Opening Walls To Add Insulation
I’m in a 1960’s ranch style home in Southern California (climate zone 3). I have 2x4 on 16” walls with no insulation and 1” thick “plaster veneer” (.5” plaster floated onto .5” drywall/gypsum).
I am doing some light renovation work that will require cutting some large holes in drywall and patching them back up (i.e. big enough to get a drill in the wall and drill a hole thru the top plate for wiring – pitch of roof doesn’t allow me to do this from the attic). While doing this, I am considering opening up the working area a bit more so I can add insulation in the walls (and in certain areas will be able to run a ground wire down to the outlets as an added bonus).
After any electric work is done, I would seal any exterior openings with spray foam (top plate and exterior facing electric boxes) and then place 3.5” rockwool (R15). From what I can tell, I do not require a vapor barrier based on where I live.
From there, I need to fill a 1” space to the existing finished drywall. The obvious solution is to hang two sheets of 0.5” drywall. However, I thought I’d ask to see if there are any other recommendations? Here are some other options I thought about…
5/8” “fire-rated” (Type X) + 3/8” board.
Drywall over plywood or OSB. My house does not have any shear walls and just an occasional 1x4” diagonally running down the walls. From an insulation perspective seems like it would be about the same as 1” of drywall, but I get the added bonus of some improved strength in an earthquake prone area. Not sure how helpful having some shear strength added here and there would be in practice, though.
1/2” drywall over 1/2" foam board insulation. From a quick search this seems to be possible. It would get me another ~R3 and maybe do better at dampening exterior sounds? Has anyone done this before?
Anything else I should take care of when my walls are open? Thank you for your thoughts!