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Weight of cellulose to attic

smokey059 | Posted in General Questions on

I’m trying to decide how much insulation I can safely blow into the attic and where the point of cost vs benefit come together. I’m right on the border between zone 6 and 7. I’ve insulated the walls to around r 39   and  sealed the wall  good by taping and caulking . The ceiling has 1/2″ plywood ringshanked nailed to the trusses 24″o.c. Then 1×4 strapping 16″ o.c. screwed to the trusses and finally 1/2″ drywall screwed to the 1×4. 
I’m currently thinking 27 to 30″ of cellulose. I calculated the weight of the cellulose to be 2 to 2.5 lbs  psf at those depths. The bag says it weighs 25lbs to cover an area to r19 (50sf 6″). So I just took the r19 value multiplied it  times 4 or 5 to get the 2 to 2.5 psf which would be r76 to r96.
The plywood weighs 40lbs a sheet and the drywall weighs 40lbs per sheet which I figured comes in at 1.25lbs psf for each or a total weight of 2.5 lbs psf. The 1×4 strapping I figured adds about .5 psf to the ceiling.
This would bring the total weight for the plywood, strapping, sheetrock and cellulose (if adding to r96) to approx. 6psf.
I called the truss manufacturer and pulled the specs and they said the trusses bottom cord dead weight was 10psf. 
So im thinking that I can safely put 30″ into the attic and only be at 60 percent of the load. Now im not an engineer its just my calculation so I was hoping someone could  tell me if I messed up on how I figured the weight.
Also would there be much real difference in the building performance between r76 and r96. The cost seems small for the added cellulose. The labor is me so well u know.

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  1. Expert Member
    Dana Dorsett | | #1

    >"I’m right on the border between zone 6 and 7"

    That means that by 2050 you'll probably be in the middle of zone 6, given the rate of climate change. You might already be in zone 6 if using this century's weather data rather than late 20th century data.

    >"I’ve insulated the walls to around r 39"

    Is that R39 using 2x12 studs, a double-studwall, or ... ?

    >"...would there be much real difference in the building performance between r76 and r96"

    A whole-assembly R of R75 would be about the maximum that's financially rational for zone 6, so even though it's cheap that last 5-6" isn't buying you much, so there's not much point to piling it on all the way to R96. See Table 2, p.10:

    (Read the whole first chapter.)

    1. vt_guy | | #7

      How are you calculating projected fuel costs over what time period to arrive at what is “financially rational for zone x”?

  2. smokey059 | | #2

    I read the link and I see that r75 will be fine. Do the numbers for the weight look accurate?
    As far as the wall goes I just added up the r value of each layer in the wall assembly. I don't know the true r value. The walls are 2x6 filled with roxul then a taped layer of plywood caulked to the framing. Then 2" of foil faced polyiso with seams taped followed by 1x4 strapping and finally the sheetrock. I also caulked the stud bays before the roxul went in. All the electrical boxes are outside the plywood air barrier. The wall should be air tight but I don't know the true r value. The thermal bridging should be broke by the polyiso. Thanks for the link

  3. Expert Member
    Dana Dorsett | | #3

    Open blown cellulose is almost always at or under 2lbs per cubic foot even in a worst case, usually under 1.5lbs if following the manufacturers' blower setting. A realistic number would be around 1.5lbs per cubic foot at initial blow, a bit denser after settling, or 2lbs as a worst-case nu

    A 12" layer of cellulose runs is about R40-R45, so 2 feet deep (R80-R90) would be 3-4 lbs per square foot and initially R80-R90. An initial blow of 24" will settle to about 22" over the first decade or so and very little thereafter, which would be the R75 to R80 target range you're looking for. Call it 3lbs per square foot if everything is nominally set up correctly, 4lbs per square foot if it's off. Some installers intentionally shoot for 2lbs per cubic foot nominal density to limit the amount of settling over time, but it's quite a bit more material, and not really necessary. A 1.3-1.5 lbs density initial blow is about right for most.

  4. smokey059 | | #4

    Thanks again Dana for the breakdown made it easier to understand. So even if I don't get the density of the blow perfect I'm still going to come in at only 4 lbs psf for an initial blow of 24" which added to the plywood, strapping and sheetrock will be around 7psf. With 10psf Bottom cord dead weight I should be within the safe limit.
    On a side note I read today on this site the penalty of just a small area of the insulation depth being less than the desired depth and how it brings down the overall r value of the whole assembly. The trusses on this structure were raised heel but not to a 24" depth. I think their 8". I did cover the top plate for its 5.5 inches to that 8" height with pieces of xps and foamed them in leaving the 2" for the soffit vent chutes but that still leaves another maybe 12 to 14" along the eaves that will not be 24" deep of cellulose. Could I extend those foam pieces out a distance to bring this area to the same r value as the 24" of cellulose. This area that's less than the 24" needed for full cellulose depth is approx. 60 sq ft of the total 2000 sq ft structure. How bad of a penalty is this going to be? Is it worth adding some ridged foam in these areas before the cellulose goes in. Thanks.

    1. Expert Member
      BILL WICHERS | | #5

      Having tried doing this (boxing around the top plate with rigid foam), I can tell you it’s surprisingly difficult to do without the foam separating. Cellulose, using Dana’s numbers, is a little better than R3.5 per inch XPS is rated R5 per inch, but with lower performance with time, so let’s say R4.5 per inch. You adding about R1 per inch with the rigid foam, but you’re likely to create voids and have some pieces come loose. What is your overall gain? Probably nothing; maybe even negative.

      I’d use the rigid foam to build baffles in the usual way to protect against wind washing, then get as good of a cover with the blown insulation as you can over the top plate. Blown insulation has the advantage of being very good at conforming to irregular surfaces, so it’s much less likely to leave any air gaps where performance robbing convection air currents can flow.

      If you can get to the top plate, it would be worth double checking that you have everything well air sealed though. A small air leak can rob you of a LOT of insulating performance!


  5. smokey059 | | #6

    Thanks for the input. I guess ill just double check for any leaks around the top plate and then fill with cellulose since extending foam out won't really gain anything. Ill have to live with a little less r value over that area. Thanks again for the help

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