I'll be building in northern Michigan, zone 6 & 70PSF snow load.  I started out thinking dense pack cellulose but the dearth of experienced, with deep cavities, contractors has caused me to consider alternatives.  Initially I was planning 2x4 walls but with my house depth (36') and the snow load they are simply not strong enough so I must use 2x6 bearing walls.  I'm sold on the benefits vs cost of "service cavities" so the inner, non structural exterior wall will be sheeted on it's outer surface with cheap OSB & detailed as the primary air barrier/vapor barrier.  The ceiling will also have a "service cavity" topped with OSB which will also be sprayed with liquid vapor barrier.  I had planned on 3 of 3 1/2" cavities,outer, inner, & middle (10 1/2" cavity cavity r=45).  However, since I need 2x6 exterior bearing walls something must change!  The options are use cavities of 5 1/2", 2:, 3 1/2" with the center occupied by a 2" Roxul board (11" cavity cavity r46) and make all the outer walls 2x6.  The Roxul boards are about twice as expensive per r-sqft as the bats.  Another possibility is  stick with bats & adopt an even thicker wall (12 1/2" cavity cavity r 53) so I'll either have 5 1/2", 3 1/2",3 1/2" in the bearing walls or 3 1/2",5 1/2", 3 1/2"  in the non bearing walls.  With a 20% framing fraction the first option calculates r34.3 whole wall OUCH!  The second, however is r42.7 for the bearing walls and r45.9 for the non bearing walls & since  64 % of my walls are bearing walls I'll end up about r43.5 for the average.  The third option, which I'll probably go with, is 5 1/2",3 1/2",3 1/2" for the bearing walls and 3 1/2",3 1/2",3 1/2" for the non bearing walls this averages out to r 40.5 over all and keeps more interior space.   Now after that wordy introduction the question!  How does one use bats in a cavity between two stud walls?  The only answer I've come up with is: delay sheathing the outer wall until after the inner wall & probably the roof is up then "cram" the middle cavity bats in through the open outer wall followed by installing the outerwall's bats sheathing etc.   Comments please, while it's still a "paper tiger'.

Why are you so interested in Roxul batts? Isn't it rather expensive to use as just a general filler insulation?

The only insulation that's cheaper is fiberglass. Lowes prices for Roxul bats are lower than the best price, which is not at Lowes, for cellulose material only. Mineral wool is both greener and in many ways better than fiberglass.

Lowes prices for Roxul bats are lower than the best price, which is not at Lowes, for cellulose material only.

I'm not following that at all. Cellulose is popular because it is so cheap. A random check at the big orange store showed 3.5" "ComfortBatts" at $40 for 60 sf of coverage. That's more than $2/cu foot for the Roxul product. Cellulose was going off at something on the order of $0.50 per cu foot. Roxul also likes to call their products "rigid" or "semi-rigid". When you use the word "board" it makes me think of the rigid, which is at least twice as expensive as the batts, pushing it up towards the price of foam.

What's your air sealing plan?

Did you price blown-in fiberglass?

around here (iowa, zone 6, 7400 hdd) blown cellulose and fiberglass run the same cost now. Most contractors will blow fiberglass in deep walls without any fuss at all since they do it often for farm metal buildings in purlin spaces. I would have them check into that option. Remember, batts, rox, whatever will still not fill the cavity like a blown product. So your r values will reduce more due to gaps and most likely locations of compressed insulation you are not accounting for that would be eliminated with the blown product. I know you read Green Building Advisor frequently, so I am surprised you are not sticking with a blown product. Rox is a massive pain to deal with as well. Cutting, installing, etc....very annoying.
Also I would probably not stress out over an r35 vs r40 whole wall. I do not know the size of your home or wall exposure, but I am willing to guess a 1600-2000 sqft single story will only have a 500-800 BTU change between the 2 values. Focus on other areas as you know such as the roof insulation, rim joist area and windows. Your windows will be a larger source of heat loss (or gain) in zone 6 and an r30+ wall.

In support of my assertion that cellulose is not as low priced as some here seem to think!
http://www.nationalfiber.com/docs/DirectionsforInstallingDensePackCelluloseBehindInsulweb1210.pdf This is a major cellulose makers coverage chart for 30 # bags of their premium priced product. 30 pound bags are NOT what you get at HD or Lowes, maybe 19#. but shopping the www one can find truck load quantities of 30 pound bags at $12 to $15
using $15 means 24.5 sqft of r13 costs $15 or $.047/rsqft Using the Roxul price given by ICFhybrid above $40 for 60sq ft of r15 comes out $.044/r sqft LOWER COST!!! If you can get 30# bags of cellulose for less than $14.18/bag then cellulose is lower cost. Lowes sells 19 # bags of cellulose for $10.16 that's $16.04 per 30 # bag equivalant or $.05/r sq ft.
The Lowes price for Roxul is $36.76 for r15 covering 62.6666sqft or $.039/r sq ft. So using the same big box store for material cellulose costs 20 % more! To match the Lowes actual price for Roxul, you've got to pay under $12.44/ 30 # bag of cellulose!
Yes, Izerac, I'm well aware of the importance of #1. air sealing, #2 careful choices of windows type and quantity.
#3 no weak spots in thermal envelope (rim joist & other possible thermal bridges). FWIW it'll be about 2382 sqft
Actually the change to 121/2" cavity, about 14 1/2" overall, bearing walls can be done by just enlarging the foundation saving all the interior space.

I think loose fill vs dense pack accounts for the differences in the cellulose pricing above. I'm using Roxul for fire and sound barrier on interior walls, but don't you want something that kills air movement a bit better for insulation on your exterior walls?

Li-
I will not double check your numbers nor question your numbers. perhaps as ICF states, there are different types of insulation designed for different applications, thus different pricing. Go to Greenfiber's website and you will see they have a bunch of different types. Most of the products available off the shelf at Lowes or similar are for loose blown attic applications. Their "free" blowers also will not provide enough pressure to dense pack walls.
With that being said, how much SAVINGS are you actually talking about here? It can not be that much at all, especially in the scope of a house. I understand, every dollar can count, but it would save you a lot of time and give you a better performing wall if you have someone come in and net and blow it. I am not pro cellulose, rox, blown fibers, etc...I am pro netting and blowing with an air tight exterior sheathing plane (zip or similar).
I think you can simplifiy your wall structure to save more money then you are talking about here. I know some on GBA push utility chases, but many super insulated and PH homes have them without. Stick with a tried and true double stud wall with zip, framing, blown insulation, and air tight gyp and you are good to go.

Posted By Liebler on 20 Jun 2012 04:17 PM
In support of my assertion that cellulose is not as low priced as some here seem to think!
http://www.nationalfiber.com/docs/DirectionsforInstallingDensePackCelluloseBehindInsulweb1210.pdf This is a major cellulose makers coverage chart for 30 # bags of their premium priced product. 30 pound bags are NOT what you get at HD or Lowes, maybe 19#. but shopping the www one can find truck load quantities of 30 pound bags at $12 to $15
using $15 means 24.5 sqft of r13 costs $15 or $.047/rsqft Using the Roxul price given by ICFhybrid above $40 for 60sq ft of r15 comes out $.044/r sqft LOWER COST!!! If you can get 30# bags of cellulose for less than $14.18/bag then cellulose is lower cost. Lowes sells 19 # bags of cellulose for $10.16 that's $16.04 per 30 # bag equivalant or $.05/r sq ft.
The Lowes price for Roxul is $36.76 for r15 covering 62.6666sqft or $.039/r sq ft. So using the same big box store for material cellulose costs 20 % more! To match the Lowes actual price for Roxul, you've got to pay under $12.44/ 30 # bag of cellulose!
Yes, Izerac, I'm well aware of the importance of #1. air sealing, #2 careful choices of windows type and quantity.
#3 no weak spots in thermal envelope (rim joist & other possible thermal bridges). FWIW it'll be about 2382 sqft
Actually the change to 121/2" cavity, about 14 1/2" overall, bearing walls can be done by just enlarging the foundation saving all the interior space.

Your assertions are incorrect at a professional level, and you will never find a similar price that an insulation company can get insulation for. Even lumber yards serving pro contractors often can't get near prices of insulation distributors serving insulation contractors for the same product. When sourcing materials I was quoted 7.25 no min., 7.50 truckload min and 6.47 no min for premium cellulose from both 2 yards and an insulation distributor and was negotiated based on quantity goodwill, normal street price of $9+. Again, lowes is not a good representation of the market, especially when considering they are liquidating stores. Often an insulation company can do the job for what a consumer would pay for just the product if they wanted, of course this means they have a lot of margin to play with and results in a wide range of quote possibilities.

Greentree,
Thank you for that insight.  But you haven't told us the contractor 'goodwill' price of mineral wool, so we are comparing 'apples to oranges'.  I figured margin would be similar at the same store or channel & this would lead to 'proper' comparison of relative cost.  Nun the less,  if my local inspectors will waive, code required, fire blocking with cellulose I'll certainly get quotes from any willing dense pack contractors (with your prices as guidance). With the "service cavity" walls, bats still make sense in the service cavity. Also holes would need to be sawn & resealed in the primary air barrier & no netting would be needed. Regardless of whether insulation is bats or blown with the OSB on the outer face of a "service cavity" serving as primary air barrier, delaying exterior sheathing allows 'detailing' the primary air barrier before insulation. However, testing the primary air barrier requires exterior sheathing, at least around windows & doors. With blown insulation this could be before insulation but not with bats.

like I said, you are penny pinching the insulation, yet want a fairly complex service cavity solution using double the sheathing amounts with extra labor required.

"like I said, you are penny pinching the insulation, yet want a fairly complex service cavity solution using double the sheathing amounts with extra labor required."
I disagree! Price will not be the driving force of my insulation choice! Lack of available contractor support to competently 'Dense Pack" to achieve reliable long term effectiveness and code requirements for 'fire blocking" drove consideration of 'alternatives' The complication of the service cavity is motivated, obviously, by cost and long term performance as an air / vapor seal. My goal is better than PH leakage both initially and ever after. The added sheathing is less than $500! If I chose ADA I would spend much more than that on drywall gaskets and premium, non standard sealed electrical boxes, sealants etc.

Posted By Liebler on 21 Jun 2012 12:28 PM
Greentree,
Thank you for that insight.  But you haven't told us the contractor 'goodwill' price of mineral wool, so we are comparing 'apples to oranges'.  I figured margin would be similar at the same store or channel & this would lead to 'proper' comparison of relative cost.  Nun the less,  if my local inspectors will waive, code required, fire blocking with cellulose I'll certainly get quotes from any willing dense pack contractors (with your prices as guidance). With the "service cavity" walls, bats still make sense in the service cavity. Also holes would need to be sawn & resealed in the primary air barrier & no netting would be needed. Regardless of whether insulation is bats or blown with the OSB on the outer face of a "service cavity" serving as primary air barrier, delaying exterior sheathing allows 'detailing' the primary air barrier before insulation. However, testing the primary air barrier requires exterior sheathing, at least around windows & doors. With blown insulation this could be before insulation but not with bats.

I would suggest you look at the last 2 issues of jlc for a little guidance and an example of not needing to over think what you are trying to accomplish, you have no need to reinvent the wheel here. Cubic foot your looking at 1.19 dense pack cellulose to 3.00 mineral wool, loose fill as was mentioned is just under .50 cu ft., sorry to tell you your "findings" have no merit.

Greentree,
Please explain what the "cubic foot" numbers you've given mean. Please help me 'properly' compare the cost of insulating materials needed to achieve the same r value for a square foot of wall. If you are saying that mineral wool costs $3.00/cubic foot that is simply BS The price at Lowes, or Home Depot or Menards is almost exactly $2.00/ cubic foot!, I'm sure an insulation contractor has lower prices available to him.   $3/cuft of mineral wool is even high for 8# density board (with one phone call I got $2.76 from a stocking distributor) which is more properly compared to foam boards. If those are costs per installed cubic foot the fact that it takes .925 cu ft of mineral wool bats to get the same r area product as DPC needs to be factored in.

Checked menards pricing, guess I am full of bs, 40 sq ft bag of 4" is 49.98 so that's 3.74 a cubic foot. That's 24" wide, if I wanted 16" wide I would pay more. A 19 lb bag of their crap cellulose is 6.26, 30 lb is 9 something so you can extrapolate that on your own. Loose fill fits better than cutting batts, constructing homes should not be done by those that are obtuse.

Good luck with your project; may god have mercy on those you hire.

FWIW Menards sells Thermafiber brand mineral wool. When a major cellulose producer markets a competing technology (mineral wool) they make sure to protect their main business by pricing the competition out of competition. Lowes OTH sells mineral wool produced by a company that does NOT sell cellulose. If you want you can pay 87% more at Menards!