OK, now I understand. I would use only pressure treated or metal structural support and styrofoam for insulation. Then, green wallboard (moisture resistant). I would install it with screws, but not tape the seams. Cover the seams with a nice piece of wood trim which could easiy be removed accessing the drywall sections. Would only be necessary to locate the screws for the center studs for the section when removing one. As for an entirely removable section, I believe it would be necessary to attach the drywall to plywood or another surface. My concern, based on the conditions you describe, is that the plywood would warp and ruin the wall. Another thought - install a flush door (either hinged or a door panel) where the problem areas need inspection. Hope this helps a bit.

"With the air tight IC recessed light cans you should still build air sealed enclosures around them because they leak a lot apparently. LEDs are a great choice for these locations especially without enclosures as they have a weaker "mini chimney" effect." in all my projects new and existing I've found many ways to seal IC cans. retrofit with inserts. then seal housing to sheetrock ceiling...from inside the house before putting trim kits in place. even 'air tight' trims don't stop the oversized cuts in sheetrock from leaking. I use a mastic tape made by Hardcast #1402 for all my holes in ceiling. more on this later. this seal described above is the best with no measurable air leakage. building boxes..maintaining the 4" air space on all sides & top of can light..also works, cheaper cost..harder install. problem is the 'legs' of the can lights that are attached to ceiling joists & properly sealing around them. this is where the air leakage occurs. I never use foam..because it shrinks over time, you have little control over where it goes & unless it is fire rated..shouldn't be next to heat producing areas. and yeah..I'm overly cautious about this. this method can be well done..and still leaky. foam installs I've seen online in videos put fg batts around recessed lights and foam them. I don't like this at all, and it is never done on my jobs. best of all is to use ICAT only, and use the hardcast tape to seal the cut between housing & sheetrock. Hardcast brand mastic tape #1402 is a 3" wide strong mastic tape. I perfer it over any other brand on the market. cut it into 1 1/2" wide strips and use it in full width depending on size of opening to be sealed. sticks to just about any clean dry surface...to the point of pulling paper off sheetrock if you try to remove it. so use it carefully. opeings at bath fans, supply boxes, stove vents, recessed lights..I use hardcast. use it once..and you'll understand why. not the cheapest on the market @ $22- $35 per 100' roll. but absolutely great stuff. while it is hard to describe the sealing of supply boxes with hardcast from inside the house..a picture shows it well. I'll attach one to this post. flgargoyle .. foil bubble foil is an overpriced product that performs no better than the cheaper radiant barriers. in my house with a tin roof..I have a single sided foil scrim fraft paper like what is used for ductwrap. I used button cap nails to attach it to the undersides of my rafters. with gable end vents..and 3 hurricanes later I've had to do a little repair on it..but it was worth the few bucks it cost me & the 2 mornings it took to install. good benefit in summer little to none in winter. my attic has fg batts on attic floor, one layer in joist bays the other unfaced layer laid across joists. no ducts, no equipment no recessed lights and a damn near perfect air barrier from living to attic space. ducts are in fur downs inside living space, vs air handler in side living space also. a bit over sized @ 2 tons with an whole house april air dehumidifier..also in conditined space. 4" media filter at return. 15SEER unit the addition I re-roofed last summer, batts in rafter bays 1" R-7 foil foam sheating lathes & gavalnized tin. as rafter/ceiling joists are 2x4 (yeah...go figure) the batts are R-13 with the R-7 gives me R-21 for this large bedroom. prior to this install the room was used for storage as it was too hot in summer & too cold in winter and too expensive & uncomfortable to use. with radiant barriers there always has to be a mininum of a 3/4" air space. thus the lathes on top of foil foam board under tin in addition & opposite install in big attic with rb under rafters with foil facing into attic space. gotta run...but wanted to share my take on things. oh here is the pic of the supply sealed from inside the house with hardcast tape. best of luck

3,2,1. The dormers will drive all of the decisions. New building codes require more insulation now, and you may have to raise the roof plus add the dormers in order to get the required head room and the required R value for the insulation levels. If you've got to completely rebuild the top floor, you'd be wasting money and time doing any other project until that project is done. Even if you can build adequate headroom dormers with the required insulation levels out of what's there, you'd still be wasting effort to insulate what's there without creating the dormers first. And taking down all of that old charming wood will be required to insulate properly. So, you've got to get the addition sorted out first.

Everyone - Thanks for your very helpful feedback. There actually is a separate HVAC system in that finished 3rd floor space. It's a small unit that is built directly into the top of one of the walls that provides both A/C and heating, as needed. The space is about 320 square feet, and it would be heated/cooled via this unit. I don't have the manufacturer of model, but it's to control the environment in that space without relying on the main HVAC system. It sounds like the consensus is to add the R38 insulation (upgrade). Once we move into the house, we can always evaluate the extent of the temperature extremes and possibly add foil after closing, although it wouldn't be as good as a full-on radiant barrier installed during the construction process. Perhaps things will change over the coming months and these materials will be available again. Thanks so much for your expertise and feedback! If anyone else has an option, please feel free to weigh in! :-)