I've been researching for ways to get the best bang for my buck attic insulation to go in my new ICF home in progress.  Note that all of my ductwork is within the condition space (open web floor trusses over my basement), so going with spray foam on the undeside of the roof deck to make the attic conditioned never has really been a consideration due to the high costs and unneccessary extension of the conditioned space.

My focus has been on blown cellulose, but at depths needed to get my target R49, it exerts quite a bit of pressure on the gypsum ceiling board which will be attached to the underside of my roof trusses 24" OC.  I got to thinking that if you can wet spray cellulose with adhesive in wall cavities, why couldn't you do the same or similar in an attic?  The adhesive should help bridge across the truss chords and reduce the pressure on the ceiling board. 

And then I discovered stabilized cellulose attic insulation.  This is essentially what is sprayed in wall cavities, but lower amounts of water are used when spraying in the attic.  The stabilized cellulose doesn't settle as much as the loose fill over time due to the included water activated adhesive holding the fibers together.  It also exerts much less pressure on the ceiling board for a given R value.  And there is less dust involved.

I found Applegate Bora-Spray Stabilized Cellulose installed to R49 in the attic is ~14" deep, but only exerts ~1.5 pounds per square foot of load on the ceiling board.  This is less than the typical 2+ pounds per square foot for loose fill cellulose at this R value.  This 1.5#/sq ft will be well within the rating 2.2#/sq ft of the USG Sag Resistant Sheetrock that I intend to use on my ceilings.

I was just throwing this out on the forum since my searches on here for stabilized cellulose didn't turn up much info.

Has anyone here used the wet spray stabilized cellulose in their attic?  If so, please post your experience with it.  Also, what about the USG brand Sag Resistant Sheetrock for ceilings?  According to their literature on the web, it resists sag better than 5/8" sheetrock or other brands of 1/2" reinforced gypsum ceiling board.

Applegate Stabilized Cellulose

USG Sheetrock brand Sag-Resistant Interior Gypsum Ceiling Board

Arkie
There are plenty of options. The easiest may be to strap perpendicular to the trusses with 1X3, 16” OC. Glue and screw the drywall and then you can handle the added weight of cellulose without issue. You have effectively cut the unsupported span from 22 ½ to 13 ½” Even ½” rock would be ok. Make sure you account for the extra depth with any ceiling fixtures. This also allows you to shim out any slight waves due to a truss being out of line a bit.
Cheers,
Eric

Strapping under the roof trusses was considered as a possible option, but that is additional work and expense. Plus, I would likely have to shim up my trusses that amount since the ICF forms I'm using will give me just a little over 9' clear space from the top of the sub-floor to the bottom of the trusses. If I strapped under the trusses and lowered the ceiling 3/4" without adjusting the wall height to compensate, it would likely require the drywall hangers to trim every piece of wall board throughout the main level of the house which adds additional labor expense.

Besides, where do you get 1x3 boards? I've never seen them locally. Are they available in your area or do you have to rip a 1x6 to get two 1x3? If I had to do that, I would probably just use commonly available 1x4s.

I'm sure wet spraying the stabilized cellulose will cost a little more than just blowing in loose fill cellulose in the attic, but I think the benefits (less weight, less settling, less dust) will outweigh the small additional cost. Plus I will have wet sprayed cellulose sprayed in one wall in my basement that separates the living area portion from the unconditioned garage portion plus a couple of interior walls on the main level will be insulated for sound control plus the 2x4 walls for the main level garage will also be insulated with sprayed cellulose, so the equipment for wet spraying cellulose will already be on-site.

Just an update for me. Actually, the cost for damp spraying the stabilized cellulose was lower than I expected. I got a quote today for Applegate Stabilized Cellulose blown in my attic (~2200 sq ft) at R49 (~14.5" initial thickness) for $0.72 per sq. ft. This is all borate cellulose and is blown with a small amount of water to activate the adhesive. For my unheated garage ceiling (~700 sq ft), I was quoted $0.50/sq ft for R22 stabilized cellulose (just enough to cover the bottom 2x6 truss chords by ~1"). For the stud walls in my garage, the cost is $0.75/sq ft for the 2x6 R19 walls and $0.60/sq ft for the 2x4 R13 walls. Walls are a little more costly per sq. ft / R due to the extra work needed to trim the insulation flush with the studs and vacuum up the leftovers.

My current plan to further reduce the load on my ceiling is to staple 1/2" wide polyester strapping (~600 pound break strength) to the top of my 24" on-center truss bottom chords approximately every 6"- 8" the length of the house. Total cost for this strapping is <$100. Once the stabilized cellulose drys with the activated adhesive to become a consolidated mass and then shrinks some as it drys, this strapping through it should help support the weight and reduce the load on the ceiling board to near nothing.

That sounds like a good price- I hope I can get a similar deal on my place. I just hope your trusses are straighter than mine are. I will strap my ceiling so I can shim it out. Otherwise, my ceiling would be a mess. The stabilized cellulose would be good, though because of less dust, and probably less settling once it sets.

staple 1/2" wide polyester strapping (~600 pound break strength)

I suggest that the stiffness of this strapping is more important than break strength. The concern is to prevent visible bowing, not an actual break.

I agree that stabilized cellulose makes more sense than adding more reinforcing (like OSB or thicker gypsum).

Posted By jonr on 30 Oct 2013 10:40 AM

staple 1/2" wide polyester strapping (~600 pound break strength)

I suggest that the stiffness of this strapping is more important than break strength. The concern is to prevent visible bowing, not an actual break.

That is why I selected polyester strapping vs. the more common and less expensive polypropylene.  From various sources on the net:

Polyester strapping is the strongest plastic strapping material of the two (polyester vs. polypropylene). The highest initial tension can be applied and retained over a longer period of time compared to other plastic strapping materials.

I don't plan on applying a lot of initial tension - just pull it tight by hand from end to end and secure it at both ends then hit it with a 1/2" staple at every truss chord.  There will only be ~1.5 pounds/sq ft of load applied by the cellulose, so assuming one strap is supporting 8" x 24" or 1.33 sq ft of cellulose between the trusses (this neglects what is being supported by the truss chord), that is only 2 pounds of load on each strap.  Seems pretty insignificant to me.  Plus the strap will be buried in the lower 1/3 of the cellulose, so temperature should remain stable over its life.  I had initially considered poultry netting, i.e. chicken wire, for this purpose but because of electrical wiring running through the attic and the possibility of stepping on a run of romex and mashing it into the wire netting I thought the plastic strapping would be a better option.