We are looking at buying a house, this house has a room above an un-insulated garage. The room has a slanted roof. The supports are what looks to be 4x10's or 4x8's. I don't yet know if there's a ridge line vent either. The soffits are vented and air does come up through them. We obviously want to maximize the space. This house is in Maine. Should I leave a vapor barrier (1"~) from the soffit up to the ridge vent if it exists? What's the best way to insulate this? I will not be using spray foam. The room would be heated with a propane wall-heater. Thank you

Hi all, new user here although I’ve used info found on this site many times for diy projects I’ve done and thank everyone for the bounds of information available here, finally signed up as a user and need advise on my insulation work. ive read a million articles and posts about this topic and just want to ask if the method I’m hoping to use will work! Im gutting my second story (1/2 story) with knee walls and vaulted slant side ceilings. It currently has no ventilation, and about R13 of fiberglass stuffed behind everything, needless to say the roof melts snow instantly and ice dams are EVERYWHERE, so I’m just gutting the drywall out as it was a poor job anyway to fix this problem. my original plan was a hot roof with closed cell spray foam encapsulating the under side of the roof and end walls, problem with that is, budget :( So my New hope is a vented and insulated area. For venting I will have open soffit vents on both sides, a ridge vent across the roof peak and as a bonus there is a fable vent at the peak already. So this creates circulating airflow as long as I don’t fully block the slant ceiling (vaulted area). my plan to save money was to do as follows, please advise me if my plan is a bad idea, my only true concern is if vapor/moisture could be a problem with my plan. fully vented as stated knee wall area FLOORS already have fiberglass in cavities, going to add 2” XPS rigid foam to entire floor and seal with can foam around perimeter. knee walls- 1” XPS rigid foam across BACKS of studs, sealed with can foam inside cavity, fill cavity (2x4) with R15 fiberglass batts (faced or unfaced??) Slant ceilings (vaulted)- 5” deep cavities, use 1” rigid foam, cut and cobbled into the cavity 4” back from inside stud edge. Seal them in with can foam. This creates a 1” opening behind the foam board as an air baffle so air can pass from lower attics on sides to upper vented attic space. Then stuff remaining cavity again with R15 fiberglass batts. Top ceiling- 1” rigid foam on back side of studs, air sealed with can foam and again stuff cavity with R15 batts. Finally use another layer of 1” rigid XPS foam board to line the entire interior (between studs and drywall) This method will give me far more R value than I have, create a fairly air sealed space with the cut and cobble method of sealing the rigid foam with can foam, give me ventilation throughout the unfinished space, and the interior sheet of rigid foam will help stop thermal bridging across the studs/rafters. My #1 concern and question is, will the double layer of rigid foam with fiberglass sandwiched in between them have any moisture problems? Am I creating a vapor barrier on both cold and hot side and going to sand which moisture into the fiberglass? Or because it’s only 1” foam on each side and the cold side is ventilated, will moisture not be a problem for me? Thank you greatly for any input, comments, suggestions that you may have!