There are gaskets you can get for cans and you can caulk the penetration with the trim off also. I'd do that first and see if it helps. The knee walls are a problem but I bet the cans are the real issue. Radiant barrier is not worth a retrofit usually. During new construction, the OSB with barrier is just a small upcharge. I think it was like $200 for our house and so it is worth it. I have a friend who had a similar house to mine built but with no extra insulation etc. He is having a similar problem as yours and he has gotten several quotes including foaming the rafters. His plans include finishing the attic someday (why I don't know since he has a finished basement and is over 6k) so he is thinking of foaming the rafters. I encouraged him to get his cans sealed - sorry I don't know what he did and summer is over anyway... It frustrates me to no end when I hear this story over and over. Comfort issues that are really failures of sealing and insulation. GCs are so oblivious to energy costs but the comfort issues should come back and bite them. Cans into an attic are just always an issue. They are so rarely handled ideally especially in the South. But you can get a lot of mileage from the gaskets and caulking the edge when you take the trim off. I bet you can fix 50% of your problem. You can certainly reduce air movement 80% but you can never have the space taken up by the can to be any better than r-2 that lowers your r-30 by a significant amount. If you can get above the can (and I am guessing you can't), then you can seal 100% and insulate some. The knee walls are an issue for 2 reasons which you probably already know. They often around here are a r-13 batt with no enclosure. Batts work pretty poorly when not enclosed in a space. Also r-13 wall against a 140 degree attic is pretty bad when you are trying to maintain less than 80 inside. The rigid foam encloses the batt, deals with the thermal bridging at the stud and adds another r-4 to the wall. It probably cuts the heat gain by 50% by a combo of all these benefits. It really should be code. The 4x8 sheets cost $12 or so.

make sure you nail or screw them down. my stepdad put flooring in his attic and did not nail any down. it was only where stuff was to be sitting, not for walking on. well, after moving stuff around and throwing out a bunch, the floor is now walking area. last time i was up there one of the boards went out from under my foot and i ALMOST went thru the ceiling. the boards are now nailed down. in many areas we doubled up 1/4" and it is fine. any more than a 16" spread needs to be 3/4".

Keep storage over garage. To install flooring over the living space you would compress the insulation and you need what you have in texas! I like to screw the plywood decking down in case you ever need access to anything below. Size of framing is important, because of weight/span. I don't think anyone starts putting stuff in the attic with the idea of storing a lot of stuff..but a lot of stuff sure finds its way there over the years. Always be careful working in the attic, Make sure there is enough light to see where you rest you tools and put your feet. oh,,,and cut plywood in half two 8x2' pieces so that it goes easily into the attic. I have to cut into 4x2 because it is heavy,,but your big guy wouldn't have that issuel best of luck.

Hi I'm new here and really don't mean to start a war.... Here in Barely Indiana I know summers are not as hot but when you take in consideration how long the material has been stored in attic it adds up. My mom has kept material stored in her attic for more than 40 yrs some of it has been up there for close to that long. She did recently go thru all of the material up there. NONE of it seemed to be ruined. I know for a fact that she has used material that has been in that attic for many many years and the item she made has held up as well as anything she bought new fabric for. Not just a few times but many many times. Based on that I'd say short term for a summer most fabric would be fine stored in an attic even in Texas. I'll add that NO the attic might not be the best place to store fabric but it has worked for my mom. Now her boxes that she stored fabric in did have holes in them so they could breathe that might have made a difference as well. The holes was cut in the boxes before she got them but it would be easy to put a couple of 1 inch holes in any cardboard box. In her attic there was knit fabric of of kinds from the double knit to the sheerest tricot and tshirt material. Cotton broadcloth, polyester and blends. Fake fur, lining fabric you name it she had it up there. A very few pieces had permnament fold lines in it which is fine for making a try it item to check pattern for fit etc.But if you leave a piece of fabric folded for 30 yrs anywhere it prolly would have done the same thing I can't say as the hot attic did that.