One option is XPS "blue board" available at local home centers. It has a similar R value to spray foam of the same thinness, with possible lower cost. it comes in a 2" thickness. Make sure you use a small foam kit to seall all of the edges. R value is great, but you also need to stop infiltration. This would go in easily if the garage ceiling is "clean" after taking down drywall. If you have things in the way, like pipes and wires, spray foam will be better. I have used the 2-part Zerodraft foam kit - it is great. I would take down the garage plywood ceiling and spray *one kits worth of foam* until it runs out. You need just and inch or so to air seal the floor. Then, reinstall the fiberglass batts for increased R value. And of course put in a drywall ceiling and tape it very well. In the USA, I get it from EFO.org, but you should have a more local supplier. Good luck! Here is a link that might be useful: Zerodraft Canada

love this forum that you can do a little diy advising! ok the mold in the supply boxes and on the grills probably isn't from the duct takeoffs @ the plenum. it takes a long time for mold to travel that far..and if there isn't moisture & food source it won't spread to that area. it isn't always mold..sometimes it is just dust. so mold/dust buildup at the ducts @ plenum is lack of proper air seal. mold in supply boxes & on grills is lack of proper air seal at the cut in the sheetrock ceiling where supply grill is attached to the box. nine times out of ten..you can solve the issue by sealing supply boxes to sheetrock. see attached picture. hardcast brand #1402 mastic tape is used to seal supply box to sheetrock. this tape is 3" wide. I split the 3" into 1 1/2" lengths. stick to sheetrock and fold into supply box. as the tape adheres to many surfaces you need to be careful to keep it within the area covered by the supply grill. once it sticks to the sheetrock..you'll lose paper if you try to move it. surfaces have to be clean & dry for tape to adhere. and once in place you'll want to press it in place as it is somewhat pressure sensitive. the tape is strong enough to keep the box tight to attic floor on the attic side when installing the supply grill. wash the dust off the grills, wipe inside the supply box & into the duct. reinstall the supply grill. oh and when you remove the supply grills..take a sharpie and mark what direction they faced (throw to window..dirveway side whatever note you need to put them back in same position) this way you'll put them back as they were. I've tried several other brands of mastic tape. hardcast 1402 is the best. the others are 2" width and mastic is thinner, resulting in less strength and useful life. why would I chose spray foam for my floors? I've sealed floors several ways, foam is the easiest albeit more costly, once prep work is done. there isn't enough room to physically install foam sheathing to the bottom of the floor joists. I'll seal any big holes..like under tub then do 3" foam. I've looked at froth packs like tiger foam..but having worked with so many foam companies locally, will just have them do my floor when they are in my area. the software is pricy. we lease it yearly. once upon a time we bought the cd..but too many people abused it. so now it is a download & upgrades to original install. a lot of mastic isn't always an indication of a good seal. it doesn't take a lot if you put it in the actual leaks. you can feel around the duct take offs on the plenum while unit is running and feel some of the leakage. also checking plenum connections to the equipment at the same time. I use mastic tape to connect plenums (after screwing them in place) much easier to seal under the plenum to equipment with tape than painting mastic upside down.

Getting Great Stuff foam off your hands can be challenging, but here are a few methods you can try: Use soap and water: Try washing your hands with soap and water, as this may be enough to remove the foam. Use warm water and lather your hands with soap, then scrub the foam off your skin. Use acetone or nail polish remover: Acetone can dissolve Great Stuff foam. Apply some acetone or nail polish remover to a cloth or cotton ball and rub it onto the foam residue on your hands. Be sure to wash your hands thoroughly with soap and water after using acetone. Use a pumice stone: A pumice stone can help scrub the foam off your hands. Wet the stone and rub it over the foam residue in a circular motion. This may take some time, but it can be effective.

Thx for all the helpful replies and I hope everyone had a great Christmas. I have been reading about this until quite frankly, I feel way more knowledgeable but still undecided. It seems to me the only reason I would spray the roofline like you all mentioned is because the ducts are up there and potentially my AC unit could be. I believe I understand that if I use OC I need to have a supply and return up there etc. If I use CC then maybe I don't. So I'm not an AC person. I get that ducts leak. I'm guessing that this doesn't matter whether you use the rigid ducts like my current house has or the flex ducts. Is the only goal here to keep the temp difference nominal so the unit doesn't run as often? Because if there is some duct leakage, that is just wasted right as it escapes out the ridge vent? Air in the attic doesn't get recycled back into the house does it? I've been sitting here reading on Greenbuildingadvisor.com and to me the biggest issue is roof leaks and being able to locate them which has happened on every house I have owned due to storms when the roof is sprayed. So I still do not get why not spray foam the attic floor with CC, and do a blower test to make sure its sealed up. Then check and address those lines? Also since I'm not an ac person, I would much rather the AC unit not be in the attic. But I'm not sure if I located in the garage storage location, if lines could be ran the 55' to the opposite side of the house. I "think" if they put in the attic, at least looking at some other houses that were done, they would put it central like over the LR and then run the lines out bot ways with two returns. One from each side of the house. I have seen a couple houses like this and then the attic floor was sprayed with CC. The walls had OC in them.