Weight of insulation on sheetrock

[Alan Gage]

Insulated the attic on my house this weekend. R-75 - 140 bags - about 3000 pounds.

As you can imagine it took quite a while so I had plenty of time to think while I was up there. One of the things I started thinking about was if all that insulation could cause my sheetrock to sag over time. In all my reading and research I don't recall seeing it mentioned anywhere so I'm hoping it's not an issue.

My house is about 1000 square feet so that's 3lbs/sq. ft. of insulation on the sheetrock. Ceiling was done with 5/8 sheetrock. Trusses 2 ft. on center.

Any thoughts?

Alan

[dug]

That is a lot of weight, almost 100 lbs. on a 4 by 8 sheet! As evenly distributed as it is  I would doubt that the  5/8 sheet rock would sag, although I would be extra certain that is is screwed down well!

[whit]

I didn't think that stuff was that heavy. I've done attics before too and never really thought about the weight on the ceiling. I think your worst enemy would be getting a leak in your attic and getting the insulation wet. Now that'll bring your ceiling down.

[Arky217]

I'm building my retirement house, about the same size as yours, and I also will be blowing cellulose above the ceiling.

My trusses are also 2' on center and the drywall is also 5/8".

I also was concerned about how much weight the drywall would support after reading a post that someone put R60 (about 18") on their ceiling which was also 5/8" on 2' centers.
They said that they noticed a slight bowing of the drywall between the trusses.

I posted the question on the DIY forum of how much weight drywall would safely support under these conditions, but I really didn't get a definitive answer.

However, one poster from the Building Science dept. posted some links to show how adding cellulose past a certain point was not cost effective.
Here is my reply to that post:

"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 to 12 inches."




But my real concern has been how much weight can the screws take before there's a danger of pulling through the drywall. Of course, as one poster here said, if you get a leak and the drywall gets wet, then probably not much.

But even barring leaks, it seems to me (even though I've not put up drywall before) that
the proper depth of the screws is critical for retaining strength. I mean, everything I've read says that if you go too deep and break through the paper, then that screw's holding strength becomes almost nil.

Well, I'm not to the point of installing my drywall yet, but when I do I am going to use a very unorthodox method (on just the ceiling drywall) that will probably cause professional drywall installers to laugh their heads off.

I realize that ceiling drywall has been installed for decades, sometimes with even less screws or nails than recommended, and is still on the ceiling.

Even on those rare occasions where the drywall has fallen, it probably was due to incorrect attachment or too few screws or nails.

However, just thinking about how all that weight is pushing the paper covering down on the taper of the drywall screws, it seems to me that it's a method of attaching ceiling drywall that needs to be critically correct.

Anyhow, it's my retirement house and I plan to live there until I die, so for my peace of mind about worrying whether those 80# sheets of drywall with another 60# to 80# of cellulose on them will one day fall on my head, I've decided to use this method of attachment.

I will be using 3/4" dia., .020 thick recessed center aircraft washers, also called Tinniman washers. They allow a drywall screw to be completely recessed so that the screw head is flush with the washer.

Of course, the washer will still be .020 above the surface of the drywall, and I'm sure it will be noticeable after the mudding but probably not so much so after the texture is applied.

At any rate, the look won't bother me, and by doing this, I'll be sure that the limit of the drywall's strength won't be the screws that hold it up.

And since cellulose is a relatively cheap insulation, I may actually put more that 10 to 12", maybe more along the line of 16 to 18".
If so, at least I won't have to worry about the screws pulling through.

Anyhow, as they say, "That's my story and I'm sticking to it"

(All you drywall pros, are you laughing yet ?)

Arky

(p.s.- That's just the way us old Kentuckians tend to think,even if we do live in Arkansas, "Build it stout so you have no doubt") 

[KWillets]

My old place has some sag in a ceiling area that I thought was due to insulation, until I crawled up there and found there is nothing bearing on it in that particular area (there were a few shortcuts in the gut renovation before we bought it).  It's probably due to the 32" rafter spacing. 

It's very shallow, and it hasn't changed much in the ten years that I've been looking at it.  My guess is the first few weeks will tell you what it will do.

[KWillets]

Another ceiling question I just thought of:  how are you dealing with truss uplift?

[fishing_guy]

Not outright laughing, but I am smirking a bit. 

A better option would be to glue and screw.    Glue every truss.  Screw/nail the perimeter of the sheet.  Then put 3 sets of 2 screws in the field, with the 2 screws being about 3" apart (or so).

{_X_______________X__X________________X__X__________________X__X____________________X_}

It'll hold, unless it gets soaked.  Even your washers wouldn't hold back wet drywall.

Good luck.

[MushCreek]

I can't really speak for the OP's question, since I have no idea, but it is an interesting discussion. As for the article Arky cites, there is one major flaw. Until the ceiling framing is well covered, the insulation hasn't reached full efficiency. If the joists are 2X8, say, you would need 7-1/2" just to reach the top of them. With the heat bridging going on, it would take at least a few more inches of insulation. A foam barrier between the drywall and framing would eliminate this, of course. There's building codes, too. If they state that you need R-60 insulation, you have to put it in, whether it's doing any good or not.

I plan on strapping my ceiling, using 1X4 furring strips on 16" centers, perpendicular to the trusses, which will be on 24" centers. I might put up rigid foam between the strapping and the trusses for the thermal break. The strapping will secure the foam, give closer spacing for the drywall, and can be shimmed to level the ceiling. Believe it or not, where I'm building in SC, you're still only required to put R-19 in the ceiling, R-13 in the walls. I will end up about double that.

Everyone says that drywall is the one thing a DIY'er should contract out, and I will, but I'm going to do the foam and the furring strips myself so I can make sure that the ceiling is good and flat to begin with.

[Squirl]

I don't have the citation, but I remember from a renovation book I read a while back about adding a second layer of drywall for heavier loads, increasing insulation on 1/2 inch drywall, or wider spacing than 24".  Since I am going off of memory without the citation, I would research it a little more yourself first.  I would also be concerned with sag from the joists themselves.


I must say, R-75, very impressive. Sounds like you are getting close to a zero net energy house.

Insulation does suffer from a diminishing return at some point, but it depends on many factors that are specific to every situation.  Cost of energy, future cost of energy, energy needs, cost of time, risk, and cost of money are all factors that articles vary widely on.

[dug]

I will be using 3/4" dia., .020 thick recessed center aircraft washers, also called Tinniman washers. They allow a drywall screw to be completely recessed so that the screw head is flush with the washer.

Of course, the washer will still be .020 above the surface of the drywall, and I'm sure it will be noticeable after the mudding but probably not so much so after the texture is applied.

I have not done much sheetrock work but when doing it in my house I found that any protrusion, however small, makes mudding next to impossible. I was driving about 40 screws per 4 by 8 sheet, which in Alan's case this would mean that each screw would be holding up about 4 lbs., not a problem IMO.

[MountainDon]

I believe the "problem" here is being over thought. Forget about worrying what might happen if there is a serious leak; if that happens the drywall will become mush no matter what.  Ceilings have been drywalled for years, for decades. Most never fall or sag, those that do will pretty much have some other contributing factors. Every home in my subdivision has 24" OC trusses and drywall/sheetrock ceilings. They were all installed with screws. Ours has not sagged or broken and that is with the original blown in R30 plus another R20 that I blew in 6 - 8 or so years ago.

I have used the pattern suggested by fishing_guy in the field of the sheets; two screws about 3 inches apart, with the screws just dimpling the paper. I used an auto stop depth tool, whatever its real name is; it made the job easy. 

[Squirl]

So you piqued my curiosity.  I am planning on doing a similar design and install to R-60.

So I went looking for answers.  I found usg.com.  It is the company for the name brand drywall called sheetrock.  They have more information than you could ever want about drywall.

First for the warranty.
Deleted, wrong product.

Then the brochure:
4. When panels are used as a base for water-based spray-applied texture finish, the weight of overlaid
insulation should not exceed 2.2 psf.
http://www.usg.com/rc/data-submittal-sheets/panels/sheetrock/sheetrock-interior-ceiling-sag-resistant-submittal-WB2195.pdf

I found more than I ever wanted to know about installation and design here, including double panel installation:
http://www.usg.com/resource-center/gypsum-construction-handbook.html

So with all my reading the best number I could find was 2.5 psf load rating without sag for single 5/8 panel 24" O/C.  Maybe you can navigate around the site and find something I missed.  Although it doesn't give the load ratings it does state that double layered drywall is stiffer, with less sag, and greater sound and fire resistant.

[Squirl]

From the green fiber website I found that typical R-60 coverage weighs 2.1 - 2.4 lbs per square foot depending on type. 
http://www.greenfiber.com/technical_documents_architects__building_officials.html

[fishing_guy]

So with all my reading the best number I could find was 2.5 psf load rating without sag for single 5/8 panel 24" O/C.  Maybe you can navigate around the site and find something I missed.  Although it doesn't give the load ratings it does state that double layered drywall is stiffer, with less sag, and greater sound and fire resistant.

Not to quibble about the "facts", but in looking over the sheets, two things come to mind. 
1.  This IS under their warranty section(CYA).  In the way it was written, when a water based texture is applied the sag becomes an issue.  This would be because the texture is thicker than paint, take longer to dry, and therefore saturate the paper.   I would doubt that primer/paint would be as much of an issue.
2.  The "sag" is based on a factory test designed to make the product fail.   They made it easy for you by combining the bold type and the x axis of the graph.  The test was conducted at 90 °F and 90% relative humidity  and the fact that the x-axis is in DAYS.  That is steady, no break heat/humidity designed to make the product fail.  I think that heat/humidity would make a person break.  We get some hot humid days in Minnesota, but out state record is 88% humidity...for a 2 hour stretch.(http://minnesota.publicradio.org/collections/special/columns/updraft/archive/2011/07/hottest_most_humid_mn_day_ever.shtml).  It would be my guess that any Humidity/Time reduction would cause an exponential reduction of sag quickly to 0.

p.s.  As MD said, you (or the tapers) would be cursing you as their (expensive) knives caught the edges of the washers and put nicks in them.  I would also bet that everyplace the thicker mud would be would be a potential crack somewhere down the line.

[Alan Gage]

Thanks for all the great info guys. Did some more googling and it looks like it may or may not be a problem. I guess I'll just wait and see (and hope). My 5/8 sheetrock did have fiberglass strands in it for extra strength. I don't know if all 5/8 sheetrock has that or not.

Another ceiling question I just thought of:  how are you dealing with truss uplift?

I'm keeping the fasteners in the ceiling sheetrock back a bit from the walls so it can hopefully flex when the trusses lift. I looked for truss clips when attaching the partition walls to the trusses but couldn't find any around here. Instead I used some 5" long screws. I predrilled through the wall plates with holes big enough so the screw was a snug fit but the threads didn't grab. Then ran the screws into the trusses but stopped about an inch or so from driving them in all the way. So when the trusses lift the screws can slide in the plates so it doesn't put any pressure on the walls.  They did a pretty good job of holding the walls by themselves but once the ceilings and walls are sheetrocked it should be really solid.

Alan