I plan to use this under an acrylic bathtub for some insulation. I was wondering if it would also assist in making the base more rigid, more firm?

Thank you for the question. Using OSI QUAD Foam for the intended use described will not add rigidity to your acrylic bathtub and would not be recommended for that purpose. Assuming you have full access, this may work for adding some insulation, however, overhead applications may be messy and attempting to fill blind cavities or enclosed spaces, that are not fully accessible, with these types of foams generally do not work and the end results are typically hollow spots that did not cure properly, due to suffocation. If you have blind cavities this product should not be considered.