Adding insulation to tested assemblies

[IsaArch]

Hi,

Starting down the rabbit whole of finding a 1 hour roof/ceiling tested assembly that allows insulation without having to add the extra gypsum layer.
My assembly is simple with roofing above plywood sheathing on 2x wood framing and drywall ceiling.

So, both GA and UL testing notes that when mineral fiber and glass fiber are added to ceiling assemblies, we need to add another layer of gypsum. Taking the prescriptive fire resistance approach from IBC 721 also notes that adding materials that might change the heat dissipation and that are not listed on the tables will trigger the need to find a tested assembly so...

1. Has anyone found a 1 hour tested assembly for 2x wood framing with 2 layers of drywall only?
2. Does adding mineral wool bypass the GA and UL note since these list fiber specifically ?

Thanks!

 

[Yankee Chronicler]

Mineral wool is mineral fiber.

 

[Yikes]

I typically use the generic assembly in IBC table 721.1(3), item 21 for 2 layers of gyp board. Regarding the adding of fiberglass insulation, IBC 721.1 “General” says:

 

[Wooly]

One option is to install a WRB to the underside of the roof truss and use resilient channel. This gives an air gap dissipating the heat requiring only one sheet of GWB. You should find designs using this quire easily. There is the issue of not having contact contact with drywall and potential air leakage. This can be controlled at the perimeter and the WRB gives you the thermal and air barrier contact you need. Just run the WRB a few inches down the exterior walls and tape the seams. GA file RC2609 as an example.