It's called "Hot-wet degradation", it happens worse in a warm environment, but it's still a problem in the UK. Essentially the Matrix (that is the non-fibrous part of the composite) absorbs moisture - 6-10kg is possible for a Europa sized aircraft in one year in the British weather, I've seen it happen.
The effects of this can be: -
- Overweight / out of CG aircraft
- Cracking on exposure to sub-zero temperatures
- Corrosion of embedded metal components.
- Loss of mechanical strength.
- Rotting of any embedded wooden structural members.
Don't panic about leaving composite aircraft out for a few days or weeks, but permanently it needs to be in a dry environment. Some aircraft will tolerate it reasonably well, but if I was spending £40k+ on a Europa, I'd jolly well want to protect my investment.
And bear in mind that a Reliant Robin only has a composite body, not composite primary structure.
G