Insulating the garage attic.
"Heat goes to cold. Period." Insulation slows the process, so absolutely insulate the lid. I assume you will insulate the walls, too (if they are not already so). Putting boards over the trusses will defeat the purpose somewhat. Dead air is not a great insulator. Blow cellulose into the spaces and save yourself the time and money of installing paneling; you'll get a better insulation up there, too. Blow as much cellulose as is recommended for your area (plus 2", if it were me), but make sure your sheet rock can support the extra load. 1/2" 'rock, 24" OC will not support a ton of insulation; sorry, but I don't have the numbers, so see an good builder or engineer. Likely, a city engineer can answer that in 2 seconds, and free. Then, what about a vapor barrier? Recommended for your area? If none is recommended, then do whatever it takes to install an AIR barrier. That may mean re-doing the sheet rock to specs, or, worst case, tearing it out and re-doing it. If demolition is not your thing, then perhaps a vapor retarding paint will help.