Jonr - That would assume cellulose installers are abundant around here, and would also assume they know what "stabilized" cellulose means. I hate to sound so bitter, but I've been contacting subs since March. It took one contractor 3 months to finally get back to me when I finally said I would appreciate at least the courtesy to tell me they were not interested after I talked to them on the phone twice and sent a couple of e-mails. I'm just getting tired of pursuing what I want.

I resorted to going down the phone book and started down the list. I've left so many messages. Only 13 insulation contractors made initial contact. Of 13 only 5 actually called me back after I sent information on what I want. Of the 5, only 3 do cellulose, of those 3 only 1 was interested enough to come out to our house to measure and give me a bid, and he bid R50+extra R13 in attic, and R15 in walls, but all he does is cellulose, dry pack in the walls. The OSB would be my responsibility, he doesn't do that. He did say drywall will not support anything over R38, which is min. around here. I guess R50 is not minimum like I thought! The other 2 are not comfortable with the "hybrid" system I want. The rest gave me fiberglass BIB bids!

I have 2 bids for the spray foam (out of 8 foam insulators I contacted). My problem now is my budget. Our appraisal came in way lower than expected, so our budget is $100 grand less than original. Hard to keep pushing for higher insulation at such high cost when we have to trim so much everywhere else as well!

To hire it myself and with 2 separate subs, one for cellulose and one for the spray foam, I'm looking at 12 grand for insulation at R100 in attic, R28 in walls, and that's not counting labor and materials for the extra OSB in ceiling and what we already paid for the 1" iso already installed on the exterior. It's very hard to fork out the extra when a couple of generals have given us bids that they can insulate our house with a fiberglass BIB for $3800-$4500, granted much less R in walls and R38 FG in the attic!

How can I calculate that the additional 8 grand to get higher insulation is worth it? I definitely am not getting any quotes from HVAC guys that say it makes any difference in equipment (but don't get me started on HVAC subs that don't do heat loads). I doubt I would see my geo bid drop 8 grand. And even if it meant one ton less, would that mean enough operating cost savings to pay back that investment in 5 years? Any longer than that, and I'm not sure I can justify it with our budget. If money was no object, I wouldn't care.

If I can compromise and go to R50 in cellulose, do I need the OSB at 24" o.c. trusses? If I don't, again, hard to justify having it. If I do I would plan on adding extra attic insulation in the future and get to my R100 eventually!

Will it be huge mistake to leave the skim coat of foam out of the assembly, and go with a FG BIB in walls? I still think I must press for cellulose in the attic, not loose fiberglass.

Thanks for the advice and discussion.

Will it be huge mistake to leave the skim coat of foam out of the assembly, and go with a FG BIB in walls

Don't leave out the spray foam. It's just too good. It really is.

In wall assemblies they'll dense-pack the BIB fg to at least 1.8lbs or more the additional air-tightness of the foam won't matter a whole lot from an air infiltration & thermal performance point of view.

But it it might make a difference from a non-wicking condensing surface & moisture control point of view. Whether the flash foam is worth it (or even absolutely necessary) for moisture control depends on the climate and wall stack up, including exterior insulating sheathing R & type, rainscreen vented siding or not, etc. (and strangely, I don't remember all of the particulars... ;-) )

And you're right, there's no fiberglass solution that has the same air-retardency & stable R at the temperature extremes as cellulose.

The pressure,on the drywall, midway between trusses is  NOT reduced!

There is no question that some pressure IS distributed to the joists, even with no glue. The question is is it practical. How much glue, does it require two passes (say a 8" layer with lots of glue, let it harden, then the remainder), cost, etc. Maybe netting is better.

I got another insulator get back to our contractor and said they think the skim coat would create a double moisture barrier, either with the fg or cellulose and would be concerned with mold growth. Is this true? I believe they use the white paperlooking mesh for the fg bib, not poly. Not sure what they do for cellulose.

Dana1 - my stackup is FC siding, tyvek housewrap/taped, 1" r-board/no taped seams, 1/2" OSB, 2x6 studs. I want 2" cc foam with cellulose fill. Would this create a moisture trap? Would 2" cc and fg bib be a moisture trap? I'm in Iowa. This contractor claims they install the FG to 2.4lbs. If that's the case, would the spray foam be needed? Would I at least want it in the attic?

Thanks.

Most blowing membranes are a true mesh, and very high permeance, though there is at least one manufacturer that is a class-I vapor retarder (can't remember the name right off). Name the mesh product! Most of the papery stuff is pretty high-permeance. Unless I know otherwise I'm going to assume it's more permeable than latex paint, but I'll lay out how it works either way:

While an interior vapor barrier wouldn't create a TOTAL moisture trap, the drying capacity from the fiberglass through 2" of foam and the R-board would be pretty slow. If the BIB is truly very low-perm (under 0.5 perms) you can go with fg only, and it wouldn't be any worse from a moisture point of view than kraft-faced batts on a wall with 1" R board, which would be OK, but not the most resilient, and air-leaks from the interior side could have mold consequences.

Atlas R-board has the usual sloppy spec of roofing iso for permeance of " < 1 perm ". But that usually means that it's at least 0.5 perms. Even without a rainscreen gap between the FC & R-board it'll have at least some exterior drying capacity. With 2" of cc foam you also have about 0.5 perms (maybe a bit more), for a total of less than 0.3 perms between the fg layer and the exterior, so it's better to give it some capacity for drying toward the interior. (Kraft facers on some batts run about 0.4 perms- there's some seasonal drying capacity, but it's not much.) With a low-perm interior it drys only toward the exterior- the sheathing is going to be OK, but any moisture finding it's way into the fiberglass layer will take awhile to dry. It's still probably going to be enough though, since the condensing hours at the foam/fiber interface would be very few.

With 2" of cc foam and only latex paint,NO vapor retarder at the gypsum, the sheathing would have about 0.5 perms of drying toward the interior which isn't bad. Blown fg has an an ultra-high permeance (even at 2.4lbs) , so if you put on a couple of coats of standard latex, no vapor retarders, it would still be more than 2 perms between the foam and the interior space, 0.5 perms between sheathing & interior, which IS enough to protect the sheathing from interior moisture drives while still providing reasonable drying rates in both directions. And with that much foam more than half the center-cavity R is to the exterior of the condensing surface (the fg/foam plane), and the foam won't wick moisture toward the sheathing. It's a solid stackup from a moisture point of view. If a bit thin on total drying capacity for the sheathing, but it's enough.

With only 1" of R-board on the exterior it isn't enough exterior R at center cavity to protect the sheathing from interior moisture drive with an all fg cavity fill & no vapor retarder in your climate, but it's close. (You'd need R7.5 min on the exterior to get there, R10 would be better.) If you drop to a 1" flash-foam on the interior you'd be running about 1 perm between sheathing & interior, but the foam would still be a non-wicking condensing surface, and with R13-exterior/R18-interior at the foam/fiber interface the annual number of condensing hours would be limited- you would never get liquid water running down the foam or wicking into the fiberglass (unless you're actively humidifying the interior to 50% RH or something.) Only if it stayed below +10F for over a week would you have any liquid moisture accumulating, and it would all be in the fast-drying part of the assembly. This too is a safe & resilient stackup using half the volume of cc foam with a bit better drying capacity, as long as the BIB is high-perm. If the BIB is very low perm (<0.1 perms) the membrane has to stay ultra-air-tight.

The difference in whole-wall R value from a 2" foam + fg solution is less than R0.5 due to the thermal bridging of the framing.

It's hard to rationalize the cost of cc foam in attics with a deep cellulose fill for anything other than air-sealing. I would NOT use open-blow fg in the attic with or without foam layers, since it's density is too low to prevent loss of performance at the temperature extremes. Going with R50-R65 cellulose is a much more likely bet. Going to R100 by any method just isn't rational in a house with your wall-R. That might be a necessary and cost effective method for reaching PassiveHouse energy use levels, but that's clearly not in the cards here. Your whole-wall R is coming in at ~R21-R22 (~1.5x code-min), and it would have to be at least 2x that to hit PassiveHouse levels.

Assuming you're heating with heat pumps in a relatively low-cost electricity market and factoring in your wall-R, going to R50 with low-cost cellulose in the attic has at least some long term economic rationale (it meets IRC 2012) and a decent comfort rationale, but taking that up to R75 might not. If your power prices hit 20cents/kwh at some future date you can always top it off with another 6" or so, provided the trusses have been designed to accommodate the depth. Attics are one place where retrofitting more R long after the fact can still be low cost (unlike walls, foundations, or slabs.)

Thanks Dana1. That all makes sense. I was thinking that one reason cellulose would be better in the walls was because my r-board wasn't as thick as it should be and if moisture did get in there, cellulose would help rather than fiberglass? Or is that backwards in that cellulose would make it worse with my stack up?

I totally agree that the attic more can be added later, but I'm getting very high quotes for the labor to do OSB in the ceiling to support the weight. Mostly because no one seems to do that around here, so they up the bid. A couple of pages back I posted the load ratings for my trusses and it seemed the discussion was that they were good enough to support up to R100 (which I can't get everywhere because of my low slope roof since there's no room for that much cellulose). But with no OSB I don't think I can add anything later because drywall won't support it.

The foam in the attic is more for air sealing and also to help max R a little towards the edges of the roof where I don't have the height to pile insulation.

My concern is that if I give in to the contractors, and once the walls are closed they are not coming open again for who knows how long. I don't foresee going back to retrofit more insulation and it doesn't seem that people do that often either. To add more in the attic would be possible, but without something to help keep drywall from sagging, it would mean more involved retrofit than just blowing more in.

Oops. Forgot to ask:

Do I need additional support for R50 cellulose in attic or will 5/8" drywall hold that weight without sagging at 2' o.c. trusses?

do you strap the ceilings there? In New England we always strap ceilings prior to sheetrock; evens out the joists/trusses. I understand that is a somewhat local practice and actually not needed in many circumstances, but the strapping does help support things like insulation.

Bob's recommendation is a good one- screwing 1x furring 16" o.c. to the bottom chords of the trusses reduces the span for the gypsum for low money and flattens out the ceiling in general.

I strap 12" on center to make sure and use 5/8" board. You only get to do it once. I wrote before about my first R-50 install with 1/2" screwed to 16" on center rafters. Saw the ceiling 10 years later and the ceiling had a very interesting cupping, looked like it was intentional. The 5/8 with furring strips will take care of it.

Bob - strapping is not common practice here. Drywall goes directly onto the bottom chord of the trusses.

Thanks for the tip. I'll see what the labor bids will be for that, but I think we can do that ourselves easier than OSB. Since you say it goes before drywall, does that mean you do it after electrical rough in? I'm thinking the electrician would need to know so that the boxes are at the right height?
Hope that's not too dumb of a question.

Would strapping be good enough if I ever wanted to add more insulation and go higher than R50? If I did prevail and get the spray foam in the ceiling, wouldn't that sort of glue the 1x to the sheetrock and help somewhat to "strengthen" the sheetrock? I seem to recall some university test with a 2x4 cannon that showed a skim coat of cc added strength to wall for hurricane simulation. Granted, that was sheathing, not drywall... but similar concept.

Thanks Smartwall. That answers part of my question. I was thinking that if we did strapping ourselves, maybe take the time to cut some "fillers" and nail them parallel to the trusses so drywall could be screwed more on a 16"x24" grid, to be safe.

We've been nailing (ring shank nails) the strapping @16"OC for years with up to R-60 cellulose & have never seen sagging anywhere. We normally do it before raising the interior walls so prior to electrical rough in.

Sounds like your a diyer. Buy a collated screw gun from Ebay it will pay for itself on one job, under $100 for a corded gun, screws are available everywhere even the homaramas have them

Ok - so can't convince the insulator that can do a flash n fill to do it because of their concern for moisture trap and mold. He's going to give me a bid for full foam cavity.

I think it's the half lb foam, and other than knowing it has less R than 2lb foam, i don't know much about it. Assuming that came back cheaper than the flash n fill bids I have using 2 separate subs, is a full 2x6 cavity of half pound foam a good option with my stackup (1" iso, OSB on the exterior)?

Thanks.

Half pound foam is fine, but the sheathing would still be at some risk of wintertime moisture accumualation, since at 5.5" thickness would have a vapor permeance between 4 - 10 perms. Demilec's half-pound goods spec 4 perms @ 5.5" , and is one of the lowest-perm versions, Icycnene is one of the higher-perm versions at 10 perms @ 5.5lbs. Others are in that range. Using a ~0.5 perm "vapor barrier" latex primer is probably the cheapest solution, but Certainteed MemBrain film (looks like poly, but has a variable permeance) between the gypsum & studs/foam would also work, and has higher drying capacity than a vapor retardent paint solution. With either you'd be counting on some drying toward the exterior through the R-board, which is fine, with a total drying capacity (counting both sides of the assembly) of over 1-perm, which is fine.

With R-board sheathing at R6 and a half-pound foam cavity fill your whole-wall R is going to run between R19-R20, depending on your actual stud spacing/framing and the exact foam product used, and it'll be about as air-tight as the flash'n'fill approach. That's still well over code-min. It's probably going to come in cheaper than the flash'n'fill with dense-packed fiberglass too.

We've gotten several bids in.
We can comfortably fit in the budget:
1)5.5" open cell walls, no VB, cellulose blown R50, 2lb foam on 5" truss heel. (I'm waiting to hear back on how much extra to add VB, but I think that would still fit in the budget).

For 2K more than #1 (we could fit in the budget if really justifiable somehow, but would be going overbudget and would have to figure out how to cut somewhere else), we can have:
2)5.5" dry pack cellulose walls, blown in attic R50. No spray foam skim coat.
3)5.5" open cell walls, no vb, 1" 2lb skim in attic + R50 cellulose, 2 lb foam on 5" truss heel.

For 4K more than #1, we can have:
4) same as 3, but with 2" 2lb skim in attic. This one really seems hard to fit the extra investment, unless I'm missing something to have to go this route?

I know the infiltration in the attic is a good thing to control, but I don't know that an extra 2 or 4 grand more than the cheapest one would be worth not having to be crawling into the attic with a caulking gun and spray foam gun to seal things up before cellulose. I do worry on the exterior edges of the attic since there's not enough space to fit 15-16" of cellulose for R50 min between sheetrock and baffle until 26 inches in from exterior edge or so. Since we have hip roofs, that's 20% of my attic "floor" would have less than R50 all along the exterior perimeter (Except right over the double top plate would be is getting 5" of 2 lb foam in that mini-energy heel area) Should I be concerned with this or not?

Any opinions/discussion?

2" of closed cell is way more than a skim coat, and hard to rationalize on any basis.

I'm surprised that #2 is more expensive than #1 (is that dense-packed rather than merely dry-blown?)

If the insulation tapers down to only 5" at the eaves how could you even make code-min with ANY type of insulation? (I s'pose 5" of aerogel might make it...)

Making the vent-baffles out of cut'n'cobbled polyiso wiill be cheaper at a comparable R-value if DIY. Using reclaimed roofing iso or factory rejects would make it comparable $/R to cellulose. You can seal it all up where necessary with a few kits of 2-part 1.5lb Froth-Pak (available at box stores in 12 board-foot kits), which goes much quicker than with 1-part foam or caulking guns.

Thanks Dana.
#2 is dense packed cellulose (not wet sprayed). It was hard to find many around my area who do dense pack or wet spray in the walls. I only found 2 and one was way more expensive for the same thing.

I don't know, but I don't think anyone cares about insulation being to code in the whole roof. Maybe they just measure how deep it is in the middle where the attic access is, and since it all looks the same level they don't think about the fact that at the edges it is tapering down to the wall! But not all houses around here are hips, so the gables do allow for more insulation to be at the edge vs. my house. I've been to several houses under construction this past year and noticed that none of them have true energy heels on them either, and they are using loose blown fiberglass. My friend complains about her kids bedroom being freezing already up in the second floor, and temps are nice!

For the eaves, the cost to spray form there was pretty cheap.