How much cellulose can a drywall ceiling support ?

Arky217

The trusses on my house are on 24" centers.
The ceiling drywall will be 5/8" Fireguard attached perpendicular to the trusses.
The insulation above the ceiling will be dry blown cellulose.

Assuming the drywall is attached securely enough to the trusses so that there is no danger of the drywall pulling off of the screws, how many lbs./sq.ft. will the drywall support before there is a noticeable bowing between the trusses ?

Also, how many lbs./sq.ft. will the drywall support before the drywall is in danger of bowing to the point of breaking ? (again assuming that the attachment to the trusses is not a limiting factor)

Arky

chrisn

I would guess MUCH more than the cellulose will weigh

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Ron6519

Why don't you mention where you live and we'll let you know how much you need for the climate. Then the question would be, I need 18" of insulation, will the ceiling support it?
Simple question, simple answer.

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Arky217

I live in Western-central Arkansas.

According to a chart on Greenfiber's web site (they are the manufacturer of the cellulose that is carried in the stores in my area), I am in zone 3.

For zone 3, they recommend a ceiling insulation between the range of R-30 (9.57", 0.878 lbs/sq.ft.) to R-60 (18.37", 2.059 lbs/sq.ft.).

I have read in several posts on different forums that for 5/8" drywall spanning 24", that you should limit the amount of cellulose to R-40.

I also read where one person put in R-60 cellulose over 5/8" on 24" centers, and he said he noticed a slight bowing of the drywall between the trusses.

I would like to blow in as much as I can without compromising the integrity of the drywall.

A slight bowing wouldn't bother me as long as it didn't get worse as time went by.

Just wondering what others who have blown in more than R-40 have experienced.

Thanks,
Arky

DrHicks

Wow - they're recommending up to R-60 for Arkansas? I wonder what these same people would recommend for Minnesota, where I used to live!

With 5/8th inch drywall, you should have no problem blowing in R-30 to R-40. People do it all the time. Insulation does not have a lot of weight "per square foot."

gregzoll

I am 207, and actually could lay on it, with no problems. Of course, that is spreading the weight across a long distance. Now if I stood on it, I would go through within 5 minutes, due to having so much weight in such a small area. The point is, gypsum will hold the weight. Min. should be no less than 12 inches up in an attic, 16 to 18 is even better.

Arky217, if the person that had R-60 noticed bowing on 24" centers, that means that the gypsum was not properly secured to begin with. Has nothing to do with the weight of the insulation.

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Gary in WA

Good questions, 2.2# per sq.ft.----page #9, Table #4, here: http://gypsum.org/pdf/GA-216-2010.html
That's if using correct fasteners and application. Check out Table #1, also, for board orientation.


A quick chart: http://www.energyguide.com/library/E...SubjectID=8375

If using Toughrock Fireguard, it has 150# flexural strength (from f.g fibers in core). Contact them for failure limits. http://www.gp.com/build/product.aspx?pid=5451
Another: http://docs.google.com/viewer?a=v&q=...U1o3kfoOlKl-7Q

Zone #3 is R-30, per IRC: http://publicecodes.citation.com/ico..._11_sec002.htm

Doubling the code minimum of -9” of cellulose (R-30 settled) to R-60 would give you only about 2% better; http://www.enersavesystems.com/pdf/E...Insulation.pdf


Gary

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DrHicks

Thanks for pointing that out! I wasn't aware that the percentage of improvement was that small. I guess it's just too easy to assume that double the thickness is double the insulating value...

Gary in WA

You're welcome. A few years ago, I found that site while researching my "Insulation" posting: The "biggest loser" in fiberglass insulation....

Imperative to air-seal the attic or wall before insulation: http://www.jlconline.com/cgi-local/v...170a32100a05c7

Fiberglass is efficient at finding the air leaks for you: http://www.finehomebuilding.com/PDF/Free/021105092.pdf

Gary

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Arky217

Wow, GBR, that energy chart is very revealing.

Using Fourier’s steady-state heat flow equation based on a 40 degree temperature difference, it appears that after just 4" of cellulose, you only achieve 1% less heat loss with every additional inch of cellulose.

And after 10", it shows that you gain nothing by adding more cellulose.

Of course, it stands to reason that the gain would be somewhat more with a greater temperature difference. Perhaps there is even a very significant, maybe even logarithmic, change in the heat loss gain as the temperature differential widens.

Not disputing the equation, but I wonder if it actually tells the whole story in actual use.

One thing it does not take into consideration, for example, is air migration through the cellulose. Even though cellulose is a very good air blocker compared to some other insulations, such as fiberglass, certainly the thicker the layer the more air migration it impedes.

The article states that it is difficult to justify the additional cost of adding insulation thickness beyond 5 inches.

To me, even discounting the air migration, it seems to defy common sense that from 1" to 5" (a 500% increase in insulation), you only gain a heat loss of 15%.

In other words, at 40 degrees differential, if you had 5" of cellulose above your ceiling, and you reduced it to 1", I wonder if you would not see more than a 15% increase in your heating bill.

(Just basing that on practical experience of having lived in a house with just a couple of inches of cellulose in the ceiling versus having lived in one with 8 to 12".)

If that energy chart does tell the whole story, then it would appear that you would be wasting money to put not only no more than 5", but actually no more than 2 to 3" since the heat loss gain is relatively negligible beyond that amount compared to the added insulation.

And if that's true, then there's been untold dollars wasted on unnecessary insulation for many years.

The heat chart may be theoretically correct, but as for me, I think that I will opt for 10 or 12 inches.

Arky

Gary in WA

A similar idea: http://www.buildings.com/ArticleDeta...1/Default.aspx

Couple of good reads on this site: http://www.buildinggreen.com/auth/ar...on-Questioned/

Gary

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