Hello, I have background of 35+ yrs in construction - mostly residential and some commercial. Insulation and venting, factory built fireplaces (formally certified by the Hearth Institute for gas appliances and wood burning fireplaces), sectional overhead garage doors and their electric operators.
If you are successful in sealing a building air-tight,(and you very much want to be, to a degree), you must bring in fresh air if you are combusting the interior oxygen, or setup your heater to use exterior air for it's intake.
Most never seal something 100%.
In this case an "Air to air" heat exchanger is ideal. I'm not well versed on furnaces, sorry, but I do know that a small furnace running at capacity for a long while will be more efficient than a large one that cycles on and off constantly.
High efficiency gas heat is generally the least expensive to heat within most areas, with Natural gas being less than Propane (buy propane in the summer when it's the least expensive if you must).
Do not use "brush seals" only to seal a sectional door, some air will pass thru them. Use any type of "flap seal" or 2 seals combined.
A 2" thick polyurethane foam injected garage door section averages around an R-18 (polystyrene R-10), 3" thick polyurethane - approx an R-25, if you can find an affordable 3" version (or one at all).
Use sectional doors that have an internal compression seal to make the joints air-tight, and true thermal-break btw the interior and exterior steel skins. The only good insulated door has a solid insulation core for it's entire width, interior and exterior skins (99.9 % use painted/galvanized steel), and no internal framework. A pan door ,(single sided steel with frame stiles(vertical posts where the hinges attach) also makes up ALL non-insulated sectional steel doors), with a piece of foam in the framework is an EXTREMELY poorly insulated door (R-0.5 to R-3 ) and leak air profusely , and sliding doors are very hard to seal and too heavy if insulated properly.
When you use spray foam on a building's interior it is to seal and add some R-value , but it's not enough to stop radiant heat loss, you'll need more insulation, but spray foam would be very expensive to gain the R-value needed, extremely heavy, and may delaminate over time.
Supplement with fiberglass or cellulose. Some spray on a 1/2" and go over that with fiberglass batts. It well depends on how the building is constructed on how you CAN insulate it.
Blown-in ceiling insulation cost much less than batts and covers so much better. Our attics looked like a good heavy snowfall had hit ! We used pure white fiberglass and some manufacturers warranty the R-value for years after installation of this product.
You lose most heat thru the ceiling, upper walls, and other leakage.
I'm in Indianapolis and in a seminar I attended approx 25 yrs ago they recommended an R-50 in ceilings to use an electric heat pump in our zone, and a gas or electric "backup" furnace for more heat !! Likely upgraded since then.
R-38 in ceilings should be a minimum anywhere (heating or cooling), our upgrade was to a minimum R-44 , with any more being less cost effective (but utilities never lower pricing - so...). Insulate cold air returns too - if they are not within the envelope being conditioned.
Tim King / Garage Door Service of Indiana, inc., and formerly d/b/a Indy Insulation and Fireplace.