Can a fully strapped (to the trusses with 1"x3") gyproc ceiling support the weight of 28 inches of blown cellulose (R-100) insulation?

Thanks

Consider using stabilized cellulose to distribute more weight to the trusses.

For those, like me, who don't know what stabalized cellulose is, wikipedia.org says:

Stabilized cellulose Stabilized cellulose is used most often in attic/roof insulation. It is applied with a very small amount of water to activate an adhesive of some kind. This reduces settling and decreases the amount of cellulose needed. This can prove advantageous at reducing the overall weight of the product on the ceiling drywall helping prevent possible sag. This application is ideal for sloped roofs and has been approved for 5:12 (41.66%) slopes.[2]

Your truss designer needs to be made aware of the additional load, but that extra weight (assume ~5 lbs / sq ft) shouldn't be hard to accommodate in the truss design.

The spacing between the furring and the thickness of the gypsum matters with static loads that high.

The furring needs to be either screwed into the truss chord or nailed with ring-shank nails.

It would take 3/4 (19mm) gypsum to handle the load a furring spacing ~16"/40cm on center, but with the furring at ~12"/30cm you could drop to 5/8"/16mm gypsum. If you go thinner it might handle the load in the short term, but is likely to sag over time.

Some builders prefer to use 12mm OSB screwed or nailed directly the truss chords to support R100 cellulose for the higher loading capacity, detailing it as an air-barrier. The use 2x furring on a standard spacing to support the gypsum, which creates a service-cavity for running the wiring.

I'll add that with enough adhesive, cellulose can support itself in vertical and even upside down (sprayed from below ceiling) applications.

It would probably take more than the standard amount of adhesive in stabilized formula to be self-supporting upside-down at R100.

Geez, this is really bothering me, having my ceiling sag! This year, next year or in 10 years, it doesn't matter - my wife will kill me!

Can I place 1/2 plywood or OSB **on top of** the bottom truss cord and then put the cellulose on top of that? I'd VB the ceiling, strap and then drywall.

My truss wall cellulose insulation will be open to the attic cellulose, i.e. they will be in contact and connected in one continuous envelope.

Maybe I should just use fiberglass attic insulation?

Thanks for any advice on this.

Posted By robert.thompson on 07 Dec 2012 01:22 AM

Can I place 1/2 plywood or OSB **on top of** the bottom truss cord and then put the cellulose on top of that? I'd VB the ceiling, strap and then drywall.

Yes.  It just takes a little extra effort to cut the OSB to fit around the trusses.  Take some one part canned foam and seal the edges of the OSB.  You shouldn't need a vapor barrier or strapping with this method.

Another option would be to put 7/16" OSB screwed to the bottom side of the bottom truss cords, then strap under that perpendicular to the trusses with 1x3 or 1x4 every 16" or 24".  The OSB will support the cellulose while the strapping would eliminate any possibility of sag of the OSB over time from being reflected in the drywall below the strapping.

Blown cellulose works far better at the temperature extremes than blown fiberglass in attic applications due to it's much higher air-retardency. SFAIK there are no high-density fiberglass blowing wools for attic applications that even come close. Rock wool is more air retardent than fiberglass as a general rule, but in an open-blown application like attics it doesn't match cellulose. There are suitable versions of each that can be dense-packed to a reasonable air-retardency in wall applications though.

Putting the OSB atop the truss chords is more a bit more labor intensive, but it works.

Thanks people.



Is there an inexpensive alternative to achieving R-100 ceiling insulation or is cellulose, plus additional structure, the cheapest?



Thanks

It's hard to beat open blow cellulose on a price/performance basis.

R100 may not be called for in every high-R building, but there's usually a good enough case to be made for R75 if you have R50+ walls and

This may be a crazy idea, but what if you did 25 inches of blown FIBERGLASS, then capped it off with 3 inches of cellulose? Seems like it would give you everything you wanted and MUCH lighter.

People have done the fiberglass with cellulose overblow approach, but I've not seen it/done it. As a retrofit it might make some sense (and a 6" overblow is better than 3", since it's hard to get a consistent 3"), but I have no data on how much that increases the settling of the fiberglass over time, which may be an issue given the much higher density of the overblow.

Rob-

What does your own computer model say the yearly savings will be in going from R-60 to R-100 in your ceiling? You might better increase your live expectancy to justify the increased insulation!

Hi Lee:

Happy New Year!!!

I am tweaking my calculator to include Air Infiltration and Ventilation.

All the heating calculations were, and still are, based on J.D. Ned Nisson's book, 'The Superinsulated Home Book' I don't know if things have changed over the past 28 years, I doubt it, although I did hear about a new-fangled thing called 'World Wide Web' - I'll have to look into that...

Rob.

Rob-

What I was trying to get at is that going to R-100 worth of ceiling insulation will cost you more than you will save in 17 years, which was your original criteria for selecting a level of insulation. Using your model, which I think is quite reasonable, and using your values for your house, I compute a total heat loss for the 5/10/20/40/60 house of 55,842 Btu/year, costing $1229. for heating costs, with an insulation cost of $6922.

Keeping everything the same except increasing the ceiling insulation from R-60 to R-100, the heat loss drops to 53,937 Btu/year, with a heating cost of $1187, with an insulation cost of $7930. Spending the extra $1008 on insulation saves $42/year, so a simple return on investment requires 24 years, exceeding your original goal of 17 years payback. This ignores the fact that you will probably spend a lot more than the $1008 to reinforce the ceiling to keep it from drooping and then paying for the extra cellulose.

Now there is nothing wrong with going to R-100 ceiling insulation, but you must change your selection criteria from "17 year payback" to "mine is bigger than yours!" I don't know if you have any sun where you are building, or if the Canadians and/or locals in that area give rebates for solar PV, but that could possibly be a more effective use of your extra spending money.

Interesting. Also account for interest rates, which typically changes 17 (or even many less) years to "never".

Posted By jonr on 03 Jan 2013 05:42 PM
Interesting. Also account for interest rates, which typically changes 17 (or even many less) years to "never".

That depends on what (if any) energy price inflation numbers you want to factor in too, eh?  There are no hard-stone images in the crystal ball.

Net Present Value analyses over long time periods should be done with a few difference scenarios, and decide how much (if anything) you're willing to pay up-front as a hedge against potential energy inflation.

I agree that there are a lot of assumptions needed. At 17 years of ownership and energy prices running twice the inflation rate, 24 year straight line payback almost worked (with my other assumptions).

Looks like straight line payback within the period you will own the house is a pretty good rule of thumb.