Care of Phicen and similar bodies

I'm after some advice specific to the UK with regard to storing Phicen and similar bodies.

I had mine in a "fairly well insulated" shed for several years and the older ones...maybe 6 years old ish...started to split.

With the new storage shed build being imminent I was wondering if anyone had any tips for storage.

My plan was to insulate the new storage shed, but I'm wondering if there is any point as this doesn't seem to have helped so far.
I'm thinking that I could just insult the storage box that will have the Phicen style figures in ?

But is insulation the answer ? If I understand correctly the best thing is to keep the humidity constant. That's easy in a house ( where my female figures are currently stored ) but SWMBO has decided that they have to move back into the shed.....

So.... What's the answer ?

As ever... Thanks in advance.

Also if this thread is in the wrong place then Mods please feel free to move it.

I think maybe a combination of both Chris. Insulate the shed, as this will also protect and keep the dam away from diorama walls and such. But maybe keep the girls in a plastic tub also, especially during he colder weather?

Cheers Dal