Humidity gets talked about and heat does not, and heat is faster. Damp takes weeks to ruin something. A hot afternoon in the wrong room takes an afternoon.
What heat actually does
Vinyl warps. Not from direct sun alone — a stack of records in a hot room will dish without any sun reaching it. Stored upright, never flat, never stacked.
Cards and comics cockle. The board absorbs and releases moisture as the temperature swings, and the edges go wavy. A sleeve does not prevent this; a stable room does.
Anything with glue lets go. Shoe soles, box lids, book spines, anything assembled. This is the one that surprises people, because the object looks perfect until it is handled.
Candles and wax soften and lose their shape, and they take the surface of whatever they are resting on with them.
Chocolate and anything edible blooms, and then it is not sellable at any price.
Plastic yellows and deforms, and that is permanent.
Electronics with old batteries swell. Check anything that has sat with a battery in it.
The three things that fix most of it
Get it off the top floor and away from the outside wall. A loft or a west-facing room is the worst place in the house, by a long way, and moving a box is free.
Stop the swing, not just the heat. A room that is hot and steady is kinder than one that is cool at night and hot by noon. The swing is what moves paper and glue.
Upright and not under weight. Almost everything on this list fails faster under its own stack.
Before you post anything in a hot month
Ask whether it will spend a day in a metal van. Wax, chocolate, anything with glue and anything moulded should either not travel in that or should travel with a cold pack and a warning. Collection exists for precisely these.
Questions people ask
Is air conditioning necessary?
No. Shade, airflow and a room that does not swing do most of it.
My records are already slightly dished. Can that be fixed?
Sometimes, carefully, with weight and gentle warmth. Say it is dished when you sell it either way.
What about a cupboard that gets hot?
A closed hot cupboard is worse than an open hot room, because nothing moves. Leave it ajar.
More: storing collectibles, monsoon and your stock, selling through the monsoon, why some things cannot be posted.