The sweating and rotting would come about because a moisture barrier is blocking moisture from escaping. It's got nothing to do with the Kingspan. Feel free to add as many as you need or want. Do you live in a humid environment ? In Colorado, I'd never worry about is as I live in a very dry environment .

So plasterboarding tightly underneath and putting floorboards direct on top should be fine? And I’m from the UK, I tried the UK DIY sub but only received some vaguely related replies and not what I was looking for so I thought I’d check this sub. Lots of rain where I’m from, all year through ha.

Thanks for the reply!

I think I'd call up one of your local insulation companies and ask for guidance. I'm guessing you'll be good, but moisture is a serious issue and getting your layers wrong can cause problems.

Make sure the insulation is not the full depth of the truss, so when you put the floor down, air can flow over the insulation. You attic needs to be ventilated.

You should use vapour barrier tape (e.g. alu tape) over the gap between insulation and wood to elimitate moisture migration from the inside. On top, on the other hand, the more vapour open the construction is, the better. Full timber floor boards wil be ok for this. OSB would not be ok (it cannot absorb or transport moisture).

If the roof on top is insulated, the moisture problem is already largely mitigated, but a vapor barrier from the inside is always advisable in the UK climate.

If it was a normal cold roof attic you would need ro ventilate the loft space above the insulation to remove any moist air. If it's a summer house and you don't make any moisture you may be OK. It's the moisture from showers and dishwashers that xreate moisture In our houses, no so much an issue I outside buildings without steam in them, it might still be worth ventilation the space above the Insulation though, and try and make the ceiling air tight with a membrane.

Unfortunately the insulation is the full depth of the truss. I guess I could put battens over the truss to basically make a raised platform for the attic floor so that air can then get underneath and to the kingspan, but is it still fine with the plasterboard tight against it underneath? Thanks a lot for your response by the way, much appreciated

Good idea, thanks!

The roof itself is felt (membrane) & battened, so I think that’s fine? I am considering putting a wood burner in this summer house though, so I think that could potentially introduce vapour issues — or is that incorrect? Thanks

Huh, TIL that not only is Kingspan not only not just the name of a rugby stadium, it's product that's so common as to be generic.

I'm not sure that a wood burner would introduce water vapour, you need to speak to the underlay manufacturer. Call their technical department and explain the build up.