Replies

1. 
   Steve Vigoren|Jun 11, 2015 05:49pm|#1

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_par025.htm
   如果你有一个不适宜居住的阁楼没有存储, this table gives spans around 18 feet or more depending on the species and 2x6 or 2x8. I plan to use this table and talk to my inspector prior to building. I am also thinking an L or U bracket screwed into the joists through the sheathing and the into the clear span truss which will be directly above the joists. I will discuss that with my inspector and builder prior to attempting.

2. 
   Jerry Liebler|Jun 12, 2015 09:57pm|#2

   To have an OSB ceiling air barrier layer AND a ceiling service cavity under raised heal trusses you have two choices. One choice is build the exterior walls , set the trusses on them then cover the bottom chords of the trusses with the OSB, then hang ceiling joist from the trusses, build interior partitions under them to support drywall. This method can get the roof on quicker by delaying framing the interior partitions but how to securely and economically "hang" the ceiling joist is far from obvious.

   The most straight forward way of implementing a ceiling service cavity and continuous air barrier is to use separate ceiling joist supported by walls (both interior and perimeter) with OSB laid over the ceiling joist and trusses above the OSB. This is what Thorsten Chlupp has done. If the largest space between interior walls can be limited to about 19 feet, 2x6 16" OC framing of the ceiling can eliminate any need for temporary walls or posts and the trusses are installed above a solid OSB deck. This second method may use a bit more lumber and requires interior partitions be done before the roof but truss uplift will never be an issue.

   FWIW I'll use the second method, my perimeter walls are double 2x4 24" OC . The outer perimeter wall's top plate is directly under the ceiling OSB while the inner perimeter wall has double top plates and supports the ceiling's 2X6 framing. The outer face of the inner wall is covered with OSB which is continuous to the ceiling OSB.

3. GBA Editor
   Martin Holladay|Jun 13, 2015 05:39am|#3

   如果有人使用杰里的第一个方法,不要忘记to install strips of OSB over the top plates of the walls before installing the roof trusses. The OSB strips should overhang the interior space, with a lip that is wide enough to tape the rest of the ceiling air barrier to.

4. 
   Brian W|Jun 13, 2015 10:24am|#4

   Thanks Steve, Jerry and Martin. I'm leaning towards the second method Jerry described. I also want to price out the cold climate building method to use in conjunction with this.http://www.cchrc.org/sites/default/files/docs/REMOTE_Manual.pdf

   Trying to figure out what would be cheaper - zip sheathing and tape for walls and ceiling air barrier or OSB taped with Siga wigluv and covered with Tyvek Commercial D. Is it recommended that the zip be covered with a house wrap also? I know Huber wouldn't say so, but I've read a few people on here say they would.

   Also, being in climate zone 5, and using this "no interior insulation" method, would it be better to have a more vapor permeable exterior insulation such as rigid mineral wool? Because I want to avoid using any interior vapor barrier. If I went with XPS, would it be worth it to use polyiso as a first layer where it could retain its insulating properties being separated from the performance degrading cold towards the outside?

   I like this cold climate insulation practice, and also was curious if it could save me money on drywall. Since the air barrier is on the outside. Many interior walls would not have to covered. Am I correct on that.

5. 
   Eric Habegger|Jun 13, 2015 11:26am|#5

   Don't forget that straight OSB isn't necessarily airtight. That could factor in depending on how tight you are trying to get your new home. See this:

   //m.etiketa4.com/blogs/dept/musings/osb-airtight

6. 
   Brian W|Jun 13, 2015 12:00pm|#6

   I saw that Eric. I was hoping that the Tyvek Commercial D covering the taped OSB would aid in reducing air leakage, along with the layers of rigid insulation.

7. 
   Brian W|Jun 13, 2015 01:27pm|#7

   Can someone please explain this picture to me?

   //m.etiketa4.com/sites/default/files/Screen%20Shot%202011-07-24%20at%2011.58.17%20AM.png

   How/when is the air tight sheathing above the service core attached? Does this mean that all the roofing members/trusses are built on site above it? I'm confused how a manufactured truss would fit in this assembly, or if it even will.

   The original commenter and poster of this pic (in Joe Lstiburek's venting article) James Morgan said: "the notion of a ceiling service core below the air barrier is a great one - future-proofs the installation by allowing easy electrical remodels below the air-seal layer. Here's a close-to-standard stick-frame alternative which meets Joes' requirement for depth of insulation at the attic edge above the wall but which avoids the structural redundancy of the truss above the encapsulated ceiling joist. Just have to figure out how to tell the client they're going to to pay for a fully-sheathed attic floor but they can't store anything up there."

8. Expert Member
   Malcolm Taylor|Jun 13, 2015 04:49pm|#8

   Brian,
   The same detail could be used with trusses. You either stick frame the roof or install the trusses from the sheathed flat roof deck. As you say, it isn't cheap. Whether having a service chase below the roof is worth the cost is a open question. I think you can discount the benefits it affords to easing the construction of the roof. I've never taken more than two days to set and sheath a trussed roof.

9. 
   Jerry Liebler|Jun 13, 2015 04:56pm|#9

   Brian,
   The "picture" shows essentially what I describe as the "second" choice. The "attic floor" sheathing is laid upon the ceiling joist and completed BEFORE any roof members are installed.. It also shows stick framing of the roof with discrete rafters and no trusses. With this construction engineering attention must be given to the junction of the rafters to the attic floor to assure adequete transfer of horizontal forces of the rafters to the attic floor diaphragm.. My preference is to use, factory fabricated, deep energy heel trusses to extend the exterior wall vertically and accept bit of redundant structure in the bottom chords of the trusses which will carry NO vertical loads from the attic floor, insulation or ceiling and simply sit on the attic floor.

10. 
    Steve Vigoren|Jun 14, 2015 08:04pm|#10

    Jerry's number 2 in comment 2 is what I plan to use. Since I am in zone 7, I will probably use poly sheeting on top of the OSB and under the energy heel trusses for air sealing.
    //m.etiketa4.com/sites/default/files/better.JPGshows this with a truss instead of rafters.

11. 
    Brian W|Jun 14, 2015 08:44pm|#11

    Yeah, I'm wondering if I'm reaching the point of diminishing returns if I go for the sheathed service chase below the manufactured heel trusses. I've got 2357 sq ft of ceiling I would have to build. Wondering if I should just go the airtight drywall approach for the ceiling and avoid all punctures - possibly going with sconses instead of can lights.

12. Expert Member
    Malcolm Taylor|Jun 14, 2015 10:43pm|#12

    Brian,
    At our local prices you are looking at roughly an additional $3000 labour and materials to include the ceiling.

13. 
    Brian W|Jun 14, 2015 10:52pm|#13

    Good to know. Thanks Malcolm.

14. 
    Jerry Liebler|Jun 16, 2015 10:45am|#14

    My question for everyone: If one is building a full attic floor over a ceiling for air sealing and attic insulation support, and will have a truss roof, which is more economical; A., using deep energy heel trusses to, in effect extend the walls or B. build a perimeter of short walls and use flat bottom trusses set on these short "attic" walls?