Roof, spray insulation, new construction
alfie
| Posted in Energy, Heating & Insulation on
What is a good way to insulate a roof over an unvented Attic space, new construction?
Let’s say the roof rafters are 2×10’s (or trusses), and we are spray insulating in the space between the framing.
It sees to me that a 1†layer of rigid over the sheathing would be a good way to get more insulation, as well as insulate over the thermal bridge areas. What happens after that, in building up the roof?
References to details would be welcome.
Replies
That depends why it is unvented and how the design deals with moisture and interior air quality overall.
All this stuff weaves together and doing things piecemeal with no plan and theory for all of it can create trouble.
generally when I use foam over sheathing it is because it is a re=-roof situation and we ate upgrading without tearing things apart inside. We run purlins over the foam and then steel roofing or more sheathing and shingles. What this is doing is creating a "cold roof"
For new work, if you are already going to sray the foam between rafters, there is no need for another layer of it on the exerior.
When we are using cells, bibbs or batts for the rafter space, we will often run the polyiso foil faced on the bottom of rafters for a thermal break, then strapping, then the sheetrock.
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Thanks for your thoughtful response.The attic is unvented because it is conditioned space. I have been following Building Science Corp for a while, and am implementing what they describe much better than I could. Hopefully that responds to your concern about moisture and air quality. Somewhere in the mix of the overall plan there is/will-be a suitable mechanical ventilation unit.Glad you mentioned the re-roof situation. I have that to deal with too, on a different roof. The roof framing is 2x6’s. It has held up fine for 40 years, and will assumably continue to do so. Half the roof has a finished cathedral ceiling, and I would prefer not to rip up the ceiling inside. I was thinking of removing the roofing shingles and the sheathing, removing the existing fiber glass batts, and spray insulating the cavities. Then, I would replace the sheathing, install perhaps two inches of rigid insulation, then sheathing again, membrane, shingles. That sound about right?
sounds expensive. I normally add the insulation by just going over the sheathing. rEmember that tearing off that can end up doing enough jostling that you damage the ceiling from above, and there is a limit to how much longer you want to leave the house open to the possibility of rain with working from above that way.If the 2" doesn't add enough, consider SIPs over existing.depending on
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Thanks. That may just be a more practical approach.
You describe what sounds like a conventional 'flat' roof situation. Insulating over the roof deck/sheathing is standard using e.g. expanded polystyrene and then membrane roofing over that (there may be a thin layer of roofing base that goes over the top of the polystyrene).
I agree, stripping the sheathing off sounds like a lot of work/expense that you MAY not have to do to get a good result ... roofing and insulation.
If you have a flat roof deck, you can apply tapered insulation and cants to get the slope necessary to drain properly.
I've seen polystyrene as thick as 12+ inches in commercial applications. This isn't new technology ... find a good roofing contractor and/or look for details on this type of application.
Thanks. The roof is a 5/12 slope. So far it sounds like insulating on top of the existing sheathing makes more sense from an insulating point of view.I had thought perhaps I'll beef up the framing if I open up the roof.In some sections there is a "sag" in the roof. From below it looks like there are built up trusses of sorts about 8 feet apart, with a support post in the center. Perhaps the framing between those "top chords" or rafters is spaced too far apart, or the in-between members are not adequate. Once the roofing is removed, I'll have a better understanding of what might be happening. See attached JPG.
Looks to me like the fly rafter and its supporting ladder is sagging. The hump appears to be over the end wall. I forget what that's called when the rake is not parallel to the end wall (the soffit is wider at the ridge), but it's not supported properly in your case.http://grantlogan.net
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I forget what that's called when the rake is not parallel to the end wall (the soffit is wider at the ridge)
I've always called it a "Prow"
so is the other end the transom?
;)We called it a flying rake in Colorado. I don't know if there is any official term
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I was thinking what SeeYou said, too. The eave support looks like it was either maybe poorly constructed or is simply sagging for some other reason.
One way to do this would be rigid insulation (e.g. 3-4 inches polyiso), new sheathing, felt and schingles. You have edge details to contend with, but that shouldn't be a major issue. I think the new sheating can be fastened using the long screws used to anchor flat roofing down with .... you can get them in various lengths to like a foot or so.
Some of these other guys might offer some other options, though.
Thanks. Looks like there is more going on with that roof than I thought.
I agree with what CU said,but I see sag in the other rafters making the whole roof look swaybacked too.Also, somebody has slopped black gumbo roof coating on that chimney instead of flashing it correctly, so you have a flashing job to do there - but you would anyways with raising the roof surface.
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I know what you mean. It was probably a poor flashing job to begin with and then somebody slopped the black stuff over it. My intent is to do it properly when I start messing around with the roof.