Summerhouse insulation

[John54]

Hello to all.
First-timer here.
Hope I've got the right forum.

I'm building a timber summer-house.
Plywood outside, with ship-lap over.
UPVC double-glazed door and window - window with trickle vent.
4x2 framing with 4" cavity-wall insulation to walls and floor - dearer, but better than loft-insulation for sound-proofing I've read.
6x2 joists to flat roof, covered with rubber - with fall, obviously.
Fire-rated plasterboards to walls and ceiling.
All exempt from planning and building regs - with exception of a toilet, for which BC have plans and been out to see me.

I read somewhere on the net that if I line the walls and ceiling with visqeen before boarding, I don't need to ventilate the roof-space because condensation can't get through either way?
I ask because I'd like to fill up the roof-space with loft insulation, meaning more heat saved. (And because I've a few rolls left from doing my loft.)
However, my builder's merchant reckons lining everything with visqeen will make the summer-house sweat like mad?

So, my questions are -

Can I fill the roof-space with insulation or do I need to ventilate the space?
Would the trickle vent in the window stop the summer-house sweating, if the builder's merchant is right?
Is lining it all with visqeen a good/bad idea?

Thanks for any advice you may give.
John.

[welsh brickie]

no dont use visqueen,use a breathable membrane like tyvek.

[Perry525]

You call it a summer house.
Yet it seems to be a bit more than that?
Do you intend to live in it?
The reason I ask is.
If you are only using the summer house on warm summer days and evenings, presumably you will have the window or door open?
In which case there will not be any accumulation of water vapour to worry about.
Water vapour will only arrive in the summer house by your being there and breathing and sweating.
No people or animals = no water vapour.
If you intend to use it in cold weather, then there is the possibility of condensation.