of 2x8's. This is not impossible by any means, but the cost will be more than double (you'd need 9½" lag screws or very precise counter-boring, for instance, plus you'd have the finiky job of drilling 7½"-deep holes straight across a 1½"-thick plank).
You will have to decide how much compromise you want to make for your situation and budget. If you want to get R-40 with sprayed foam, that'll still require a 2x6 crib for 5" of foam. You could, OTOH, use a layer of 3" + a layer of 2" for a total of 5" which would also permit you to use a crib built from 2x6's. You'd lose R6.2 by doing that, but it would likely cost you 30% less for the insulation.
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
rez asked if I would stop and comment.
To be honest, I have looked at this thread twice and have not really been inspired to react to it, mostly because I have no meaning full ideas on it.
I am not sure what this material is that you have now
I've never seen a 2x10 decking on a roof before
I don't know the overall appearance and style now or what you are aiming for, or what the budjet might be...
I feel lost in the dark on this one, but if you can add more info or a photo or two...
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