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About to embark on an attic conversion for a one and half story bungalow in Portland, Oregon. Existing rafters are 2×4 (as in 2″x4″!) at 24″ o.c. and seem to be doing the job spanning across the 22 foot wide house. Question is: how best to insulate the roof and kneewall areas? To complicate matters, the existing asphalt shingle roof has at least another five years left, so I won’t tear off immediately but plan to do so when the shingle life is gone. Will reroof with 25yr or plus asphalt shingle. Have skip sheathing now but will want to resheet with plywood then (thus may affect choice of insulation).
Had originally planned on installing 2-2 1/2″ of polyiso board between the rafters leaving a vent space, and adding soffit and ridge vents. With all the thoughts floating around on cellulose, foam and other insulations, venting or not venting, I’m having second thoughts. Also wondering if a radiant barrier might add value in this application, given that I think it may be difficult to achieve much more than an R-20 value.
Anyone with some words of wisdom or good experience with an alternative approach to this particular application?
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Roof complicates it. I would put housewrap so it will lay against underside of skip sheathing in preparation for cellulose. Sheath under rafters with 1.5 polyiso, seal (intentionaly wide 3/8" or so) joints with foam in a can. You'll get a near perfect air barrier this way. Blow cells above foam densley enough to prevent settling. (over 2 pounds pcf). I would go all the way down the rafters to exterior walls, and achieve a contionous air barrier from foam down to ceiling below. Cover this with drywall or even t&g. This depends on ahcieveing a pretty good air seal - which is easy this way.
But stop right now if the attic floor joists are only 2X4 - seldom near adequate.
*Mike: Sounds like the house I owned in Seattle. 1910 pre-craftman 2x4 rafting. Plaster inside, skip sheathing outside.I would lean strongly towards doing it all at one time, rather than try to get another 5 years out of the roof. If it is near the ends of its life, any workers walking on the roof wil do it in. And why disrupt your life twice? I'm also concerned about the work done on one side, disturbing the plaster or old shingles on the other.But here's a way to do it in two steps: If you place 2x6's above the existing skip sheathing, you could put R-11 unfaced FG batts between them, cover with OSB for sheathing and them roof with new shingles. You would have great venting from top to bottom because you would be constructing the eaves and ridge with that in mind. From underneath, spraying urethane foam will reduce air leaks tremendously. You'd have about R-40 total with little air leakage and good venting. -David
*You could furr the rafters down by attaching 2x8's to the sides of the existing rafters (only where the slope ceilings will be). I left a 1" air gap next to the roof deck (used 1" foam battens & F26) and put 1" foil-faced foam between the rafters. After the drywall is hung, we'll dense pack cellulose between the foam board and drywall. This will give me about R-28, with a radiant barrier, and good venting (continuous ridge & soffit vents).
*Bill, the attic floor is 2x6's at 16"o.c. A little soft on the 13' foot span over the living room but hoping to live with it. So your idea with the housewrap is to keep cellulose out of the gaps between the skip sheathing? (so I can install the plywood flat over the rafters later)?Are you suggesting the roof can be done safely without venting? And what about a vapor barrier in this application. Does the foam and cellulose combo achieve enough air seal that a VB is not needed? Apologies to David: I know I should do the whole thing at once but still thinking of what I can reasonably budget this year from a time and expense standpoint.
*I believe the inner foam membrane air seals very well and easily - since you can see it so well. I don't think the ventilation under the roof contributes to building performance, especially when there is good air sealing.The housewrap is not an idea I have tested but yes, my idea is enough of a wrapper to keep cells in. I expected you would ply over the skipped sheathing, not remove it. Actually, having removed siding over sheathing with spaces, you probably don't need the housewarp. I think the cells would pack against the underside of the shingles well enough so you can strip shingles and lay ply back down.
*Before you put a layer of plywood on, make sure the rafters can handle the weight. There are several threads on this board describing roofs just like yours collapsing after two or three layers of asphalt shingles were installed.I'd say 2x4 rafters 24" OC want only skip sheeting and probably wood rather than ashpalt shingles. Dave's advice seems good to me in your case. Increase the structural strength of your roof before adding material.But hey, wadda I know? (with apologies to the original)
*I would suggest you consider adding a 2x6 sister to each joist (within the existing shell). This would help carry the new (increased) roof load (new finish at interior plus new roofing/sheathing to be applied later) and would also increase the cavity space for insulation. One idea would be to insulate the existing rafter space with rigid, cells, or foam (unvented) and leave 2" of the 2x6 exposed. You could get R20-R25 depending on which material you use. This would give you a batten look at the attic ceiling that might be nice to look at. It would also eliminate problems with getting an even finish at your drywall that would be typical in this application if the ceiling is continuous over the rafters. With your climate, I don't think thermal bridging here would be a huge issue (with the exposed 2x6).Air sealing the roof-ceiling will be important. You can use caulk, foam, or tapes or a combination of all.
*I agree with Bill Conner, with one exception...I no longer seal the sheet edges with foam. The polyiso seems to want to cup as a matter of practice. Squirting foam in this space will cause humps under the drywall. Unless you use tons of fasteners, but then your poking holes in your envelope.I typically sheet the undersides with OSB or ply to create a diaphragm for strength. This also eliminates aestethic issues with fastener spacing on the drywall due to non-uniform rafter spacings. I sheet the OSB with foam, butt the joints tight, and tape them with foil tape. Then apply drywall and finish.No problems with cracking, and the wall looks beautiful with the infrared camera. Nothing but black spots where the screws are.-RobP.S. - you may also want to consider sheathing the exterior of the roof with Nail Base Insulation. This is 1/2" OSB bonded to a sheet of polyiso foam. This takes care of all kinds of ills and connections within the walls by simly capping them with a continuous layer of thermal and air barrier. Blow in the old rafter bays solid with cels.
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About to embark on an attic conversion for a one and half story bungalow in Portland, Oregon. Existing rafters are 2x4 (as in 2"x4"!) at 24" o.c. and seem to be doing the job spanning across the 22 foot wide house. Question is: how best to insulate the roof and kneewall areas? To complicate matters, the existing asphalt shingle roof has at least another five years left, so I won't tear off immediately but plan to do so when the shingle life is gone. Will reroof with 25yr or plus asphalt shingle. Have skip sheathing now but will want to resheet with plywood then (thus may affect choice of insulation).
Had originally planned on installing 2-2 1/2" of polyiso board between the rafters leaving a vent space, and adding soffit and ridge vents. With all the thoughts floating around on cellulose, foam and other insulations, venting or not venting, I'm having second thoughts. Also wondering if a radiant barrier might add value in this application, given that I think it may be difficult to achieve much more than an R-20 value.
Anyone with some words of wisdom or good experience with an alternative approach to this particular application?