Air Sealing Question – Zone 5 – Existing vapor barrier, conditioned/unconditioned meeting point – GreenBuildingAdvisor

– Zone 5 (Northern Illinois – cold winters, hot summers).

-Vented / Unconditioned Attic

In the process of air sealing my attic, but a bit confused at what the proper technique is for a few large swathes of attic walls. The vented, non-conditioned attic walls are shared with conditioned space – bedroom walls and high ceilings. I opened up one of these to validate what was actually behind it. Currently the insulation consists of fiberglass bats (paper face turned to conditioned space), covered with fiberglass boards. Some of the boards are starting to sag, nothing is air sealed.

My current understanding is that ridged foam board to replace the fiberglass board isn’t ideal due to a double vapor barrier, and the combustibility of the foam.

The options as I see them:

1. Replace any sagging fiberglass boards
   1. Air seal seams with appropriate air seal tape product (mastic + fiberglass mesh tape?)
2. Alternatively – replace some/all fiberglass boards with mineral wool.
   1. Air seal seams with appropriate air seal tape product.
   2. The cost here doesn’t seem worth it.
3. Remove all fiberglass batts –> air seal the studs –> replace with non-faced batts –> rigid foam and/or insulation board
4. Remove all fiberglass batts and board –> closed cell spray foam application.
   1. I’d prefer not to do this due to cost and inability to DIY such a large space. If it ends up being the least labor intensive option though that may be ideal.

Things I cannot or should not do:

1. Replace the sagging fiberglass boards, rigid foam over the existing batts (double vapor barrier).
2. Rigid foam over existing batts and fiberglass insulation boards