I am in the process of insulating my garage and above it a bonus room. The garage is attached and has 2.5” of continuous insulation all the way around up to the truss tail ends the walls will have r21 batts. The ceiling of the garage has an r38 batt under the bonus room and the areas that are not under the bonus room will have blown in r60. Walls and ceiling in the bonus room are insulated as well to r50 minus one spot where a proper vent goes through where the most we can fit in the space is r19.

The question I have is should there be any smart membrane used on the ceiling of the garage. The insulator has said that years ago they quit using vapor barrier under the bonus room and usually stop it about flush with the edge of the room because it caused moisture issues in the past. I had never heard this.

my garage will typically be around 50 in winter the bonus room around 70

I know that vapor barrier is generally speaking not needed but I have opted to use certainteed membrane because it’s a vapor open product on my walls and ceiling. I have a class one installation install everywhere this guys amazing with his install.

Based on his suggestion is it wise to not use the membrane on the ceiling of the garage under the bonus room? And if your saying no what’s the thought process.

the reason for the membrane is my local inspectors preferred a smart membrane over just the airtight drywall approach because he had some issues on another job with just drywall. He was comfortable with the smart vapor barrier though.

appreciate any thoughts

Thanks, Tom