Basically its going to be trial and error to find out whether you need to increase or decrease and by how much - most things in diabetes seem to be worked out by trial and error unfortunately (least for me they are)
I need slightly less (not close to even 20%) when its hot but @Antje77 obviously needs more..
I tend to be more insulin sensitive with the heat.
Like @Rokaab i don't decrease by as much as 20%, getting my calculator out. It's closer to 7%. Which equates to 1u. (Ish.)
Hi, it is also worth remembering that it can take a few days to notice the effects of dose changes for some people.
Possibly also think about what change you might have to your daily routine while there, and whether this will warrant a basal change too? As others have said, the joys of trial and error!
I guess it also depends on what temperatures you're currently used to? I hope it is a nice reason you have to be going away somewhere warm