Insulating a summerhouse

Brebal

Morning!

So I’ve been reading through a lot of old threads about this subject and celotex gets mentioned a lot. I’m considering about using that to insulate my summerhouse. My question is, will it make condensation worse? I’ve had the summerhouse a year now so seen it through all seasons. In winter it does condensate a little inside. I got one of those dehumidifier box things with crystals in which turned to water pretty quickly.
Is there anything I can do to combat that?

Also, with regards insulation... if I used celotex to insulate and make the summerhouse warmer in winter, will it also hold the heat more in summer or is there anything that will have some sort of reverse thermal effect.

I have a gym in there. Most of the year it’s alright but it’s just the extremes of summer and winter. I have a panel heater in there for winter which I turn on to take the chill off before a workout, and I obviously soon warm up, it’s still a bit on the cool side for me though. And in the few really hot days of summer, it’s too hot to work out really - I had two fans and the window open and it was still a bit much.

Any other tips please?