2" fiberglass insulation can be compressed as you indicate.

I didn't think it would be problem but I'm new st remodeling so I wanted to ask. Thank you!

I didn't think it would be problem but I'm new st remodeling so I wanted to ask. Thank you!

Q for reno8...if it is compressed, won't it lose R factor? Also how would fiberglass compare to styrofoam sheet for insulating? Might that be a better alternative?
Thx!

I usually dont offer opinion on questions directed at individuals but feeling generous today. I think its better to compress insulation. It technically does lose R value in a lab setting but in reality should improve it by stopping airflow. The technicality is based on FG filling all airspace and being airtight on all 6 sides which is rarely the case in the field.

Insulative sheathing is always a good insulation practice because it reduces airflow and thermal bridging better than cavity insulation.

Below is a compressed R-Value table from Owens-Corning.

The slight compression won't affect much.

In the OP's situation, I'd just cut and piece-in XPS and foam it tight.