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Michael:

Sounds like a nice project you are considering. I would highly recommend consulting a structural Engineer and let that person evaluate what you have and help lead you to where you want to go. My first impression is that the existing roof structure is iffy at best without adding the additional weight. If nothing else chat with your local building codes office and/or a local reputable residential building contractor.

Insulation you may want to consider, as mentioned before, is fiberglass batt insulation w/a vapor barrier. Manufactures like Owens Corning and Certainteed will show on their websites typical installation procedures and I would be surprised if they did not recommend an air space between the glass batt and the underside of the existing roof substrate. This can be achieved with Styrofoam spacers from soffit to ridge. In general terms R30 or more (10-12”) would be a minimum starting point. Also there is closed and open cell field applied blown foam or ridged foam sheets. I would also recommend opening up the exterior soffit and add ridge vents (at least with fiberglass batts). This is primarily to evacuate the heat (and moisture) that will build up between the underside of the exiting roof and the insulation. It will also elongate the life of your shingles. Blown fiberglass insulation may also be an option to you.

Ceiling application of 1x6 T&G sounds nice…best on 16”centers if not 24” centers. Six foot centers are far too wide apart to apply this type of material on. There are many different ways to add blocking or dead wood to facilitate 16/24” centers. This can even be achieved by attaching a 2x2 to the underside of the existing ceiling, plywood gussets and another 2x2 nailer for the 1x6 to attach to. The issue will be if the existing structure is there to carry the additional weight/load.

Most recessed canned light rough-ins require the insulation to be pulled back from around them…sometimes as much as 12” so as not to create a fire hazard. Surface mounted track lighting, while not what you stated, can be less of an overall issue.

There are a lot of ways to skin this cat…get someone to confirm your ideas so your job will be done right the first time around.

Best of luck,

Glenn