I want to hang 2X6's perpendicular and under my existing 2X12 joists so I can add r-21 batts under the existing r-38 batts. I live in airfreight only territory so foamboard is too expensive to add to the ceiling. Does anyone do this? Vented Cathedral ceiling.

How would I attach them securely? I looked through the simpson catalog but didn't see what I needed.

Thanks.

Look under "rafter ties" or "hurricane ties"

There are a couple of variations on the theme- the single side such as the Simpson H2.5 G90 or the diamond shapes that nail in from 2 sides such as the Simson H1 G90 (preferred.) See:

http://www.dhcsupplies.com/store/p/2575-Simpson-Strong-Tie-H2-5-Hurricane-Tie.html

http://www.amazon.com/Simpson-Strong-Tie-H1-Hurricane-Tie/dp/B000WI30LG

http://www.amazon.com/Simpson-Strong-Tie-H10-Hurricane-Pack/dp/B0044USXEM/ref=sr_1_40?s=hi&ie=UTF8&qid=1324673310&sr=1-40

You could use long wraparound joist hangers for 2x10s too, and while that would be stronger structurally, it also creates extra thermal bridging. Unless you live in an earthquake zone this isn't a high-stress application a rafter tie at every intersection should do it. The ties are only only being asked to supporting the weight of the 2x6, the finish ceiling, and the insulation. I'd use 1.25" screws rather than nails though, as the fasteners will be carrying the load, and nails may work loose over time.

What distance are you running? Assuming it is not a huge diistance maybe you could fasten a 2x6 "rim joist" to the walls and use joist hangers? Basically supporting them from the walls rather than hanging them from the existing ceiling structure. Depending upon the distance you may not even require 2x6 as you could leave a 2" space between new 2x4 cross-pieces and the existing joists.

I like Bigrig's idea better from a structural point of view, but the long 1.5" x 2" channels through the batting makes for a huge thermal bypass. It functions better thermally if it can be done with no voids where air can move freely. (That would be more of a "rim rafter" than a "rim joist" though, since it's a cathedral ceiling, eh? ;-) )

If you had access to an insulation blower and could pack it with cellulose at 2.5lbs+ density that would be great, but I'm assuming that's out of the question. It's tedious, but you might cut 2" wide strips out of 2x4 batting and tuck & compress the strips in on top of the 2x4s carefully before installing the 2x6 dimensioned batts to get the thermal break without the thermal bypass. Don't worry about compressing the R13 or R11 2x4 batt strips down to 2" thickness- it's performance per-inch will be better, and compressed to 2" it will deliver close to R8.

Low density batts R19 or R22 batts would SUCK in this type of application, but high density "cathedral ceiling" R21 batts should do pretty well if you can install them without gaps or voids.

I would carefully evaluate the spans of the 2 x 12s and the new 2 x 6s. Hurricane ties are not intended for this application. Screws need to be rated for shear value. Bigrig has the right approach. Consult an engineer.

If all it's holding up is half-inch gypsum and a 2x4 at an angle and you're using them at every intersection the diamond-shaped hurricane ties would surely handle the static shear load even if it's a sub-optimal design. (If they couldn't handle that fairly modest load they wouldn't serve the function they were designed for very well either.) But it's not as straghtforward and dead-obvious as BigRig's approach.

Thanks for the great input. It's a 16X20 building with the 2X12 rafters running the shorter 16 direction.

So the span is 16 feet. What is the spacing and type of lumber.

2X6 24 OC walls and 2X12 24 OC Ceiling.

The lumber is basic stud grade, we don't get strong stuff in Bethel AK the suppliers send the stuff they want to get rid of on the barge b/c we can't really send them back. I really like hanging 2X4's w/ the air space to cut the thermal bridging (I could do 16 oc and run them the short span) WAY exceeding what a chart would say, but if it's just to hang drywall it should be fine, yes?



 

Probably yes. It's pretty common to build truss structures similar to Larsen Trusses on to the inteior of joists though, with a comparable benefit regarding thermal bridging. See the photos on page 6 of this document:

http://www.kaplanthompson.com/_images/publications/09.06-jlc.pdf

If I were paying air freight based on weight/volume I would probably frame it with steel studs and stiffeners and hang it with pencil rod. If the ceiling is the underside of roof (cathedral ceiling) its already rated for the snow load c/w insulation and drywall so adding the weight of light weight steel stud and additional insulation is unlikely to cause any problems.

The thermal performance of steel studs are pretty abyssmal due to high thermal conductivity of steel relative to wood. (A 2x4 studwall with fiber insulation comes in at around R10 with wood studs, R7 with steel.) Steel Z-furring performs better thermally than steel studs (and it's lighter still!)

Posted By Dana1 on 13 Jan 2012 11:22 AM
The thermal performance of steel studs are pretty abyssmal due to high thermal conductivity of steel relative to wood. (A 2x4 studwall with fiber insulation comes in at around R10 with wood studs, R7 with steel.) Steel Z-furring performs better thermally than steel studs (and it's lighter still!)

Dana - your right about the steel however since its being hung on wire the through thermal bridging would only be the x-section of the wire. If I rethink this a bit I would probably use just 1.5" studs with stiffeners and hang them with eyelags and regular hanger wire (think T-bar) and cross strap it with hat track. op can hang this at what ever depth of insulation he wants to contain. The insulation would stay above the 1.5" stud. I would probably run the studs in plane with the roof slope and anchor to the end walls with a metal angle to resist any downward creep. Using the studs and hat track I believe he can use all 8' material as the grid joints could be staggered.
With a buildup like this, would you leave the v.b. on the underside of the joists or move it to the underside of the new ceiling? I believe this was a vented ceiling. Bob