Most collection advice is written for damp countries, where the enemy is mildew and the answer is airflow. Listeners in the desert Southwest write in with the opposite problem. Their boxes do not go soft; they go brittle, fade and warp, and then once a year the sky opens and everything they learned about dry storage stops applying.
Heat is the slow killer
Cardboard, glue and shrink-wrap all age faster in heat. A garage that reaches fifty degrees in July will loosen the corner tape on a box lid within a couple of summers, and the glue holding a rulebook spine lets go soon after. Keep the collection in the conditioned part of the house, even if that means fewer shelves than the garage would give you.
Resin and plastic miniatures suffer as well. Thin spears and banners creep out of true under sustained heat, and a model that spent a summer in a car boot often never stands straight again.
Sun is the fast one
Direct light bleaches box art in a single season, and the spine facing the window goes first. Shelve away from glass, or fit a blind and keep it drawn during the afternoon.
The storm that undoes everything
Dry climates still get water, just all at once. The summer monsoon can drop an inch of rain in under an hour, and roofs built for sun do not always cope. Water comes through a ceiling light fitting, runs down an inside wall or pushes under a back door, and the games shelf happens to be in its path. Move what you can save, photograph the rest for the insurer, and leave the room itself to people with pumps and moisture meters. Players around Arizona have told us that getting a Tucson flood cleanup crew in on the first day mattered more than anything they did with towels, because plasterboard soaked overnight stays wet for weeks in a closed house.
Saving what got wet
Separate the components straight away. Boards and box lids dry flat under light weight, with paper towels between layers changed every few hours. Cards fan out on a rack and should never be stacked while damp. Rulebooks stand upright with their pages slightly open in front of a fan, and can go into a freezer if you cannot deal with them the same day.
Shelving that helps
Use metal or sealed shelving with the bottom shelf a hand's width off the floor, and keep the most valuable games at chest height. Store boxes upright like books rather than in towers, so the weight never crushes the bottom one. Put a cheap hygrometer on the top shelf: in a dry climate you want to see it above twenty per cent, and a reading that jumps after a storm tells you the room needs attention before the games do.
Three nerds who talk about gaming as literature have strong opinions about where you play it. The ideal is a dedicated room with a table nobody has to clear. For most people that means the basement, the garage or the room over it, and those spaces have a failure mode that has nothing to do with dice.
Four hours in a room tells you things an hour does not
A short visit to a cellar tells you very little. Four hours sitting still in one, twice a month, tells you everything. Players notice the cold at ankle height first, then the smell, then the fact that the character sheets have gone soft at the edges by the end of the night.
That earthy cellar smell is not atmosphere. It is airborne spores from something growing where you cannot see it, and the reason it registers after an hour rather than on the stairs is that your nose stops reporting it while the source keeps producing it.
Separate the two causes before you spend anything
Condensation shows up on the cold surfaces, spread fairly evenly, worst in winter, and it eases when air moves through the room.
Water coming in from outside is concentrated. A dark patch on one wall, a line low down that returns after it dries, a corner that is always worse. Ventilation does not touch it, and a dehumidifier runs forever without winning. Groups whose games room turned out to be in that second category have been sent to firms handling flood damage specialists in Buffalo and similar outfits nearer to them, because the question is where the water enters, and that is a survey rather than a gadget.
What this does to the things on the table
Paper and card are the first casualties, which in this hobby is most of the kit. Rulebooks swell along the spine. Card stock delaminates at the corners. Printed character sheets in a folder at floor level go wavy within a season.
Minis fare better, though the primer lifts off resin in a persistently damp room and a painted figure left on a shelf against a cold external wall will bloom.
Dice are fine. Dice are always fine.
Make the room usable while it is being sorted
Move everything off the floor and away from the external wall, even by a few inches. Keep the collection upstairs and bring down only what a session needs. Put a cheap hygrometer on the table and look at it in the second hour rather than the first, which is when the room has had time to show itself with four people breathing in it.
If a session leaves anyone with a headache or a cough that clears when they go home, stop using the room until someone has looked at it properly. That is not fussiness. The whole appeal of a basement table is that you can leave the game set up, and a room that is quietly rotting is one you should not be leaving anything in at all.
The good news
A sound basement is genuinely the best games space there is: no daylight on the screens, no neighbours, and a table nobody needs for dinner. Sorting out the water first is what buys you the fifteen years of campaigns afterwards.