Replies

1. GBA Editor
   Martin Holladay | May 31, 2012 04:39am | #1

   Jerry,
   As architect Rachel Wagner explained in a recent Fine Homebuilding article, "The International Residential Code (IRC Section R302.11) requires draft-stopping in double-stud assemblies every 10 feet (minimum) horizontally along the wall, from bottom plate to top plate, the full depth of the double cavity, using ½-inch gypsum wall board or ¾-inch plywood (½-inch GWB is easier to cut and fit into place). The code also requires fire blocking such that the top of the assembly does not remain open to floor framing or attic spaces above. If you’re not using a full-depth top plate that spans across both stud walls, install ½inch GWB or ¾-inch plywood in between the top plates and fire caulk the joints."

   That's what the code says. Needless to say, what really matters is how the code is interpreted and enforced in your area by your local code official. On the question of fire blocking and draft stopping in double-stud walls, interpretation and enforcement vary widely.

   That's why it isn't very useful to get an answer here on the Green Building Advisor website. You need to call up your local code official and get an answer from the person who will be reviewing your plans and inspecting your building.

2. 
   user-1137156 | May 31, 2012 09:03am | #2

   Thanks but it is in the interest of green building to get the code changed!! Either the fire retardants in cellulose insulation make it unnecessary overkill, especially the horizontal requirement or not should be the same everywhere. Meeting the code as it is requires the addition of significant thermal bridge area and seriously complicates construction, especially if the inner wall's outer surface is a vapor barrier and primary pressure boundary with the inner wall being a service cavity.. The only path to accomplish the horizontal fire blocking would be from the outside which would require installing the blocking after both walls were up but before completing the exterior sheathing installation.
   If the code is appropriate Similar horizontal fire blocking should, no MUST, be required in foam insulation as well!

   blocking

3. 
   user-1137156 | May 31, 2012 02:48pm | #3

   I found that 1" or more of Roxul " mechanically fastened in place" Is a Canadian code approved alternative to i/2" drywall or 3/4" plywood in both horizontal and vertical fire stopping required by IRC R302.11. This definitely helps, rather eliminates, the thermal bridging as Roxul is r4+/in.

4. 
   t_t | Aug 09, 2016 06:53am | #4

   Martin, I understand that the local building inspector always has the final word.
   I've been reading the code and some articles about the code and I've drawn a conclusion. Not sure it's the correct one.... :-)

   A] Fire blocking basically means that the area between 'fire blocks' may not exceed 10ft x 10ft.
   B] Walls aren't allowed to have an 'open connection' with ceiling and floor.

   Code doesn't really care how it's done as long as some fire block is placed every 10ft horizontally.
   Just closing the wall wit wood means a thermal bridge; but smart placement of doors and windows can reduce that I think. I a door is placed on say 9.5ft the door acts as a bridge that would be there regardless of code.
   Only a little extra above the door needs to be closed. No much of extra bridging.
   Windows have a similar effect. Especially if they are of the tall narrow type instead of wide-low.

   What's not entirely clear to me what materials are viewed as a block.Would filling the gap with (nailed down) Roxul count as a fire block? Roxul is quite dense and has a can withstand up to 2150F.

   Is the above correct?

5. GBA Editor
   Martin Holladay | Aug 09, 2016 08:33am | #5

   Tony,
   As far as I know, mineral wool insulation is an acceptable fireblocking material.

   The image below came from an online document posted here:
   http://www.awc.org/pdf/education/mat/AWC-MAT110-Fireblocking-071024.pdf

   As usual, your local code official has the final say -- not me.

   .

   
6. Expert Member
   Dana Dorsett | Aug 09, 2016 02:59pm | #6

   In a double-studwall filled with cellulose, the cellulose itself can serve as the fireblocking. At ~15" of thickness even lower density cellulose is as good or better than 2x lumber as a fireblock. Quite a few high-R Larson Truss houses have been built with the attic cellulose continuous with the wall cellulose, with no other than fireblock certified tested cellulose the fireblocking material, eg:

   http://nationalfiber.com/docs/CelluloseFireBlockingandIgnitionBarrierCapabilities1112.pdf

   http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%203.html

   See section R301.11.1 Fireblocking Materials, where it lists:

   "8. Cellulose insulation installed as tested in accordance with ASTM E 119 or UL 263 for the specific application. "

   If the inspector still isn't buying it for use in double-studwalls (most will), you can use rock wool, per section R302.11.1.1 that specifically calls out double-stud configurations:

   "R302.11.1.1 Batts or blankets of mineral or glass fiber.
   Batts or blankets of mineral or glass fiber or other approved nonrigid materials shall be permitted for compliance with the 10-foot (3048 mm) horizontal fireblocking in walls constructed using parallel rows of studs or staggered studs. "

   The whole point of R302.11.1.1 was to avoid having to use more thermally bridging rigid materials on really fat walls- the cellulose will still do the trick, but if they want to see rock wool instead at that transition point it's not worth wasting your breath.

7. Expert Member
   MALCOLM TAYLOR | Aug 09, 2016 10:44pm | #7

   Jerry,
   How does a Canadian NBC alternative help you if you are building under the IRC?

8. GBA Editor
   Martin Holladay | Aug 10, 2016 06:05am | #8

   Malcolm,
   Jerry posted his comment in May 2012 -- so he may not answer your question.

9. Expert Member
   MALCOLM TAYLOR | Aug 10, 2016 10:59am | #9

   Got me! Thanks Martin.