Highest indoor temp I have seen so far this year is around 74F. This in a very poorly insulated house that started it's life as three shacks drug together in the 1940's. We are out on the Olympic Peninsula and we always run a little cooler than most of the rest of the state.

My reasoning for sheeting in the ceiling stack was primarilly to aid in air control. We have a 40' roof span so there will be a bit of movement between the tops of the internal walls and the bottom chords of the trusses. A air barrier that remains more flexible over time, particularly one covering the top corners of the drywall appeals to me. I hadn't thought of using housewrap though, I will have to look into that, thanks...

Posted By ronmar on 06 Aug 2015 08:56 PM
We have a 40' roof span so there will be a bit of movement between the tops of the internal walls and the bottom chords of the trusses. A air barrier that remains more flexible over time, particularly one covering the top corners of the drywall appeals to me. I hadn't thought of using housewrap though, I will have to look into that, thanks...

I don't recall where I saw it, in some of my research or a workshop or where, but there is a way to deal with this. Basically you don't fasten the ceiling sheetrock to the joists any closer than about 12" to 18" of the walls. The wall to ceiling sheetrock joint will then cause the ceiling rock to flex up and down as the joist moves up and down.

I trust the more experienced here will chime in with some clarifying comments.

Do some research on buildingscience.com about vapor and air movement. You can get plenty good air barrier with sheetrock and paint and still have a vapor permeable layer. You want to allow vapor to move in and out, which poly sheeting won't permit. Vapor movement does not equate to air movement.

Yep, that is how you attach sheetrock to allow the bottom chord of the trusses to float. You also use slotted clips to attach the walls to the trusses. Secures the wall laterally, but allows the truss chord to move vertically in the slot.

But since it still moves/flexes the drywall, my concern is long term ability to maintain a seal.

Yes, you can deal with the movement issue as dmaceld describes. We have used this method for years in our colder climate. The more severe the climate and the steeper the pitch of the trusses, the more differential in the expansion of the bottom and top truss chords. But it means that you are relying on the paper tape joint in the corner as you air barrier. I believe this is ronmars point. He recognizes the need to have a long lasting flexible joint at interior wall and ceiling joints. If you place framers poly (18" wide) between the double top plates prior to standing up the wall, you can then fold this poly up the edge of the top plate and horizontally over the ceiling. When the ceiling poly is applied it over laps the wall poly and is sealed with acoustic caulk for a long lasting flexible joint.
I see no reason why housewrap would not work but have not tried it.

If we have conditions such that water vapours are going to condense on the upper side of the ceiling (regardless of the material) will any vapour permeable air barrier allow sufficient vapour though to make a difference to the near ceiling insulation's vapour content if the whole attic space continues to produce condensate at that point? If it did, would you have to run the a.c. even longer to remove this vapour that is being transmitted through the semi permeable barrier thereby causing more condensate to form above the ceiling?

If the conditions prevail, it is unlikely that enough moisture moves into the house to make a difference. As soon as the condition reverse to a more drying condition, the vented attic space should provide more drying then a permeable barrier can provide so I'm not quite sure why the concern with the vapour barrier.

Even at my age, I can properly poly and seal a 3000 square foot ceiling space in under five hours. I suspect it would take two young lads three times that long to solid sheet a multi room ceiling and then you would still have to provide seam sealers that will remain adhered and flexible to OSB forever which I'm not persuaded can happen due to the expansion and contraction of the OSB to itself and to it's adjoining materials.

Experience in the muggy southeastern US (with average mid-summer dew points in the 70s F ) shows that latex paint is sufficiently vapor permeable to keep the paper facer on the insulation side of the ceiling gypsum mold-free in air conditioned buildings. 

The same would be true of OSB, which becomes more vapor open than latex paint when it's moisture content rises sufficiently to grow mold.

The expansion & contraction rate of OSB with moisture & temperature is the same as the wooden truss chords they would be attached to.  If tapes and goops work on exterior sheathing attched to studs over the long term (and they do), they'll work even better in the more benign climate of the interior side of the assembly.

Posted By FBBP on 07 Aug 2015 01:11 AM
Yes, you can deal with the movement issue as dmaceld describes. We have used this method for years in our colder climate. The more severe the climate and the steeper the pitch of the trusses, the more differential in the expansion of the bottom and top truss chords. But it means that you are relying on the paper tape joint in the corner as you air barrier. I believe this is ronmars point. He recognizes the need to have a long lasting flexible joint at interior wall and ceiling joints. If you place framers poly (18" wide) between the double top plates prior to standing up the wall, you can then fold this poly up the edge of the top plate and horizontally over the ceiling. When the ceiling poly is applied it over laps the wall poly and is sealed with acoustic caulk for a long lasting flexible joint.

Exactly! This was also very much what I was envisioning for the wall to ceiling seal. Strip of poly folded over the wall top plate before the wall is stood up. The wall-ceiling sliding clips would be placed on top of the poly(and sealed) so that the connection to the moving truss is situated outside the air seal. Sandwiching the poly between a doubled top plate sounds interesting, less chance of tearthru where the clips attach... The approach of creating one predictable air barrier and good sealing at any wall top plate wiring/plumbing penetrations appeals to me. As opposed to a possible breakdown or leak at every drywall to top plate union, every wall penetration and every wall or ceiling taped joint. Thru good detailed drywall air sealing most all of these can be dealt with effectively except the drywall ceiling taped joints on a moving ceiling. I am also of the opinion that internal moisture is best controlled thru proper ventilation and not on the possibly unpredictable or perhaps insufficient permeability of the building materials. Wow I sure drug this thread away from it's intended topic, Sorry. Perhaps I should start a new one on air sealing.

Another way to lock your ceiling drywall down at the interior partition wall top plates is to install 2x6 blocking between the truss chords over the 2x4 top plate.

"Sandwiching the poly between a doubled top plate sounds interesting, less chance of tearthru where the clips attach."
No to mention much safer! If you are a wall walker, you don't appreciate slippery poly under foot when you are moving trusses. Especially when wet or frosty! Just remember to stick the acoustic nozzle up any hole in the plates and give it a good squeeze to seal around any wires or plumbing.
We generally lap the ceiling poly down the ICF walls about one foot and seal it to the foam. The drywall then sandwiches it tight. Don't know if you have vapour hats available for electric out lets where you are but if you travel north every box store will have them. Vapour hats, h.d. 6 mil poly and acoustic caulk and there is no reason not to have a 100% sealed ceiling that will last for a long time.

Some great ideas here too bad you guys are putting it all under a topic that has nothing to do with what you're talking about . Folks that want to learn about the topic won't and people looking for your stuff won't find it

I'm going to start another thread on sealed versus unsealed attics -- I have a roof design I want to throw by you guys and see what you think. Regards

Posted By TexasICF on 07 Aug 2015 05:54 PM
Some great ideas here too bad you guys are putting it all under a topic that has nothing to do with what you're talking about . Folks that want to learn about the topic won't and people looking for your stuff won't find it

What do you care, having dredged up a 4+ year old thread?

This topic comes/goes around very often.  ronmar had a specific stackup and was looking for "...of a possibly better way..." I was questioning whether it was climate-appropriate and had some suggestions.  I don't care if ronmar were the only person reading it, but am happy to read the input from jonr, FBBP, arkie6, dcmaceld & smartwall too. There a many ways to skin this cat, with lots of details to consider, some work better in some climates than others.  It's been discussed at length on this site before- count on it.

With a 40' span bottom chord loading specs become an issue at high-R with cellulose, and even if an OSB or 5/8" gypsum layer could take the load, you need to verify that the truss chords are designed & rated for it.  With 2x4s perpendicular to the truss chords under the OSB as a lighting & wiring chase the dynamic loads get shared among trusses, but the static load goes up due to the weight of the extra framing & OSB layer, even though you can back off on gypsum weight a tad. 

The OSB gives the chord load a bit more structural strength whereas gypum would add none, but unless you want to do the engineering on that you should treat it as dead load, and use only the truss designer's load spec for the chord.

Texas ICF is right. Going off-topic doesn't help the search function. It's pretty easy to just start a new thread. This website has NO moderators but on other websites the moderator would have intervened to stop the off-topic thread.