Will, if it is warm up there it is likely that there is inadequate insulation between the too of the rooms and the loft so insulating above your head is a good idea. My first Loft Conversion back in the 70’s I insulated with soft rolly stuff of the day. Horrible to do but it was the cheapest option when I was penny poor. That was covered with Hardboard so that it did not constantly keep dropping fibers but also to keep it in place. Not the cheapest of jobs these days and not too easy getting the covering sheets through the hatch. I modified mine to be 2’6” to assist in that regard.
As for your knees, protecting them now will pay dividends in later life. Trust me I used to be on my knees all the time when I was your age and regret it now!