Siga Majrex T&G ceiling – Drywall or not?
I will soon be insulating and finishing an interior scissor truss ceiling in climate zone 7 with a vented attic above. The bottom truss chords will be insulated with rockwool batts and cellulose will be blown in above the batts. Siga Majrex will be used as an air barrrier and is properly draped over all partition walls. The finished ceiling will be tongue & groove pine and it has been my intention to place 5/8″ drywall between the pine and Majrex for added fire safety and modest sound reduction; however, I am now tempted to install strapping to create a service cavity below the Majrex to hold the insulation prior to tongue and groove installation, reduce penetrations in the Majrex, and add the benefits of a service cavity. I will need two layers of perpendicular 1x strapping so that the tongue & groove can be installed perpendicular to the trusses, as desired. It feels like a lot of layering (and weight?) if I have Majrex, strapping, cross-strapping, drywall, and tongue & groove.
So, my question is whether it is acceptable, or a mistake, to cut out the drywall. Does it provide meaningful fire protection or sound attenuation worth keeping it in the assembly? What would you all do in this situation?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would ask your local code official (if you have one). The inspectors in my area allow only T&G paneling on ceilings as long as it's a full 3/4-inch, no drywall required.
I've installed strapping when using paneling several times, I like the look of running the paneling up the roof instead of across. It's different than what we normally see but still looks good IMO.
Randy,
Or if an architect is involved...
Thanks, Randy! No inspection here, I just wanted a sanity check that it isn’t inherently irresponsible for the assembly to exclude the drywall fire protection.
I guess I hadn’t considered running the boards vertically. The flooring (also wood) will run perpendicular to the trusses, so I planned to keep the same direction with the ceiling t&g. I’ll reconsider that approach because vertical would avoid the need for two layers of strapping. Thanks!
ESL_ND,
Our code, and as far as I can see the IRC, views 3/4" and thicker wood as having a low enough flame spread rating that it doesn't need drywall behind it to slow fire spread.
The drywall is an unnecessary step in my opinion. More fire resistant yeah but wood is good enough. In terms of sound dampening I think that has more to do what’s in between the walls. I have been to plenty of places that the whole house is covered in drywall and I can hear everything.