Heat ruins things quickly and you notice. Humidity ruins things slowly and you do not.
What is at risk
Paper — comics, books, posters, cards. Cockling, foxing, and a softness in the corners that never comes back.
Textiles — mildew in a fold, and a smell that survives washing.
Metal — tarnish on silver, spotting on coins. Never clean a coin; the cleaning destroys more value than the spot.
Leather — mould on the grain, which marks permanently.
Electronics — corrosion on contacts you cannot see.
Anything sealed — condensation forms inside the seal, which is worse than outside it.
What actually works
Airtight boxes with silica gel. The cheapest real protection there is. Replace or dry the silica when it changes colour; a saturated sachet does nothing.
Off the floor and away from outside walls. Both are colder, and cold surfaces are where moisture condenses.
Air movement. A still cupboard is worse than an open shelf in a moving room.
Not plastic bags. They trap what is already in there.
What not to do
Do not clean anything valuable because it looks dull. Collectors price original surfaces; a cleaned coin, a polished brass, a washed textile are all worth less afterwards.
Do not store anything in a bathroom or an outside-wall cupboard, however convenient.
If something is already affected
Dry it slowly and describe it honestly. Naming the flaw raises the price.
Questions people ask
How do I know if humidity is a problem where I am?
A cheap hygrometer. Above about 60% sustained is where paper and metal start caring.
Is air conditioning enough?
It helps because it dehumidifies, but only while it is on.
Does this matter for things I will sell next week?
Less. This is about what sits for months.
What about posting in the monsoon?
Bag against damp inside the box. Packing.
More: shipping across India.