Insulating summerhouse with celotex advice

[barnsley7]

I'm about to insulate a summer house or actually a treehouse as it's 6ft off the ground. It's 8ft x 10ft. Joists and studs all 3x2 inch so only got
70mm space for insulation. I'm thinking 25mm celotex all around. With 45mm air gap between outside cladding and insulation. Going to use foil tape for all gaps on inside. Then 5mm plywood for inner cladding.
I've used shiplap cladding on outside and used sealant all around to stop water getting in through any gaps at the edges.

Now I'm thinking is this a mistake? Could condensation build up between inside and outer cladding and not escape?

It will only be heated once per week. Used as a cinema room.

2nd question , the floor only has 50mm space, so is it worth messing around with an air gap here or should I just put my inner cladding, 5 or 9mm plywood directly on top of 50mm celotex?

 

[barnsley7]

Photos attached

 

[^woody^]

Wood is permeable. There are no condensation issues with a timber shed.

 

[cdbe]

For once a week, the payback on the cost of the celotex and presumably some kind of inner wall lining etc would probably be forever compared to just taking a hit on the heat loss, especially if those window holes don't have any glazing in them. Maybe consider IR heating.

 

[barnsley7]

For once a week, the payback on the cost of the celotex and presumably some kind of inner wall lining etc would probably be forever compared to just taking a hit on the heat loss, especially if those window holes don't have any glazing in them. Maybe consider IR heating.

That's a good point. It may get used more frequently in the future. I just don't like wasting energy. I've not heard of IR heating before but it looks interesting. That's my bedtime reading sorted anyway!
And no I don't have double glazing, Only a single sheet of perspex on the windows but I could add another sheet