I'm in climate zone 5 and would like increase attic floor level insulation in the tight angle between the roof decking and the ceiling of the floor below (the attic has no real floor).  This space is about 5.5".  Am looking for lower cost, and found this posted...  "Yes, you can combine rigid foam and spray foam if you want to save some money. I would be inclined to install the layers of rigid foam first -- cut undersized, and with canned foam sealing the perimeter of each rectangle -- and cap everything with spray foam from a contractor's truck."

Don't people do a layer of closed cell spray foam to get a good seal and some R value and combine it with fiberglass batts to get the rest of the R at lower cost - is this what flash and batt is?  The process above seems similar except it would be better for tight spaces and the spray would be put on last - let's call it Board (rigid foam) and Spray...  It should cost less than cavities full of spray foam but you could still attain a high R in that height-limited space.  And if rigid board will be used, it seems to make sense to stack it in FIRST against the last foot of ceiling up against the band joist (I may misuse terms, sorry) because the cut board is squared and the framing's squared and it'll fit well.  Once the stack of boards is in, then "cap" it all with closed cell spray foam to provide the air seal and extra kick of R value.

So the questions are 1) Will rigid foam, let's say polyisocyanurate (or other), stacked in the angle between the band joist and the top plate/ceiling and between the ceiling joists and topped off with closed cell spray foam which would come up to the air baffle under the roof decking (and the spray would seal all the edges of the stack of boards) cause or help moisture problems (interior to this perimeter set up would be taller cellulose to the same or greater R)?  And 2)  Does anyone have a link to an article saying this is a legit or illegitimate way to insulate an area where you lack height but don't want to fully spray foam it?

Thank you very much for your time.

Gordon

installing sheet foam and sealing the edges with spray foam sounds great but is almost impossible to do well. (despite what you'll hear from lots of folks.) the bottom line is that spray foam, especially canned foam should not be relied on as an air barrier, and an air barrier is exactly what you need in that situation. Bite the bullet and have it sprayed by a high quality experienced firm with a good track record. Well worth it.

To do foam exterior to fiber insulation roof deck above the IRC has prescriptive R values for the foam required for dew-point control. These values are only valid at the IRC code-min total-R values- if you go higher-R, you need more foam, keeping the ratio the same.  Under NO circumstances can you put foil-faced rigid foam directly against the roof deck, since that creates a severe moisture trap for the roof deck, wedged between the low permeance roofing materials and the EXTREMELY low permeance foil facer.

That said, you can actually "cheat" on the IRC prescriptive foam-R with closed cell polyurethane, and going to the prescribed levels in some areas would actually be more risky, since above 2" or so close cell polyurethane becomes a fairly stiff Class-II vapor retarder, creating a similar (but not as severe) moisture trap issue to the foil facer problem.  The folks at BSC did a series of fairly robust WUFI simulations to demonstrate that while the IRC prescriptive levels for putting rigid foam above the roof deck have scientific merit, using those same values for spray foam below the roof deck does not. 

The details can be found in RR-1001 (take a close look at Table-3- even 1" is good enough in Zone 5 unless you have a  heat-rejecting "cool roof" type of roofing above).

At 5.5" if you filled it entirely with closed cell foam you'd be almost certainly net-damaging to the world from a global warming perspective, unless you went with one of the very few foams blown with a low global warming potential blowing agents. Low GWP HFC blowing agents are relatively recent and have almost no market penetration, but there are a couple of closed cell products that use water as the blowing agent. (Any of Aloha Energy's 1.8lb foam formulas, or Icynene's MD-R-200, not to be confused with their higher-R MD-C-200, which is blown with HFC245fa, at something like 1100x CO2 GWP.)  At 1-2" in most stackups even with HFC245fa blown foam you'd still be at a lifecycle net-benefit, depending on what your heating/cooling energy sources are.

By far the best place for rigid foam is above the roof deck, since it keeps the roof deck warmer (= drier), and provides a substantial thermal break over the rafters.

Bob and Dana,

Thanks for the replies. My perception is that polyiso plus closed cell spray foam in a given volume will cost less than pure closed cell spray foam, and I know polyiso has a lower climate change potential than non-water-blown closed cell foams, so I am looking to minimize the closed cell spray foam.

Dana wrote "Under NO circumstances can you put foil-faced rigid foam directly against the roof deck." What about kraft or foil-faced polyiso on the sill plate and extending onto the top of the ceiling of the level below (that is, the non-weight-bearing "floor" of the attic)? In Fig. 1 of RR-1001, off to the left where the band joist is the vertex of the ventilation air-baffle and whatever's between the ceiling joists (the attic's "floor"), I was suggesting a stack of polyiso be placed against the band joist up to an inch from the baffle, then that inch be spray foamed for a combination of extra R and the seal it would provide around the polyiso. BUT in this case kraft or foil would be directly against the top of the ceiling of the room below (no real floor in this attic), and a few kraft or foil levels would be stacked (however many boards it takes to stack to 4") on top, with the 1.5" layer of closed cell spray foam below the cold air in the baffle - does that form a condensation problem? Sorry if RR-1001 answers this - I looked and it wasn't clear to me.

And to get away from spray foam completely, could I tape the crack between the band joist and sill plate and the crack between the sill plate and the "floor" of the attic and just add more polyiso to the stack instead of making the top of the construct be spray foam? This way it's sealed with the tape and I'm getting a similar R, but am using no high climate change potential closed cell spray foam. Thanks to any responses.

Most siding would provide sufficient drying capacity toward the exterior that it's fine to cut'n'cobble foil-faced foam on the interior side, provided it's sufficiently air sealed to never convect conditioned space air. I'm bit confused by the use of sill-plate and band joist in this context. At the eaves in an attic the joists and rafter typically rest on the top plate of the studwall below, and there may or may not be a facia board blocking the ends of the joist bays or rafter bays. As long as whatever exterior plank wood you're putting the foil-faced goods against can dry toward the exterior, you're fine. But if put against something with a very low permeance exterior layer (like #30 felt under composite shingle), it needs some ventilation space.

RR-1001 is all about applying spray foam directly to the underside of the roof deck, with no ventilation gap, converting the attic into conditioned space, no venting.

Batt facers all leak copious air, and are impossible to air seal adequately. If you're using a low-permeance foam on the exterior you MUST have the minimum IRC prescribed ratio of foam to fiber R values to keep the fiber from getting wet from condensation.

I thing both Bob and I have been misconstruing you- are you talking about keeping the insulation all at the attic floor, and just stacking iso over the top plates of the exterior wall to keep the R-values high enough at the eaves by using R6/inch goods at the sloped edges where 2x6 joist elements rest, rather than the R3.5 goods in the middle of the attic? Something like this(but with sealed iso, not spray foam):

http://www.structuretech1.com/wp-content/uploads/2013/01/Foam-insulation-at-eave.jpg

YES, that works! Cutting-in foil face iso as your ventilation chutes (maintain at least 1" to meet code), stacking cut'n'cobble foam in to the roof-edge-wedgeand filling the main part of the attic with cellulose is the right way to go. At the extreme edge you'll be down to 4.5" of iso, but that's still north of R25 rather than R15-ish with fiber.

But before adding any insulation at the attic floor, you need to first AIR SEAL the attic floor from the conditioned space below, which means foam-sealing all electrical penetrations, using fire-rated caulk and sheet metal to air seal around chimneys & flues (and wrap the chimney/flue with at least 3" of rock-wool batt, tied with wire, blocking contact with the blown goods).

Almost ALL of the moisture moving into the cold vented attic in winter will be from air-leaks- the vapor diffusion issue is orders of magnitude behind that, so the air-sealing is the first most-critical step. Wherever those air leaks are, you will end up wetting the insulation in the cooler upper layers. Do the best you can. Using blown cellulose is preferred to other insulation types here due to it's high air-retardency, even at open-blow densities, but it is by no means a sufficient air-barrier on it's own for larger air leaks.

Dana,

YesYesYes - that - http://www.structuretech1.com/wp-content/uploads/2013/01/Foam-insulation-at-eave.jpg - is what I'm imagining, and almost like you said ("sealed iso not spray foam"). EXCEPT I'm imagining "sealed iso WITH closed cell spray foam." An inch or two of closed cell spray would be on top of horizontally stacked iso boards, the stack itself resting on the the top plate and attic "floor" (the spray foam would provide extra R and seal the space between the top of the boards and the low point of the air baffle where it goes into the eave if the stack is high enough). And some spray foam would be on the joist and attic sides of the board to seal the stack to the joists, seal the cracks between the iso boards on the attic side of the iso stack, and seal the iso stack to the attic "floor." This gets rid of most of the climate change potential of filling the whole joist bay end with spray foam, but still gets spray foam's sealing potential at the eaves by effectively sealing the top plate/ceiling floor, top plate/ceiling joists, and top plate/rafter joints by sealing the iso stack to the structures around it (and seals out the eve by sealing the top of the iso stack to the baffle). And yes, cellulose to the interior. IN FACT, it would LOOK like the pic you linked me too because you couldn't see the iso stack under the layer of spray foam! Thank you for putting up with my misuse of the building terms. Now that we're on the same page, is the general idea good or not?

IF generally it's good, specifics...

Dana wrote "As long as whatever exterior plank wood you're putting the foil-faced goods against can dry toward the exterior, you're fine." Will the top plate and attic "floor" be OK with the kraft or foil of the first layer of horizontal iso board directly on it? It's not exterior, so I'm guessing it'll be fine.

I went up there today and there is nothing at the end of the joist bays. I think the space just falls off into soffit. I've seen diagrams of board insulation being placed vertically on the top plate. So if the stack of iso + the 1.5" or so of spray foam is not high enough to seal to the baffle, I'm assuming it's safe to secure vertical iso board high enough to reach the baffle on the top plate between joists, seal around it, and jam the end of the iso stack against this? Or, is there precedent for this other idea I came up with in case the iso stack + spray foam isn't high enough to seal against the baffle... People write a lot about heat leaking around the top plate, and Building Science Corp. even says that heat leaking through the wall under and in the soffit can form an ice dam on the roof just exterior to the top plate. In cases where there is no fascia board, do people successfully cantilever a stack of foam boards (iso in my case) out over the top plate into the soffit until it touches the baffles under the decking (each board added would step back a bit because the baffle comes more interior as it rises with the roof) and seal it with foam? This would insulate the roof over the attic-side of the soffit from heat in that side.

Thanks again, I think I've almost got a plan (and yes, I realize the first step is a top-notch sealing job).

Stacking the iso on the top plate to block all but the ventilation gap at the eaves is fine.  There isn't much benefit to cantilevering out much beyond the exterior wall though, but a couple of inches is fine if you want to.

A cut'n'cobble approach to the rest of the joist-bays is more expensive and time consuming than it's worth- the thermal bridging of the joists cuts SEVERELY in to the performance of high R/inch insulation (a 2x6 wall might have an R35 center-cavity value, but performs at only R16 or so due to the thermal bridging of the framing.) Where you have the height, heaping cellulose to hit the desired R value is far cheaper & quicker. To hit a code-min R38 would take 11" settled-depth, (12" initial blow), but would give you an R15+ thermal break over the studs.  So stacking the cut'n'cobble  may be worth it where the distance from the ceiling-gypsum to the roof deck is less than 13", but for the rest you'll use a lot less spray foam if you use it just for air sealing, and blow a foot or so of cellulose on top of the gypsum.  Cellulose runs about 3 cents per R per square foot, compared to 10+ cents for polyiso, and in cut'n'cobble the scrap rates can be substantial.  Only if you have access to reclaimed roofing iso from demolition/re-roofing can you get your material costs down anywhere near that of virgin-stock cellulose.

Posted By Dana1 on 23 May 2013 02:08 PM
Under NO circumstances can you put foil-faced rigid foam directly against the roof deck, since that creates a severe moisture trap for the roof deck, wedged between the low permeance roofing materials and the EXTREMELY low permeance foil facer.

If you are trying to get more insulation and consition your attic without spray foam, have you considered nailing the rigid foam to the bottom of the roof support? You will need some sort of rig and soffit vent. You will also need to watch for humidity issues on the underside of the foam. However, it is much easier to air seal at the top rather than move eveything around. I am doing this with my addition and it is helping a lot with the summer heat but dont know about winter because I just installed it. It seems to do a lot to keep a lot of the heat out. The rig and soffit vent also seems to work better since before the attic air was at a higher volume than the ridge could support. Now if you put your hand under in the space behind it you can feel the air almost rushing through from the soffit to the ridge. If I could afford it I definitly would have done spray foam to the underside of the roof at whatever depth was maximum. I am less concerned about the global warming potential of the spray (since most of the global warming through CFCs is definitly up for debate now) and more concerned about reducing energy load in the long run since energy will only get more expensive and not any cleaner to produce.