You are better off using these panels to cover your kneewalls after you insulate them with batts. This will give you an additional 2-5 R-Value per inch thickness.above the value of the batts on your knee walls. 1 inch of styrofoam polystyrene is R-5, but I thought ceiling tiles are pressed cellulose which the R-Value will be somewhat less than R-5. TVA recommends a full 2 inches of polystyrene encasement on knee walls AFTER you have filled the wall cavity with fiberglass batts. In 2x4 construction that gives a final R-Value of 23 if installed correctly.
To place them in the 3.5 inch cavity between your ceiling and your roof is asking for problems especially if your attic space is air conditioned. You will get condensation and mold problems. The R-Value bump of 2-5 is simply not worth the risk of obstructing airflow and condensation problems.
Think outside the box and consider reframing the ceiling of your attic from the interior. It will make the ceiling a few inches lower, but then you could properly insulate it and also condition the room with out much fear of condensation/mold.
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