Here’s the scenario- you have 2×4 (3.5″) stud walls and you want to go the FLASH & BATT route of one or two inches of foam and the rest of the space with fiberglass batt. They do make 2″ fiberglass batts now for this type of project but they are not always available in local stores.
The problem with regular 3.5″ batts is that to fit it, you either have to cut the batt or squish the batt in after the foam has been sprayed.
It is true that squishing batt insulations collapses the air pockets they use to produce R-value.
The problem is- when air passes through fiberglass batt, you lose R-value anyway. That’s the point to why you’re putting in the spray foam- to stop the air flow. Plus, when moisture gets into the batt, you lose MORE R-value.
That means, not doing FLASH & BATT is worse than doing it with some squished batt. This is because the foam will provide a tight air seal and a Class III vapor retarder, plus an R-value of 7 (or 14 for the people doing 2 inches). When no air or moisture move through fiberglass batts, the batts perform better. Period. So, if they are a little squished but overall the wall performs much better, you’re ahead by squishing the batts.
OR – Here’s another option if you just don’t want to squish fiberglass. How about using recycled denim batt. You can squish RDB up to 1.5″ before affecting the R-value because it doesn’t use air pockets to hold the heat.
OR – Go for all 3″ of closed cell foam (or swap to open cell foam for the final 1-1.5″).
**Reminder for anyone who heats a lot in the winter – you want to do 2 inches of closed cell foam first before any other form of insulation to avoid condensation during the winter months.