I need you guys to help me make the right decision.  On the house I'm building, I have about 1800 square feet of flat ceiling, and about 1200 feet of cathedral ceiling.  The cathedral ceiling is at a 5/12 pitch.  The trusses are spaced 24" OC.  The house is ICF and I'm in southern Illinois, so Zone 4.  I'd like to stuff R-50 of cellulose up there, but I'm concerned about drywall sag.  I've heard on here that you can OSB the ceiling to provide support for the drywall, but everyone I've mentioned this too seems to have that look on their face like I'm crazy.  I've been told "overboard" when mentioning R-50 and also when mentioning OSB ceiling.  One builder said that the 7/16 OSB would sag and CAUSE the drywall to sag???  He said if he did that, he wouldn't put less than 5/8" up there.  That's a lot of weight (and cost).

Anyway, are there any other options if going with R-50 blown-in.  DW manufacturers state that 5/8" can hold something like 2.2 lb, which is the same as 1/2" "ceiling" drywall.  I believe R-50 of cellulose would be a bit over that.  They don't make high strength 5/8" that I can find.  Any insight would be appreciated.

Sean

I stuffed 2 thicknesses of 2-inch xps foam board between the trusses in my house, caulking and foaming on both sides, and then blew r30 cellulose on top (and then threw trimmings from xps and polyiso tapered foam on top of that.) The foam held the cellulose without drywall. This is super labor intensive but also super easy as a DIY. People are still going to look at you funny unless you get a terrific deal on the xps, as I did ($5 for used 2" 2x8 board.) Check craigslist or insulationdepot.com. Google reclaimed insulation as well. Multilayer caulking and foaming also assures a very tight ceiling.

It is the 24" span that causes ceiling sag concerns with high R cellulose. You can run 1x4 or ripped 2x4 lath perpendicular to the roof trusses at either 12" or 16" on-center. Then, regular 1/2" gypsum ceiling board or 5/8" drywall screwed at 12" or 16" span can easily support R50 loose fill cellulose.

Another option if you have sufficient room between the ceiling and roof deck is to use stabilized cellulose which has dry adhesive and is applied with a small amount of water to active the adhesive. This helps to lock the cellulose in place and effectively bridge across the trusses.

See previous thread I started about stabilized cellulose here:

http://www.greenbuildingtalk.com/Fo...fault.aspx

More info:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/79520/afv/topic/Default.aspx

If the roof is not already installed, you might find it easier to place some of the insulation on top of the roof trusses.  I think Dana1 has explained how this is done and still preserve ventilation above the insulation.

Thanks for the responses so far.

Alton, the roof is already on so I'm stuck with going to go the traditional route - cellulose or maybe spray foam. I'd like to stay away from the spray foam, but I'm not totally opposed to it. There won't be any HVAC in the attic. Just some wiring and plumbing vents.

ToddM, that sounds like a lot of work Plus it'd be expensive, like you said. Maybe a couple inches of closed cell spray foam would accomplish the same thing.

Arkie, did you end up using the stabilized cellulose? I'm not sure my insulation guy uses this. He actually doesn't even like to wet spray anymore.

A couple days ago, when I presented this issue to my drywall guy, I mentioned the use of either 1x furring strips at 12" OC or osb. He said that he didn't like the idea of 1x holding up the sheet rock and the insulation. Plus, with 7/16 osb costing 6.00 a sheet, I think it'd be cheaper and faster to screw osb on the ceiling. I'm worried that 7/16 might sag as well. I figure it'll cost me 500 - 600 bucks for osb on the ceiling.

What about the Blow-In-Blanket System (BIBS)? Wouldn't the mesh fabric support the weight of the insulation fill instead of putting it on the sheetrock?

Greetings,

I live near Collinsville, Il (Troy) . If you want to see an ICF house whose cathedral ceilings are insulated with RI sys then E-m me [email protected] and you can see the house near Mascoutah and talk to the home owner. He's been in this hose for about 7 years. The utility company changed his meter because they couldn't believe he was using so little energy. He also saved a bundle because he was able to use 2x8s instead of 12s. He has 3000 sf of cathedral ceil'g.

r50 cellulose and 24" truss spacing is common where I'm at and I've never seen drywall sag because of it. I install cellulose to r50 and I've never had a callback, I'm talking hundreds of homes. If you're really worried about it staple insulation mesh or housewrap across the top of your trusses to take the weight.

Posted By greentree on 14 Dec 2011 09:52 PM
r50 cellulose and 24" truss spacing is common where I'm at and I've never seen drywall sag because of it. I install cellulose to r50 and I've never had a callback, I'm talking hundreds of homes. If you're really worried about it staple insulation mesh or housewrap across the top of your trusses to take the weight.

Hundreds of homes, but how many years?

And how thick is the gypsum?

Half-inch (or even 5/8" gypsum) over a 24" span can sag visibly over 2-3 decades with the 1.5-2lbs per square foot static loading of R50 celluose.  But it'll still look pretty good in year 5.  If your plan is to move on in 5 years,  go ahead, jam in some half inch sheet rock and be done with it! :-)   I wouldn't count on insulation mesh to not sag in the center- it's not exactly structural.

The notion that 7/16" OSB would sag similarly at similar loading is simply preposterous.  Mounting furring on tighter spacing screwed to the truss chords would work too, and is less chord load to the truss than OSB, if you're close to the limits.

Dana,
You know of a trussed R50 cellulose attic that is 3 decades old with 5/8 gyp ceiling that has visibly sagged?

Trusses and gypsum here means the oldest installations are orginally wool attics from the 70's with 5/8 gyp. Cellulose upgrades to r40-50 most likely from the mid eighties on, a few original cellulose but probably not at r50 levels. No problems. My installations are 10 years old or newer, no problems over 5/8, no telling how high strength 1/2" will react.

Never heard from salesman, distributors or other contractors of having sag issues either, maybe the cold weather keeps everything nice and stiff;]

Do you have much field experience, or do you just like to read stuff? I'm just curious.

Thanks guys for your help. My insulation guy is trying to talk me out of R-50. He thinks it's way overkill and that takes a lot more cellulose to get to r50 than simply dividing 50 by 3.8, since after you get to a certain thickness, it starts compressing. He too has been doing it for many years and has never seen sag. I told him that it wasn't an immediate result, but might happen 10 years down the road, and the complaint would probably be to the drywall guys and not the insulation guys. His response is probably correct. He stated that if there's a problem, he'd definitely be called in, because everyone likes to point fingers.

On the flip side, think about that! My insulation guy is talking me OUT of buy more of his product. How often does that happen??? I'm leaning toward having him place 12", which should be less than 2 lbs per cubic foot. At full 3.8 per inch r-value, that would get me r-45. At the low end of 3.2 per inch, it would get me just over r38. My last option would be to spray foam an inch or two of CC on top of the drywall and then add cellulose to that.

Jeepster, I too have somewhat the same dilemma. I am concerned about the thermal bridging and air-sealing. I am in a much more benign climate, central CA. I will spray an inch or two of foam to air seal, put an inch of foam board below the truss, blow the gap with cellulose. I am looking for feedback here, as well, as I don't have a great understanding of the workings of insulation. I do want to make sure the foam is thick enough not to get condensation. Do you experts see any issues I have missed? Thanks for your input.

Posted By greentree on 15 Dec 2011 09:32 PM
Dana,
You know of a trussed R50 cellulose attic that is 3 decades old with 5/8 gyp ceiling that has visibly sagged?

Trusses and gypsum here means the oldest installations are orginally wool attics from the 70's with 5/8 gyp. Cellulose upgrades to r40-50 most likely from the mid eighties on, a few original cellulose but probably not at r50 levels. No problems. My installations are 10 years old or newer, no problems over 5/8, no telling how high strength 1/2" will react.

Never heard from salesman, distributors or other contractors of having sag issues either, maybe the cold weather keeps everything nice and stiff;]

Do you have much field experience, or do you just like to read stuff? I'm just curious.

I just make it up as I go along!

My family was in the construction biz between 1960 & ~1995, but my direct involvement with that biz ended in the late 1970s.   I've seen half-inch gypsum sag on 24" o.c. joists at levels well under R50 with cellulose-   I can't say I have any direct experience with 5/8" goods sagging at R50, but my gut doesn't give me the warm fuzzys thinking about it.

[edited to add]

I've worked a number of industries, but have also managed and consulted on several residential & commercial design & construction projects since 1980, not just my own.  The building-science stuff is mostly a hobby, but I keep getting sucked into consulting on high-efficiency building envelope projects (mostly retrofits of the full-gut variety) and to a much lesser degree, heating system design/upgrades.  (But my living is primarily derived from electrical engineering- I'm one of THOSE nerds. .)

Posted By Dana1 on 16 Dec 2011 07:51 PM
I've seen half-inch gypsum sag on 24" o.c. joists at levels well under R50 with cellulose-  

I wouldn't be surprised if it sagged under its own weight over time, side by side there's quite a difference between 1/2 and 5/8 rock. You can put your fist through 1/2 inch, try it with 5/8.

What I'm wondering is if interior finish plays a part as well, would a plaster coat stiffen the rock or just add to the load. Would a texture (most common here) help or hurt or just disguise sag?

OP, make sure he gives you an installed bag count as well as installed depth, make sure you poke your head and verify the installation. Can't tell you how many times I've seen the insulation taper down the deeper into the attic I am, especially hip roof corners and decorative bump outs off the main roof structure and any hard to get to areas by supposedly reputeable companies.

As arkie6 said, stabilized cellulose (ie, a little bit of water sprayed with the cellulose) sounds effective, easy and low cost.

Posted By jeepster on 13 Dec 2011 11:33 PM

Arkie, did you end up using the stabilized cellulose? I'm not sure my insulation guy uses this. He actually doesn't even like to wet spray anymore.

I haven't gotten to that stage of building yet.  Still leaning towards the stabilized cellulose as the preferred approach.

Another approach I considered was installing R13 batt fiberglass insulation with Kraft or FSK facer toward the living area before the ceiling drywall is installed. The insulation facer flanges would be tightly stapled to the sides of the ceiling joists. The facer would then support the batt insulation as well as the R38 overblow of loose fill cellulose over the fiberglass insulation.

And if you really wanted to insure that the batt insulation staple flanges did not pull away from the ceiling joists, you could rip some 1/4" x 1-1/2" strips from 2x4s and lightly nail these over the facer flanges.

Posted By arkie6 on 17 Dec 2011 12:20 PM

Posted By jeepster on 13 Dec 2011 11:33 PM

Arkie, did you end up using the stabilized cellulose? I'm not sure my insulation guy uses this. He actually doesn't even like to wet spray anymore.

I haven't gotten to that stage of building yet.  Still leaning towards the stabilized cellulose as the preferred approach.

I mentioned stabilized cellulose to my installer.  He said the stabilizer is likely starch.  He doesn't think that it would make a difference and he also said that borate is the only thing that should be in the cellulose.  He's completely stopped using the wet spray and he also said mixing water in cellulose that thick is a recipe for disaster. 

He's by far the best guy around.  I'm going to take his advise.  Heck, he does do this for a living . . . .

Sean

I found this topic pretty interesting so I did a little digging and found a 2.2 lb per sq ft unsupported insulation weight limit on 1/2" high strength or 5/8" on 24" centers. If you used two plys of either, the max weight limit goes up to 4.4 lbs per sq ft.

R50 cellulose will vary by manufacturer and blend in weight, but if you look you'll find 1.7 - 2.3 labs per square foot, within spec.

It seems like the industry gets more freaked out about water based textures and water additive insulation (like stabilized) as sag potentials because of the water. All of the technical papers and guides I skimmed made no mention of insulation load when talking about sag, mainly the texture application and a few mentions of water added insulating and cold weather condensation potential. This is from 2 different manufacturers and the gypsum association.

jeepster- I'd agree with your installer about wet spray in an R50 open blow. It's great in stud cavities, and for overtopping in attics, but it would take forever to dry at that depth, and the potential for degrading the gypsum board with moisture is higher.

greentree- R50 cellulose might (usually) limbo under the 2.2lbs/ft spec. limit for 5/8" gypsum, which means it won't break even in a mild earthquake, but that doesn't mean it stays flat forever under load. Taking anything up to the limit seems imprudent. At R50 you'd be at a high fraction of the spec using anybody's product, and if you ever needed/wanted to go higher R you'd be limited by the structural limitations of the gypsum.