Seems that whether one is better than the other is moot, neither is very good according the this NAIMA document. 

"The study made it clear that any difference between the two insulations was insignificant when compared to the overall leakage through the other components of a house. Small differences in workmanship elsewhere in a house are likely to be more significant than differences in the air permeability of wall insulation".

They do say "Drywall contributes about 77% of the total resistance of the wall, the sheathing and siding about 12% and the insulation about 11%"  this seems to be for stick-framed home.  However, they make no mention at all about the ceiling. 

For a standard cellulose/fiberglass vented attic the drywall has to provide 100% of the infiltration barrier, however the environment is not as harsh, no wind and so no rapid pressure changes.  The incentive for air movement is primarily convective and I have no feel for whether that is large enough for small gaps and cracks to create a significant heat loss due to infiltration.  It's best practice to seal the large gaps around light fixtures, electrical/plumbing penetrations, etc. which would suggest that filling the smaller spaces also provides some benefit.  A 1/16th of an inch gap along 25 ft of drywall is 18 sq.in.  That's the equivalent of a 5" circular hole in the ceiling.

stonecaveman,

I agree with your remarks when types of fiber insulation are considered in context to the whole house.  My remark dealt strictly with testing air infiltration through the insulation and nothing else.

The inspector, who has just left, says that he doesn't have a problem with drywall first approach so long as we leave him some way to get up into the space to see the installation.

Slow talking guy, gives you the impression that he's not really concentrating, but then he says something that makes it clear that he's actually understood everything you said and points out some things that you'd missed.

Anyway, so that was no help.  If he'd just flat-out refused to climb a ladder I wouldn't have to make a decision.  The crew (meaning the folks who will end up sleeping under this drywall) are in favor of cockamamy.  But they can be fair-weather friends - as soon as a difficult problem shows up they suddenly decide that other things need their attention.

There is a home builder in my area (NE SD) that builds exactly this way! The plumbers have to figure out where the stack vents are going to be and and stub them just below ceiling height. I am guessing it is a real pain for the plumbers and electricians but the idea is to have a very well sealed ceiling vapor barrier. I have am not say it is right or wrong or goofy. I have been involved in a couple of these builds (HVAC), but it didnot affect our job, but you can guess I heard a lot of pissing and moaning from the other trades.

Posted By slowroadster on 27 Aug 2010 10:12 PM
There is a home builder in my area (NE SD) that builds exactly this way! The plumbers have to figure out where the stack vents are going to be and and stub them just below ceiling height. I am guessing it is a real pain for the plumbers and electricians but the idea is to have a very well sealed ceiling vapor barrier. I have am not say it is right or wrong or goofy. I have been involved in a couple of these builds (HVAC), but it didnot affect our job, but you can guess I heard a lot of pissing and moaning from the other trades.

Thanks for that input.  If the builder has done it more than once it must not be a total pain.  I think that the idea that you'll get a better seal is sound, whether that seal is significant is another issue.

I think we'll give it a shot and see how it turns out.  We have it a little easier because we have flat trusses in the center and scissor trusses on each end.  In the center you can stand up in the attic and the plumbing is on the transition from the scissor to flat, so good access.  You can easily get into the flat trusses through the triangle created by the scissor/flat transition there are plans for an attic ladder, but we may cut that later because once the walls are up we may find a better spot for it than the one in the plan.

Here is a link to a discussion that you might find interesting:
http://www.greenbuildingadvisor.com/community/forum/general-questions/17954/airtight-drywall-approach
My main concern in using this approach with truss rafters is that the problems with truss uplift will be more difficult to deal with, not less. The only way to deal with this would be to use some sort of molding at the interior wall to ceiling interface and be sure that the molding is attached to the ceiling only.

Posted By Garth Sproule on 23 Sep 2010 12:42 PM
Here is a link to a discussion that you might find interesting:
http://www.greenbuildingadvisor.com/community/forum/general-questions/17954/airtight-drywall-approach

An interesting discussion.  The drawbacks are about the same as noted here.  I have SIP walls, so I'm only concerned with the ceiling drywall before the interior partition walls.

Posted By Garth Sproule on 23 Sep 2010 12:42 PM
My main concern in using this approach with truss rafters is that the problems with truss uplift will be more difficult to deal with, not less. The only way to deal with this would be to use some sort of molding at the interior wall to ceiling interface and be sure that the molding is attached to the ceiling only.

Well, we're about finished with the flat portion and will move onto the 3:12 scissor trusses.  So far we've found that hanging the drywall is easy.  Big sheets with no interruptions.  Mud/tape/finish should go pretty fast. 

I'm not sure why you would think that truss uplift problems will be worse.  So far, we have the drywall attached only to the trusses.  The interior walls will use truss clips.  We have one up already to form the partition between flat and sloped.  You're right that we will have to put molding around the tops of the wall rather than finish these spaces if we want to avoid any uplift issues.  BUT the ceiling above is sealed.  If you build the walls first, to avoid uplift you have to float the drywall.  If you float the drywall, you would need to seal along the tops of the walls above the drywall with polyethylene or similar.  This is non-trivial.  So, as long as you're willing to go with molding and not finish the wall/ceiling seams, then the ceiling first is ahead here.

To me, the plumbing is about even.  We have two bathrooms with one vent stack each, and the whole rough-plumbing could have been installed before the drywall.  That may be mostly just a function of our particular build and I think, in general, plumbing complexity would increase, but only slightly.

Electrical is going to be a pain.  We have good access, but we'll still end up crawling around on top of the trusses running wiring.  I think that the electrical trades would have every right to complain that their task is more difficult.  If we'd roughed in the ceiling boxes with enough copper to be sure that they'd reach the switches the task would be a little easier, but there'd still be wires to pull and holes to seal and a bunch of wasted copper.

If your comparison is ceiling first, vs. traditional drywalling, then the traditional approach obviously wins.  However, if the goal is have the ceiling be an airtight part of a tight house, then I think the ceiling first is superior and less effort, even allowing for the challenges of electrical and plumbing.  I could not imagine how you would get a good seal between the drywall and the tops of the interior partition.  This includes around all the penetrations for electrical and plumbing, etc.  You also need to seal between the interior walls and the exterior wall, to avoid air leaks there (any other gaps you can think of?).  Walls first would also take a lot more attention to detail and would be easy to miss penetrations.

I still have no idea whether the saving in infiltration will make scrap of difference.  Since it's my house, and I'm planning to live in it for many years, we are being particular detail oriented, probably excessively so in many cases.  And this may be one of those cases.  On the other hand, one of the selling points of a SIP roof is the limited infiltration, the fact that you can make the house tighter.  Presumably if the infiltration through the ceiling drywall was negligible, this wouldn't be significant. (Ditto for foam sprayed under sheathing).

With either drywall approach, there's going to be a considerable added cost/effort to "doing it right".  However, SIPs and spray foam are significantly more expensive (material cost) than cellulose.  If "close enough" is good enough then traditional is the way to go, I have no idea which approach makes sense commercially if you're going for a really tight structure.

Some more info for you...
http://www.buildingscience.com/documents/reports/rr-0403-air-barriers
and here...
http://www.greenbuildingadvisor.com/blogs/dept/building-science/air-barrier-or-vapor-barrier-building-science-podcast

I especially like the second link and the discussion that follows...

And one more for ya:
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-barriers-airtight-drywall-approach/?searchterm=airtight%20drywall%20approach

Posted By Garth Sproule on 23 Sep 2010 03:00 PM
And one more for ya:
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-barriers-airtight-drywall-approach/?searchterm=airtight%20drywall%20approach

Good sites! So the building science folk, at least, feel that sealing the drywall at the ceiling is of importance.  If the partition wall is close to a truss and you lock the edges together (as they suggest you should) then you have the potential for uplift cracking.

Here is an article on truss uplift...

Posted By Garth Sproule on 23 Sep 2010 05:11 PM
Here is an article on truss uplift...

So now you have to float it and seal it.  Not impossible, but it's a detail you don't have to be concerned with if you drywall the ceiling first.

The other problem I see is that if your partition wall falls close to a truss, how do you maintain the "don't attach for 18in" rule?  If the wall runs parallel to the trusses 24"oc, even if it's in the middle you've only got 12".  If it's close to a truss you'd need some creative framing to get something to attach the drywall to 18" from the wall.

Since I live in the US desert SW I'm probably more paranoid about this than I need to be.

I am sure your system will work fine...be sure to seal any electrical boxes, can lights, wire penetrations etc. Also be sure to attach your molding to the ceiling only...might have to glue it if there is no blocking to attach it.

As to the 18" rule, some planning and blocking between the rafters is required.