The issue comes up because of numerous real-world examples of sagging gypsum, and the difficulty of maintaining the air-sealing integrity of gypsum over the long term when making the finish ceiling (and walls) part of the primary air-barrier.

There are advantages beyond sag to using OSB or ply on the underside of the truss chords as both the insulation support and primary air barrier. Routing power/plumbing and having lighting fixtures completely inside the air barrier makes for fewer penetrations and far better initial & long term air tightness. That cavity between the gypsum & OSB can also be filled with insulation, and if framed optimally, provides an excellent thermal break.

Posted By greentree on 23 May 2012 08:43 PM
I dont know why this issue keeps coming up, R49 cellulose is within published gypsum insulation load capacities.

The reason is that people are experiencing drywall "sag" after a few years due to the weight of the cellulose. Real world usage is showing problems.

Posted By greentree on 23 May 2012 08:43 PM
I dont know why this issue keeps coming up, R49 cellulose is within published gypsum insulation load capacities.

It's because people are experiencing sag after a couple of years. They might want to rethink and republish the load capabilities.

I would like to see more data regarding using stabilized cellulose to transfer its weight to the trusses.

Posted By Dana1 on 25 May 2012 12:23 PM
The issue comes up because of numerous real-world examples of sagging gypsum, and the difficulty of maintaining the air-sealing integrity of gypsum over the long term when making the finish ceiling (and walls) part of the primary air-barrier.

There are advantages beyond sag to using OSB or ply on the underside of the truss chords as both the insulation support and primary air barrier. Routing power/plumbing and having lighting fixtures completely inside the air barrier makes for fewer penetrations and far better initial & long term air tightness. That cavity between the gypsum & OSB can also be filled with insulation, and if framed optimally, provides an excellent thermal break.

Sorry to roost this thread, but that's BS. I live, remodel and build in R49 country and you do not see ceiling sagging from r49 cellulose insulation load.

Posted By greentree on 12 Oct 2012 06:30 PM


Sorry to roost this thread, but that's BS. I live, remodel and build in R49 country and you do not see ceiling sagging from r49 cellulose insulation load.

It all depends on what YOUR building codes and practices are. In your area there might not be ceiling sag but in other areas their might be because they use different building practices. It's like saying you don't get mold issues with your building technique but you live in a desert. While that same building technique used in the Pacific N/W would cause a lot of mold issues.

That brings about this question: What drywall thickness do you use and how is it attached (on center)?

What building technique in the desert would cause mold in the pacific northwest?

Here the roofs are trussed, since you are a bit of an expert around here this will of course indicate ceiling member spacing. For panels on the ceiling we use, drum roll please......gypsum ceiling panels. You should write a book, you overflow with tainted genius.

Posted By greentree on 13 Oct 2012 09:10 AM
What building technique in the desert would cause mold in the pacific northwest?

Here the roofs are trussed, since you are a bit of an expert around here this will of course indicate ceiling member spacing. For panels on the ceiling we use, drum roll please......gypsum ceiling panels. You should write a book, you overflow with tainted genius.

Here we go again. If you disagree with something someone said, whether Dana or me, or whoever, resorting to name calling and personal attacks is childish and reduces ones credibility. Do you talk to your clients this way?

Posted By Lbear on 13 Oct 2012 04:15 PM

Posted By greentree on 13 Oct 2012 09:10 AM
What building technique in the desert would cause mold in the pacific northwest?

Here the roofs are trussed, since you are a bit of an expert around here this will of course indicate ceiling member spacing. For panels on the ceiling we use, drum roll please......gypsum ceiling panels. You should write a book, you overflow with tainted genius.

Here we go again. If you disagree with something someone said, whether Dana or me, or whoever, resorting to name calling and personal attacks is childish and reduces ones credibility. Do you talk to your clients this way?

Don't put yourself in the same category as Dana, I would have a respectful discussion with him or many others on this forum any day of the week and your knowledge level isn't a small sliver of theirs, I only resort to calling you an idiot because there really is no other way to put it.

Posted By greentree on 13 Oct 2012 08:42 PM

Don't put yourself in the same category as Dana, I would have a respectful discussion with him or many others on this forum any day of the week and your knowledge level isn't a small sliver of theirs, I only resort to calling you an idiot because there really is no other way to put it.

You were disagreeing with Dana and you stated his post was "BS", your words not mine. So your disagreement was with what Dana stated and you said his statement about ceiling sag was "BS". I chimed in to simply state that the reasoning behind why a roof can sag, I wasn't comparing myself to Dana or speaking for him.

There are many varied opinions on this forum, people disagree, it's when the disagreement turns into nasty name calling and bullying. I wish you could rethink your approach to being civil.

Sorry to roost this thread, but that's BS. I live, remodel and build in R49 country and you do not see ceiling sagging from r49 cellulose insulation load.

You may not see it, but it does happen. You might allow for some occurrences outside of your experience.

Posted By ICFHybrid on 15 Oct 2012 01:44 AM

Sorry to roost this thread, but that's BS. I live, remodel and build in R49 country and you do not see ceiling sagging from r49 cellulose insulation load.

You may not see it, but it does happen. You might allow for some occurrences outside of your experience.

So what where the conditions that you were witness to ceiling sag? Type of ceiling and insulation type/depth? Joist/truss spacing? Between myself and others we've got a pretty large pool of homes we have been intimate with and it has never been witnessed.

There have been several threads here on GBT discussing it.

Posted By ICFHybrid on 15 Oct 2012 08:32 AM
There have been several threads here on GBT discussing it.

I'm not talking about internet chatter. YOU said it happens so I'm asking you the conditions in which you have personally seen it occur.

My personal experience has no bearing on whether or not it occurs. That is what I am trying to point out to you.

Posted By ICFHybrid on 15 Oct 2012 11:40 AM
My personal experience has no bearing on whether or not it occurs. That is what I am trying to point out to you.

some of us got the point, thanks

Jesus, all I'm asking is who has seen it firsthand and the conditions.

Here is an article about ceiling sag due to insulation:

Ceiling Sag

Thanks for trying to find out some real world examples, I'm hoping to hear from an actual builder or insulator though.

Lbear, I don't know about you, but post #1 uses 1/2" drywall on the ceiling, doesn't note it as high strength and says he's getting sag from r13 fiberglass, kind of suspicious of the credibility here. As someone else mentioned "sag" can often times be confused with out of plane trusses.

Ceiling sag from insulation is like Sasquatch, everyones heard of it but no ones seen it.

Posted By greentree on 18 Oct 2012 08:33 AM
Thanks for trying to find out some real world examples, I'm hoping to hear from an actual builder or insulator though.

Lbear, I don't know about you, but post #1 uses 1/2" drywall on the ceiling, doesn't note it as high strength and says he's getting sag from r13 fiberglass, kind of suspicious of the credibility here. As someone else mentioned "sag" can often times be confused with out of plane trusses.

Ceiling sag from insulation is like Sasquatch, everyones heard of it but no ones seen it.

I don't know if it applies to drywall but "creep" is a real world thing and materials can deform over time due to stress, heat and loads. With drywall, any moisture will cause it to fail overtime.

1/2" drywall on the ceiling will experience problems, most codes don't allow 1/2" on the ceiling, 5/8" is the minimum.

As far as what I observed, here in my home I experienced drywall cracks in some areas after they blew in more fiberglass. It's R-40 right now but the drywall is 5/8". Whether or not it caused it, I cannot prove it 100% but all the evidence leads up to the fact that it happened after I added more insulation. While not "sag", it did crack in the spans between the trusses.