Loose blown attic cellulose: Compress or shift?
Given the exact same depth, which performs better (which I think is equivalent to which has a higher R value?)
a. Blown cellulose as installed shaved off to x depth, with the excess shifted to the adjacent perimeter
b. Blown cellulose as installed compressed to the same x depth so there is no excess to spread around the perimeter
Before having my unconditioned vented attic blown with loose fill cellulose, I had limited framing installed over the ceiling joists for an access platform (for access to a fresh air damper and it’s control box, installed in an unfortunate location), bringing the total height over the gyp ceiling to 14.5″ (2×8 joist = 7.25″ high + 2×8 framing for platform = 7.25″).
The code here requires R-30, but I decided on R-45 as a cost effective upgrade. I was told the R-45 goes in at an installed thickness of slightly less than 14″ then settles at just over 12″. This seemed to work well with the 14.5″ platform height, as well as other factors.
However, the actual install was thicker (and more bags than calculated). After a few months it has settled about 1″ to about +16″ at the platform framing. Some googling indicates loose blown cellulose continues to settle for 10+ years.
Since I now need to install the platform for access to this damper, I need to either
a. “shave” the 16″ to 14.5″ by sliding the platform plywood across it and spreading the excess cellulose at the perimeter
or
b. compress the 16″ to fit in the 14.5″ space.
Which is the optimal solution?
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Replies
Loose-blown cellulose insulates slightly better than dense-packed cellulose so you would have higher total R-value if you spread the cellulose around. The difference is pretty slight, though. Here are a couple of charts I made several years ago that show R-value vs. density for two different manufacturers, with some false data points added to smooth the curve, and using solid wood as data points for "highly compressed" cellulose.
Thank you Michael. That is a helpful chart.
My install is supposedly approx. 1.5 lbs/cf. The chart shows a slight increase in R value between 1.465 lbs/sf and the next data point of 2.4 lbs/sf. Increasing the density by compressing about 16" into 14.5" is compressing about 1.5", or just under 10%. If it is installed at 1.5 lbs/sf, compressing would increase the density to about 1.65 lbs/sf, still on the upward part of the curve.
The other reason to compress it, is that after quickly reading your initial reply but not yet taking the time to review the spreadsheet, I began "shaving" the cellulose by sliding the edge of a piece of plywood across the top of the platform framing. I found the cellulose does not "shave off" very well at all but rather sort of moves in chunks, which will leave gaps under the plywood, and leaves piles at the perimeter even taller than the 1.5" I would already have. So I will go back and fill in the gaps created by the attempt to shave, and for the balance, I will clear the top of the framing by sliding a 2x parallel to the framing, tilt it each way to compress it a bit, and the place the plywood down over the insulation, compressing the top approx 1.5"...
Thanks!