Cell. R50-60 ceiling weight concern
I did an archive search and came up empty so…
New construction, I’m ready to insulate the ceiling – W trusses, 24″ OC, 5/8 sheetrock, only screws used. In northern Minnesota I want more than the minimum of R39, more like R 50 or 60. I read that 5/8 sheetrock 24 OC is rated for 2.2 lbs sq. foot.
If blown in cellulose is 1.5 – 2.0 lbs. cu foot – and R 60 is rated at 15-17.5 inches that puts me as high as 3 lbs cu. foot. I sure don’t want saging sheetrock some day. All I can figure now is to put the first half of the insulation as blown fiberglass and cover with a layer of blown cellulose. Wouldn’t this compress the fiberglass overmuch? What are folk doing up in Fairbanks? Hindsight says I should have used 1×4 straps 16″OC. Too late for that. Ideas? Info? Thanks, Dale
Replies
we're blowing 20" settled ( about 24" if you don't fluff it too much)
so we're getting about 3.3 lb/cf ... we install on top of 1/2" blueboard screwed to 1x3 furring..
i've never seen any deflection..
you're using 5/8.. should be stronger.. and of course , i'd agree that a strapped ceiling gives a better drywall job...
check with USG if you have any doubts.. my gut says you'll be alright..
and i'm not sure you will get any weight savings if you use fiberglass as a base....what does the fiberglass weigh per CF ?
I was suprised at tjhat thought. The FG is slightly heavier per R-value, but not much.If I had the concern, I might look at doing what that other nergy guy was recommending around here for awhile - Spray an inch or two of foam and then top it off with the cellulose. That way, he gets the VB of the foam with some structural benefit and not need as much cellulose weighing on it.Easy answer would ahve been to use strapping 16"oc- but only New Englanders know about that secret...I'm still shaking my head sideways WOW over the thought of R-60 insulation. I just KNOW that someday, somebody will say, let's just add another floor up there...That's what they make vacumn trucks for
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here's a cathedral ceiling / attic... with 24" blown last week...
it'll settle to about 20"... we keep a bale count to check our installationsMike Smith Rhode Island : Design / Build / Repair / Restore
Use sag-resistant drywall, if you can get it in your area....lighter than 5/8 and stiffer.....
With 24" oc, the framing does not bear much of the insulation weight.
You could encourage the framing to bear more of the weight by running some horizontal [anything] from truss to truss at the top of the bottom member. Netting, strips of housewrap, strapping, that sort of thing... Would not take much to lighten the load to your target. I would imagine that even strapping at (say) 16" would take a lot of the load off the drywall.
At that depth, most of the insulation would be higher than the top of the bottom truss member.
I suppose you could also run perpendicular 2x4 blocking between trusses to give the drywall more backing, but this would probably take a lot longer, and may not be a good idea with trusses from a structural perspective.
From Posts so far:
"I was suprised at that thought. The FG is slightly heavier per R-value, but not much.""check with USG if you have any doubts.. my gut says you'll be alright....
and i'm not sure you will get any weight savings if you use fiberglass as a base....what does the fiberglass weigh per CF ?"From Dale, the originator of this post:My calculations show that
FG is rated @ .5 to 1.0 lbs/cu' and .022 lbs per R sq'
Cellulose is rated @ 1.5 to 2 lb/cu' and .042 lbs per R sq'
For Cell. I show an average weight of 2.5 lbs/sq' for R60
and 2.1 lbs/sq' for R50......
USG ceiling span tables showed 2.2 lbs/sq' for 5/8 @ 24"OC
So I will shoot for around R50 , and watch how the bale weights average out per sq'.
This is the first time I have blown Cellulose, and since I am working at the weight limit for sheetrock I reckon I could build a simple foot sq box and measure the weight as it is coming out of the blower.
I appreciate the one reply that suggested netting or various strips, I dont know of any material that would not permit a gap to form below it as the Cell. settled though. I think my main concern now is adjusting the cell blower to put out a rate that's within theweight perameters. Thanks Fellas, Dale(orininal post)
I did an archive search and came up empty so...
New construction, I'm ready to insulate the ceiling - W trusses, 24" OC, 5/8 sheetrock, only screws used. In northern Minnesota I want more than the minimum of R39, more like R 50 or 60. I read that 5/8 sheetrock 24 OC is rated for 2.2 lbs sq. foot.
If blown in cellulose is 1.5 - 2.0 lbs. cu foot - and R 60 is rated at 15-17.5 inches that puts me as high as 3 lbs cu. foot. I sure don't want saging sheetrock some day. All I can figure now is to put the first half of the insulation as blown fiberglass and cover with a layer of blown cellulose. Wouldn't this compress the fiberglass overmuch? What are folk doing up in Fairbanks? Hindsight says I should have used 1x4 straps 16"OC. Too late for that. Ideas? Info? Thanks, Dale
Consider a layer of blown in foam first.;
foam adds strength to whatever it's blown over. simple do-it- yourself test.; make a couple of mock ups of sheetrock and joists (couple of feet sq.) in one leave it bare and in another spray a can of foam (like great stuff or Hilti's foam etc.; ) make sure it is firmly in touch with the joists and say a couple of inches thick..let it dry a couple of days and then flip each over and see what effect the foam has had. I'll bet you can put far more weight on the foamed one than the plain one.