I'm in the middle of crawling around the attic and sealing up all the light fixtures, seams, etc.  In the course of doing so, I have to shift some pretty big piles of blown-in insulation around (a mix of pink fiberglass on the bottom and what looks like a subsequent layer of cellulose on top).  Eventually I am planning to come back in and blow another 7-9 inches on top, but this will probably not be till after I complete a bunch of other work.
My question is whether I need to do anything special when I replace the cover of insulation in areas I've finished work on, apart from just using the back of the rake to more or less flatten the pile.  I noticed that the undisturbed insulation areas looks different than the recently-disturbed areas.  Is this because the cellulose is slightly damp when blown?

I do believe what your seeing is cellulose settlement. When you do go back up look at those spots to see if they've gone down. In any case if I am not mistaken, cellulose acts better when "fluffed" .

I do believe what your seeing is cellulose settlement. When you do go back up look at those spots to see if they've gone down. In any case if I am not mistaken, cellulose acts better when "fluffed" .

Cellulose has higher R per POUND when it's fluffed up but at a fixed depth it is but doesn't start to lose R value per inch until it's at ~ 4.5lbs/ft^3 or denser (which will NEVER happen from settling alone.) Blown at low 1-2lbs density normal seasonal humidity cycling will cause it to settle, but it stabilizes rapidly as it sinks and the density goes up. The long-term settled depth is the R your after, but it'll start out 15-20% higher (deeper) than that.

As the density increases with settling, the amount of convection that can occur within the insulation decreases, and it's R value per inch increases. At low density it's ~ R3.4/inch, but hits ~ R3.7/inch at 3lbs density. It behaves BETTER when settled, for any given depth. Normal settled densities will almost always be under 2lbs in an open blow attic situation. Typical would be 1.4-1.8lbs, depending on vendor, depth, and climate.

The difference in appearance of the recently disturbed and undisturbed material is a combination of things: Dust & debris accumulation and slightly higher oxidation will occur in the upper 1/2" of the fiber. The stuff you shoveled/raked out has be re-mixed, with less contamination & oxidation apparent on the upper layer. It's performance will be pretty much identical to the undisturbed stuff at a given depth.

When blowing the new stuff use "stabilized" goods, even if dry blown. It contains adhesives for wet-spraying that will limit the amount of settling. Normal bouts of seasonally higher humidity are usually enough to activate the adhesive the upper layers, making it less susceptible to wind-migration from attic venting, and keeps the activated layers from settling as much.

Thanks for the clarification. By being "fluffed" meaning air was more present in the cellulose, I would of thought it would've acted as a better R. Apparently not! Learning something new every day.
When it's said that 10'' or so gives you R? does that mean that it's at max. peformance when settled at that depth.
Or 10'' reduced to ? when settled gives you R?

Posted By thagreen on 02 Dec 2010 01:50 PM
Thanks for the clarification. By being "fluffed" meaning air was more present in the cellulose, I would of thought it would've acted as a better R. Apparently not! Learning something new every day.
When it's said that 10'' or so gives you R? does that mean that it's at max. peformance when settled at that depth.
Or 10'' reduced to ? when settled gives you R?

It DOES have more R when fluffed 'cuz it's deeper when it's fluffed.  But it doesn't STAY fluffed- it settles, and that settling factor has been measured.

The coverage markings on a bag specify the settled-R, but tend to overestimate for real-world installations.  In generale it's good to allow for 15% more material than specified by coverage charts on the bag.  To meet the manufacturers spec with that coverage requires reading the fine-print:  Set up X-manufacturers blower model Y with air settting Z, bla bla bla, and for the most part if you follow the prescription to a T you'll get that settled depth and density (within 10% anyway.) 

Real world installation isn't so precise, but if you apply 10-15% more material than on the coverage chart, blowing with as much loft as is "reasonable", the settled depth and R value will meet or beat it, but on day 1 it may exceed it by 15-20%.

The best measure is to wait 10-15 years, measure the depth and assume something like R3.5-R3.6 per inch.  But for us with less patience, buying the extra material and shooting 15-20% high on installation day is close enough for all practical purposes. 

eg. If you want R35 at settled depth that's ~ 10", so set your cardboard depth markers to 11.5-12" and make sure it's that deep on installation day, independent of blower settings.  On day 1 it'll be ~R40, but in a decade or two it'll be more like R35, and won't drop much from there.  In your 7-8" overblow situation, shoot for 8" and you'll still have better than 7" settled-depth.

There are occasional reports of "fluffing"  blown insulation in the industry, but I haven't heard of any real egregious operators since the early '90s. This was more common with blown fiberglass, that sometimes takes forever to settle, and has real performance issues if installed at too low a density.  With over-fluffed cellulose the performance is OK, but the settled depth well below what the manufacturer specified.  Find somebody who's been in the biz for awhile and has a good rep, and you'll be fine. Or if a DIY, just give yourself more margin, and count the bags & square feet for a rough estimate of your installed density with the beater rental-blower.

Thanks, Dana, you've made me feel better about crawling around the attic fluffing the insulation. (... that didn't come out quite the way I intended it to... ;> )

When doing renovations/additions, what we normally do is use an electric leaf blower to blow the insulation to another area of the attic. We do the repairs/joints and than just blow it back.