How should I insulate my garage?

Wood frame, plywood exterior. Single 16 foot wide roll up door, single entry door. Central Florida climate, hot and humid. 
 

I've got a dehumidifier that runs continuously, and I just adjusted the window AC unit so that it drains outside and will run that continuously until fall. 
 

Requirements are cheap and easy over any concerns about efficacy. I don't want to make it perfect, just more stable and keep the temperature from climbing into the 90s during the summer. 

 Can I just apply insulation between the studs and leave it exposed, or will I have to drywall or plywood the interior? That would add a much greater amount of effort, since I'd have to move and take down a lot of stuff. 
 

I saw some precut foam panels for insulating garage doors, which would be an easy start, but what can I do about the gaps on the sides and top of the door?

Interior:

Exterior

My wife asked if it would make sense to insulate the outside and add siding. Definitely a literal version of thinking outside of the box. 

You could use type FSK batt insulation. FSK means "Foil Skrim Kraft" and it basically looks like regular batt insulation, except the vapor barrier is covered with foil.  It's rated to be left exposed - regular paper kraft face should NOT be left exposed because it will burn fairly easily.  However, the FSK is still going to be fragile and likely to get a lot of holes and tears in it in short order.

Also look at Johns Manville ComfortTherm batts which are poly bagged to completely enclose the glass fibers.  Looking at the data sheets, the flame spread is less than 25 which should let you leave it exposed.  However, it will still be subject to getting punctured pretty easily.  But I would go this way rather than the FSK.

There is a vapor barrier version and non-vapor-resistant version for hot humid climates.

In Florida a lot of your heat buildup is coming from sun exposure, I assume.  You're going to have to deal with that, too, by insulating your ceiling and possibly installing a radiant barrier.

The cheapest R value per inch is typically blown cellulose.

I'd start with the ceiling, and go as deep as you can. There's got to be a bunch of solar gain on that roof in FL, so that's where you'll get the most bang/$.

Once that's done, I'd start with the south and west facing walls. Rigid foam board will give you great insulation and stand up to impacts better than some cheaper insulation, but the trade off is that it costs a bit more than fiberglass batts.

You can get cheap rigid foam board from craigslist/marketplace that are pulled up from existing constructions.  I plan to buy a bunk in bulk for use on the wall and maybe even the ceiling.  Cover walls with drywall (WAY cheaper than OSB now, fire resistant) and ceiling if you have rafters with metal roofing panels (lighter easier to put up than drywall, no mudding or painting).

It seems like the best gain is adding a ceiling with some insulation 

The temperature inside of the garage is about 82 right now, it's 99 in the attic and up by the attic vent it's 116. The laser thermometer reads the inside of the air conditioner at about 58°, so it's working pretty well.

The inside of the garage door is 88°. It's on the north side and doesn't get direct sun. It'll be easy to insulate, so I'll go ahead with that. I think the foam panels just drop in.

The exterior walls (on the inside) are at about 86°. 
 

edit: Ambient temperature outdoors right now is 86°.

Exposed foam insulation is a fire hazard unless it specifically meets NFPA 286 requirements.  Otherwise it needs to be covered with a thermal / ignition barrier like drywall - I think 1/4" ply is the minimum.   It can also only be left exposed in unoccupied spaces like crawl spaces, etc.

R12 fiberglass batts and drywall.  You are going to have to move everything to get the insulation in, it won't take a ton more time to screw down some drywall.  And its pretty cheap.  And its fire rated. And it brightens up the interior.

You really don't have to, but I mudded and taped and painted with high gloss white paint, and its groovy as all get-out.

If you really don't want to drywall, find some fire retardent foam and cut to fit.

But drywall is better.

Vent the attic once you have created one, if it isn't already.

Edit:  I see the roof is already insulated?  Makes it an easy two Saturday project to drywall it.

In reply to Streetwiseguy :

Do you mean drywall the roof itself or add a ceiling? I assume you mean the latter, even with the roof insulation it gets really hot up there. 
 

The roof is vented along the peak, should I add more venting?

I really like the spray foam insulation in my shop.  Thermal barrier and air infiltration in the same product. A big bonus is that it also insulates for sound, so all of the angle grinding and compressor running that I do at night does not irritate the neighbors.

Yes it does have to be covered, but once done you have a good surface for pegboard or took hanging of your choice.

In reply to Floating Doc (Forum Supporter) :

Easy would be to rely on the insulation that is already there, and just insulate the walls,  Its hot up there because heat rises, and I figure a bunch of it is collecting up there because of the walls.  I would think about sealing off the vent if you don't add a ceiling.  Or, add a ceiling and insulation, then increase the venting up top.

If you do nothing but insulate the walls, you will improve the efficiency of your air conditioner by a huge factor.  Any further insulation will make it better, but will be less noticeable.  Kinda like gas mileage.  Going from 10mpg to 20 is huge.  20 to 30, nice but not as big.  30 to 40?  Won't even notice, unless you are driving a tremendous amount.

Doc, I'm dealing with the exact same questions as you. My additional question on ceiling insulation is if I should add baffles to allow air to flow from the eaves, up the ceiling, and out the roof vent. I store a ton of stuff in my shop rafters and really don't want to put in a ceiling unless necessary. About 2/3 of my shop is concrete block with the remaining 1/3 being frame (currently with no insulation).

Last year I added gable vent fans at both ends which helps get the hottest air out of the shop but (as you know), once the FL heat and humidity kick in, the only thing to fix it is some good AC.