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Is it worth an upcharge to go from 1.5 lb./cu. ft. density to 2.0 lb./cu. ft. density on fiberglass BIBS?

Rick Van Handel | Posted in Energy Efficiency and Durability on

I’m having my walls blown and I was questioning the installer about installed density. I was looking for a 1.8 lb density as that’s what most of the fg blowing wools seem to require. However, he is installing Owens Corning L77 wool that does meets rated r-value at a lower density than most other blowing wools I’ve looked at.

On the fact sheet, Owens Corning provides r values in wall cavities at 1.3 pcf, 1.5 pcf, and 1.8 pcf. At the top of the fact sheet, there is also an r-value provided at 2.5 lb pcf density, which ends up being about r=4.57 per inch.

To me the dense packing doesn’t seem as “hard” as I was expecting. Contractor is looking for an approximate $800 upcharge to go to the higher density, which would be 2.0 lb pcf. This upcharge would be for a 4256 sf coverage area.

The rest of my stackup is as follows:

29 gauge corrugated metal siding with 1/16″ rain gap
15# asphalt felt WRB
1/2″ cdx with all joints taped with zip tape
dense pack fiberglass (see above)
membrain vapor retarder (detailed as air barrier also, with tremco sealant at plates and overlaps)
interior 1/2″ plywood

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  1. Rick Van Handel | | #1

    Zone 6
    Zip 54944

  2. GBA Editor
    Martin Holladay | | #2

    There is no simple answer to this question. The answer depends on your R-value goals, the time frame that you expect to stay in your house before moving, and your expectations concerning future energy prices.

    If you want to determine home much energy will be saved on an annual basis by this upgrade in R-value, you need to use an energy modeling program like BeOpt.

  3. Expert Member
    Dana Dorsett | | #3

    The marginal improvement in cavity-R is severely undercut by the thermal bridging of the framing. You're looking at less than an R1 difference in "whole-wall-R" improvement after factoring in the thermal bridging going from 1.8lbs to 2.5lbs in a 5.5" deep 2 x 6 cavity, and it would barely move the needle going from 1.8lbs to 2.0lb.

    At 1.8lbs most fiberglass blowing wools are comparable to 3lb cellulose in air retardency, but not as retardent as 3.5lb cellulose. But since you're properly air-sealing the cavities at both the sheathing and interior it hardly matters- 3lb cellulose or 1.8lb fiberglass is plenty. I'm not sure what the extra 19 cents per square foot would be buying you. That's the about the same cost as 1/4" fan-fold XPS siding underlayment which would give you a full R1 in whole-wall R improvement, and the fan-fold could be also detailed as the WRB.

    BTW: A 2x6 R20-ish wall is about R5 short in whole-wall R to meet IRC 2012 & 2015 code in wall performance for zone 6.

  4. Rick Van Handel | | #4

    Dana and Martin, thanks for the excellent tips and analysis. I forgot a critical component of the wall stack up. The walls are 2x8 framed and are strapped horizontally on the inside with 2x4s (Mooney wall). The total cavity depth therefore is 8.75" with a cavity fill r-value of approx r-36. I have a very low framing were spaced on smart framing intervals, door & window headers have 4" of xps foam, windows are u=.19 triple pain. I'm really hoping for a whole wall r value of r=30+.

  5. Rick Van Handel | | #5

    Studs and Mooney strapping is also on 24" centers. Single bottom plate. Bottom and top plates are caulked to sheathing. All exterior penetrations are foamed and caulked. Soffit baffles are stuff and foamed with corbond.

  6. Rick Van Handel | | #6

    I wasn't sure if the extra density would be cheap insurance given the wide and deep stud bays. The wall framing is 14' high, so I added fire blocking so the max height of each cavity was under 8'.

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