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.

I"m researching this sealed attic concept for a renovation/addition in Virginia. It seems to me to make sense that if you seal your attic and your mechanical equipment is up there it will have less heat or cool loss - and the upstairs will be improved as it will be against a semi conditioned attic thus resulting in less heat or cool loss. I guess if you only insulated your ceiling - just the fact that your ac unit is running in a 130 degree attic or your furnace is running in a 30 degree attic - means you are losing cool/heat that you are paying for. A sealed attic (spray foam roof deck) would seem to be more efficient and result in lower energy usage. the closed cell/open cell decision and the the thoroughness of the sealing of the envelope would seem to be critical. Also - what are the real ramifications of roofing lifespan reductions due to hot sheathing. Will the sheathing really be that much hotter than when it isn't insulated under the roof sheathing? I suppose the heat cannot flow into the attic - but at some point doesn't the attic heat up to the point where there is no more heat flow and the roofing is as hot as it would be with a sprayed deck? HOw might metal roofing effect this consideration? What about solar PV or Hot Water above a sealed attic (seems like the solar hot water could only benefit - i guess the pv panels could get too hot if not spaced enough to 'cool' off. any thoughts?

Since you're planning to use a height-challenged attic as living space and you don't want to touch the roof, you've limited your choices. The ceiling becomes a cathedral ceiling and the best way to reach Code R levels is high- density closed-cell foam followed by one-two inches of extruded polystyrene foam board. But with typical 2"x6" roof rafters (the sloped boards), you'll still be below recommended insulation levels. Not to mention problems pointed out by other posters: limited headroom, possible leakage from the roof, inadequate ceiling joists that will become floor joists. In addition to the excellent article linked by energy rater, you might want to look at further items at the Building Science Corp. site: Unvented Roof Assemblies for All Climates, Building Profile: Very Cold Climate. According to the Canada Housing and Mortgage Corporation, recent research indicates there's not much difference between conditions in vented and unvented attics. Here is a link that might be useful: How Much Insulation is Needed

Mongcot Thanks for replying. I don't think you screwed up the math but the main difference is that I decided not to go so thick. The contractor said he would spray in r-38 in the ceiling. ?From what I understand that would be overkill. I had no option if I went with him other than to go with that much spray foam, He didnt want to do the job unless it was $6k+. No other contractors in my area . The contractor also would not take doors and windows out of the EQ. I have 6ft sliding glass doors and large windows. I spent $3k will do approx. 3" on the underside of the ceiling, 2" in walls and under floor. That cost also included the fast kick gun. I have 2 large emglo air compressors already sitting in tandem. I read that 4" of lycene foam was used in making freezers , If that's true I figure I should be ok. Am I right in thinking that I don't need "R-38"(6" or so)? So my options were $8k for what sounds like overkill(wish I could have used half as much for $4k ad had someone else do it but he wouldn't budge. or batt insulation for half that much . The pallet arrived late with the gun missing . A new gun was supposed to be here last friday. its tuesday no idea when the gun will arrive so I am sitting here ready with a pallet of foam. This looks like a lot of work and want to get the drywall up . but hopefully it'll be done with no major setbacks in teh next couple weeks