A house, described the way our trade describes it, has a conditioned envelope: the volume of air the system is designed to hold at a set temperature, bounded by insulation and sealed surfaces. An attached garage is almost never inside that boundary. It usually has no supply register, no return, an uninsulated roll-up door facing the weather, and a slab with nothing under it. The wall between the garage and the kitchen is an exterior wall doing exterior wall work, whether or not anybody in the house thinks of it that way.
The practical consequence is that the garage does not track the thermostat. It tracks the outdoors with a lag, and the lag is the part that fools people. In summer here it warms all day, peaks in the evening after the sun has been on the door for hours, and gives that heat back slowly overnight. An unvented attic above it behaves worse. Then a monsoon week arrives and the humidity in that space swings across a range the living room never sees.
Homeowners store things out there anyway, because it is the only room with spare shelves. Paint, adhesives, batteries, pet food, wine, camera bodies, tool cases, and sealed containers of whatever was bought in bulk. Heat cycling and humidity swings are hard on most of that list, and the failures are quiet ones: a seal that stops sealing, a coating that hazes, a label that lifts, a fastener that seizes. Nothing announces itself, which is why the item usually gets blamed for being cheap rather than for the shelf it sat on for four summers.
It is worth checking what the maker of anything valuable actually asks for. The phrasing tends to be some version of dry, clean, and away from temperature extremes, which is close to what a Las Vegas gear retailer puts on its own site. Summerlin Armory offers cases and storage tools alongside its ammunition and accessories and spells that guidance out in plain language. Readers of ours who keep anything with that requirement in the garage may want to read it and then take another look at where the shelves are.
From our side the options are ordinary. Insulate and weatherstrip the garage door, seal and insulate the attic hatch above it, add a small dedicated system if the space is genuinely used as a room, or accept the garage as outdoor space and move the sensitive things into the conditioned part of the house. The last one costs nothing, and it is the one people argue with us about on a first visit.