Attached Garage Separation Wall Insulation

[Super400]

I'm getting close to finishing my 26x26 attached garage project addition. The garage is in Michigan, Zone 6. My question is regarding the wall the separates the house and the garage. It is constructed with 2x6 studs that have a cavity depth of 5.5". Building code requires that I remove the existing OSB and install drywall for fire code. Currently for insulation there is R-19 fiberglass in the wall cavity. I want to make a more robust insulated wall assembly since it's the barrier between the house and garage and I was thinking of doing the following below. Wanted to get some opinions from members on the forum if there are any issues with my plan.

Garage Separation Wall:
*Work will be performed from back side of wall (into garage) as interior side has finished drywall, so I don't want to open up the wall form the inside of house.

1.) Cut and install 2" of R-7.7 expanded polystyrene (EPS) insulation in each of the stud bays tight up against the backside of the interior drywall. This EPS is the stuff you get and Home Depot that has one side with a metallic-reflective polymeric facer. Air seal edges of EPS in stud bays with caulk or spray foam for an air tight seal.

2.) Install 3.5" of R-15 Rockwool Comfortbatt mineral wool insulation behind the EPS.

3.) Caulk face of studs with a sealant or adhesive then install the drywall for the fire barrier code requirements. Sealing the face of the studs I'm hoping would add another level of sealing so the cavity doesn't have any air movement so R value is maintained but also so no fumes can get into the house from the garage.

Some may say just skip the EPS and add R-23 Rockwool since it has a depth of 5.5 and it would fill the cavity. Problem is in my area I can only get the R-15 Rockwool with a 3.5" depth so the EPS will fill the rest of the cavity and since the EPS will be placed up against the drywall (warm side) and edges sealed it will function as the vapor barrier per code where I live.

I'm just wondering if anyone sees any problems with the approach I'm taking, i.e. condensation issues in wall cavity, etc. etc.? I know there are cheaper ways of probably doing this but it's only a single 26' wall and saving a couple hundred bucks isn't my objective. I really want an above average separation wall that has good insulating properties as well as isolating the garage air from interior of the home.

Thanks in advance for any thoughts you can share.

 

[nmk_61802]

What's your exterior wall insulation? If the garage is insulated itself, there is not a lot of reason to increase the insulation in the separation wall. The garage will always be warmer the the exterior, and all heat-loss/ gain will come from the other walls/ attic. Air sealing via caulk or foam is a good idea.

 

[Super400]

Right now there will be no insulation in the garage exterior walls. At some point I may add R-19 rolls (which is really R-17 when compressed into cavity) in the walls and blown in insulation in the attic. I know my insulation plan for the separation wall seems like overkill, but I wanted to use the Rockwool for the sound reduction aspects and adding the 2" of EPS to fill out the 5.5" cavity is easy and really doesn't cost much and didn't think having more insulation than needed would be a bad thing, but that's why I wanted to ask the folks on this forum. If I could have got the Rockwool R-23 (5.5" depth) insulation I would have just used that but unfortunately that product is not available where I live.

 

[Scotto]

That seems like a lot of work for minimal gain (you're already at R-19). I'd just put up drywall over the OSB to add the flame retardant. You'd have the added benefit of having wood everywhere to hang things.

 

[Rusted Nut]

That seems like a lot of work for minimal gain (you're already at R-19). I'd just put up drywall over the OSB to add the flame retardant. You'd have the added benefit of having wood everywhere to hang things.

This!