Humidity is moisture affecting your guitar, while heat (or cold) more directly affects the material's nature to expand or contract in response to temperature changes. Heat and Humidity do not necessarily go hand in hand, but they tend to do so (ie: more heat will support more humidity; cooler temps generally support less humidity in the air).
If moisture gets into a piece of wood (any wood), it will tend to expand the fibers of the wood. What I have faced a lot building cabinets and stuff is moisture leaving the wood over time and it shrinks, leaving gaps in joinery. This effect is pretty much independent of temperature changes in any normal home. It's a longer term effect for wood to both absorb and release moisture.
Temperature changes are more immediate and can take effect moving from a warm room to one with AC for example.
I don't know, maybe you are worrying too much? I live in Nova Scotia where it gets dripping humid for 2 days, then dries, then humid again, rains for 3 weeks straight, gets freakin-ass cold, temps change 10 or 15 degrees celsius from day to night... I know people with expensive guitars, but I don't know anyone with a humidor. :)
...so ever since then, I always hang on to the buckle.
