Paul in your current build you have no ceiling correct ? So when you look up from inside you see the rafters and roof decking ? Because you can't get a lot of insulation up there stapling a radiant barrier in place before insulating might be a good idea . Just a suggestion and I am no expert .

I'm going to have to have something up there to hold the insulation in. Probably put in the insulation batts held in with spring rods between rafters and then put up drywall ceiling. I don't want to see the insulation but it's a lot of work to screw up the boards. I'd need to rent a lift for a weekend probably.

You guys got me thinking on the insulating roof stuff.

About 11 years ago I ripped off the old tar/gravel roof of my garage and replaced all the plywood and use peel and stick on top, which has held up really well.

The old garage had a plaster ceiling - no insulation. I removed that during construction and never replaced, so just have open rafters now.

It gets pretty toasty in there, even with garage doors cracked open.

How much of a difference would it make, IYHO, if all I did was install insulation on that ceiling. Like between the rafters. Stapled in place? I guess I would also have to install an actual ceiling afterwards, right?

I'm thinking also (at some point) a window unit or mini-split or some kind of ventilator fan(s).

Two car garage with fiberglass door, 2 windows, and 2 man doors.

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You can add batt insulation between the joists and it will help. You can run furring strips perpendicular to the joist to hold it up there or use the spring rods that are sold at HD. The spring rod is just a flat piece of metal that bends between the jousts and hold the insulation in place.

I'm not going to predict how much difference you're going to feel but you'll notice it. The problem is with a flat roof you don't have any air movement above the insulation like you would if you had an attic.

Foil backed will stay up there easily as I think I see 16" OC joists. Foil needs to face the airspace to work, so in your case, facing the interior. So now you have a chrome ceiling. ;):D

OK thanks for the replies and info, Paul and Milt. Chrome ceiling sounds nice if it helps with the heat buildup in there. :D

I don't know why I never thought about insulating the new ceiling. Thanks to threads like these, it helps those like me who are somewhat ignorant and inexperienced in the methods of construction.

I agree with Milt , foil backed insulation would probably work best for Paul and Baz . Baz in your case your trusses appear to be at least 2x6 if not 2x8 . The more insulation you can get in there the better within reason . In a perfect world with no $$$ restrictions for both of you spraying a closed cell foam might be best . Closed cell helps strengthen the structure , insulates very well and is somewhat impervious to roof leaks which may be good for your relatively flat roofs . Your flat roofs really don't vent well which is another good scenario for closed cell foam . Generally speaking when a " conventional " 4/12 or 5/12 pitch roof is done in spray foam they spray right over the ridge vent . The goal being keep as much heat out as possible . I'm no expert just what I read when researching for my man cave .

Baz not sure which way your garage door faces but if you are happy with it functionally I would suggest insulating it . If you are not happy with it I would replace with an insulated door they do make a difference . Especially when you start thinking about adding AC . Plenty of kits out there for adding insulation to existing doors . With a fairly well insulated ceiling and walls , garage door and decent windows you have the potential for a successful AC conditioned work space .

Thanks for the advice, rfuerst. I measured them last night and they are 8" and on 16" centers.

I never thought about doing a spray foam. Interesting.

This looks like a good winter project to do, so I have plenty of time to figure it all out.

One thing is my garage door faces north and is in shade all day. I realize that doesn't factor into it's ability to hold temperature inside the garage but at least it's not letting the heat get in from radiation. I do like it - fiberglass and very light. Easy to raise when door opener isn't wqrking. :) Plus I like the way it looks.

Spray foam is very efficient but may not be worth it for a garage. There are plenty of youtube videos out there about it. I have a contact that quoted me these prices; R-20 for $2/SF and R-30 for $3/SF. But I was getting a deal with not minimum charge and no set up. As they say..."Your cost may vary". It's not a do-it-yourself-job of course.

As Rick said it will add strength to the roof system and if you did the walls its as if you glued everything together. But lets say your roof does get a leak and your plywood rots out. You would have to cut out the plywood and patch in some new pieces. Could be a big PITA.

Then you should keep it but I would look into insulating it . There are ' blanket " type kits or you can go with solid foam and cut it to fit . Neither will add much weight but you will feel a difference inside the garage .

Thanks for the additional comments and thoughts, guys.

To be honest, there's a chance I will be modifying my garage - making it a big larger - in the not too distant future. That will depend on whether or not I build a separate structure in my side yard or not. If I do that, I can probably leave my garage as original, which is my preference. So the insulation that makes the most sense for now is the non-spray type.

That garage is built like a bunker too so don't think I need any strengthening, per se.

If I might add something:
The foil will prevent upwards-heat from frying the shingles from both sides.

But a southern white coating will prevent the top asphalt material from heating up at all.

If I might add something:
The foil might prevent upwards-heat from frying the shingles from both sides.
But a white coating (towards the southern side) will help prevent the top asphalt material from heating up.