The installation instructions will state the minimum free air space required around the fixture. It will normally, also be on a sticker on the fixture. You may need to make a dam or box to keep the insulation outside of those dimensions. While similar, each model will be a little different. The alternative is to buy an IC-contact rated version. They are usually not much more, but you may not find that version in a remodel version. This would require you cut then patch the ceiling rather than just cut a hole and insert the lamp.