Went to a large homeshow here in Maine today and came home with thoughts on Foam insulation now instead of a double wall with dense pack cellulose.

What are you thoughts on Icynene vs dense pack cellulose

There were other foam companys there but it seems the off gassing could/would be bad, the icynene guys say no off gassing and no harmfull anything.

What are you thoughts on installtion costs vs double wall and dense pack cellulose, r values, long term effects, worth it ?

Said it before, and I'll say it again.
I'm a big fan of Icynene, and spray foam insulation, in general.

come on Wes, give me more insight :) I know you have more

I too am a big fan of Icynene, and spray foam insulation in general.

But as big a fan of spray foam as I am, there are issues with getting reliable results when applying half-pound foam in very thick walls- enough so that some local codes limit the total depth to 6"(?) putting you in the R21 range before thermal-bridging- figure on ~R17-R18 clear-wall values with most conventional framing:

http://www.naima.org/pages/resources/library/pdf/RP067.PDF

If your goal is true super-insulation, (R35 clear wall values or better) you can't (legally/safely) get the whole way there with spray foam.

Spray foam makes it easy to build a perfect air-barrier, though, and 2" of 2lb foam is a near-perfect VAPOR barrier as well (for good or ill- it must be placed properly in the whole assembly.)

In comparison to cellulose, half-pound foam offers essentially zero thermal mass benefit compared to cellulose. Cellulose has specific heat ~0.4 BTU/lb, twice that of concrete per lb. If dense-packed to 3lbs/cu-ft it comes in around 1/6 that of concrete per unit volume. In a 12"+ thick wall that ends up being a factor significant enough that will be measurable in the energy bills.

Spray foams offer NO hygric buffering, which inevitably causes humidity being absorbed in higher quantity by the framing & sheathing materials (higher mold-risk), whereas cellulose is the KING of hygric buffering, and draws OUT humidity stored in framing/sheathing wood.

Spray foams need attention to thermal-barrier finish wall issues, whereas cellulose has no such requirement.

If you find a void (via thermal imaging or other) in spray foam it's a total PITA to fix (sometimes impossible), whereas should a similar void be found in dense-packed cellulose it's usually fixable with spot-re-blowing with a single 1-1.5" hole.

Half-pound foam provides less sound-abatement than even low-density fiberglass batting, whereas dense-packed cellulose (or even conventional 2-hole lower density cellulose) makes a HUGE difference.

If you're going for clear wall values of well over R20 assemblies it's easier/cheaper/better to do the bulk of it with cellulose. (Plan-B would be layers of 3" fiber-faced iso board @ ~R20-21/layer. But it'll be more expensive, and provide no thermal mass or hygric buffering capacity to speak of.)

So, while I'm a big fan of spray foams, I'm also a big fan of dense-packed cellulose- especially in fat wood framed high-R walls. (If there's any irony in it, it was discussions with several FOAM contractors skeptical of the claims/recommendations of other foam contractors that pushed me toward dense packed cellulose in certain complicated retrofit situations. When multiple foam contractors were saying "Don't let that other guy take your money, it'll be lose-lose- your best option is cellulose." it eventually sunk in. :-) ) Where appropriate I'll use it all (even the lowly fiberglass-batt), and IMHO, 3lb+ cellulose is your BEST option for primary wall insulation in a super-insulated house.

If you're going for R18-R24 clear-wall using 2x6" framing (with an inch of iso sheathing) it's hard to beat simplicity & speed of application of half-pound foam though. It's at least as fast as sprayed cellulose, and forms a perfect air barrier when done right.

As for the "no offgassing" argument, well...

I did my attic rafters in Icynene (tm) 2 years ago, and I can still smell it when I stick my head behind the kneewalls on a warm day. It may not be toxic, but they aroma can hang with you for awhile. (But if it's behind the required thermal barrier applied with air-barrier techniques you won't smell it.)

newbiejohn - I was debating the same issue, and that's what led me to my planned hybrid wall - 10" total thickness, 6.5" dense pack cellulose, 3.5" icynene .5lb spray foam. I feel this combo will give me the best wall in terms of air barrier, insulating value, and cost effectiveness.

I'll tell you about my combo I will use on my next house, 12" thickness, 2*4 wall with R-13 batt, 4" thick closed cell board foam, 2*4 wall with R-13 fiberglass batt. Thermal bridging really eats the effectiveness of whatever you put in the stud bays, so R-13 fiberglass is as good as anything, plus its cheap and easy to install. In my area, based off estimates I have been given 2lb foil faced board foam is about 1/2 to 1/3 the cost of the same thickness of spray on foam. I will then use foil faced tape and caulk to create my air barrier on the warm side of the wall, effectively giving me all the air sealing qualities of a spray on foam, without having to work around wiring, which is in my inner stud cavity with fiberglass. I get all the advantages of foam's high R value and air sealing, but wiring will be much easier, especially if I have a redo after the fact.

However, for my floor joists cavities, I will use 0.5lb spray foam, just for the simplicity of installation.

If you can use cellulose in walls, use it, it is the best bang for the buck.
Installing spray foam in a wall cavity is a great concept, and the R value is great, if you are using higher density foam (not Icynene).
BUT, you should have a layer of foam on the exterior to act as a thermal break to the framing.

I would not hesitate to use cellulose in a flat attic. And in walls, too. To get the best performance, once again, the thermal break is imperative.

I do not think Icynene is worth the cost, given the added performance that higher density foam affords you. It is not going to pick up or hold water.
It's insulation value is about twice that of Icynene. And, if installed properly, it is tight. Any outgassing is limited. It does not smell after a couple days after installation, since it skins over and is not as porous as low density foams.
It is also expensive and some folks would rather not use it. That is understandable. Then cellulose is the next best choice.

The performance advantage is so marked, I will invest in the foam when appropriate.

Currently living in a 200 year old 1100 square foot home that is R-65 foam. Used 100 g oil and 1/2 cord of wood for last winter for heat and hot water.
Most of the time it was 75F.
Next year, no oil!

Hi John
We have done my partners home in Atlanta with Icynene foam  $2,65/sf. The positive advantage besides insulation and better R-value is that the whole attic is more tight now. But check for quality as I also have seen bad quality in brand news homes offered. Always compare costs first.
Hoowood

I read with interest your recommendation on insulation types and combo's.  With a mixed climate (central VA) stick frame 2X6 outerwalls 2X4 internal and web floor joists 12:12 pitch roof. I t was recommended 1" foam sheathing (thermal break), for 2X6= 2"   foam spray in (close cell 3 lb.) followed by spray in cellulose.  For web truss flooring still open to ideas for noise and insulation and for Attic 2" underside trusses and cellulose for ceiling floor.  All unconditioned space in attic.   Any better ideas and comments would be appreciated.
House main level and finished full basement. 2395ft main level 2100(conditioned space) LL.
Lisa