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've found that rooms over garages are very hard to condition. there are a couple of ways to address this, walls shared with attic space are called kneewalls. on one side is living space (conditioned) on the other side attic space (extreme heat/cold). Southface has great info, check page 5 for visual of air sealing these walls. http://www.southface.org/default-interior/Documents/airsealingkeypoints.pdf with these attic rooms surrounded on 6 sides by extreme attic temps there are two ways to address them. one is pictured in link. convnetional insulation in stud bays, foam sheathing, caulked nailed & taped in place from top plate to bottom plate of kneewall. the detail on link also shows how to seal for air movement under floors of these rooms by using foam board, and placing it between floor joists & sealing air tight. I personally like to cut the foam board to fit tightly & air seal with caulk. I see people using great stuff..but am not a fan of the stuff myself. too much expansion to get a good seal, and over time, it shrinks. this is a process that is time consuming & detail oriented. easiest in new construction prior to any insulation being added to attic floor. more cost effective than spray foaming the walls and between floor area. but labor intensive. so its a easy fast process (spray foam) or a longer lower cost higher labor install with foam board. the benefit of creating an unvented semi conditioned attic space works great in my hot humid climate where in the summer attic temps reach 130+ degrees in attics. as we often put our ducts in the attic, the foam creates a less extreme temp for less heat gain. install of foam is crucial. temp mix has to be correct chemical mix has to be correct, temp of area sprayed is a factor. foam has to meet code requirements for your area. if code says R-30 in attic, enough inches of foam to reach that R-value has to be installed. seal has to be complete in an unvented attic. roof connection to attic floor has to be sealed. install should be level, no belly in or out between rafters. faces of rafters should be covered. none of that average fill stuff which means the spray can be less in some areas & more in others so that it 'averages' out to xx inches. low spots & voids will cause problems. testing for leakage, marking leakage sites with spray paint and having foam company spray to seal is the way I do my unvented attics. this all has to be established, in writing prior to any work being done. shop companies carefully. avoid 'average' fills, Kvalues Qvalues. R-values that is what code looks for. there is No exception for spray foam. I'm with Manhatten all the way on spray foam in walls. long payback when a better option is to put foam sheathing on exterior of walls & pay close attention to air sealing details. search southface site for air sealing pdfs..great stuff there! I've never recommended spray foam in walls after running the numbers for my climate & seeing 25+ year paybacks. and I've done these numbers lots of times, not just for one house. (btw hey Manhatten..long time no see) if you chose to use these details of air sealing for the tradeoff for this better insulation is SMALLER hvac system. it only makes sense people if you weight the benefits of upgrades against each other. get a load calc for the house including all types of insulation, window info, air tightness tight not leaky or average & the size needed lessens. bigger is not better, it is just lack of correct education in the trades & misinformatin to the homeowner to sell them more equipment than needed. you don't size the system for the 10 days a year of 50 people in the house & then pay for that the other 355 days. two stage units, variable speed air handlers are great things that vary with the needs of the house as they change. these are good investments. if you are truly interested in making your house efficient, hire an energy rater. specific to you information with inspections, testing & verification. unbiased working for you. resnet.us is a good place to start. new construction, existing homes we do it all. best of uck.

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

The moisture problem on the roof deck may occur when the deck is colder than the air rising into the attic. How is a whole house dehumidifier going to solve this problem in the winter. During the winter houses in SE PA are struggling to raise their humidity level to 40%. Many are running humidifiers in the winter which will further aggravate the problem. I have looked at a lot of new houses in the NJ and PA area. I can't recall any having HVAC equipment in the attic. I have never seen a house with a spray foamed roof deck either. I am skeptical you will find an insulation contractor who has experience doing this in a new residential home. If you do find one, you know the questions to ask. See if his answers make sense.