Perhaps I’m missing something but why don’t you just counterbatten the undersides of the joists with treated 4x1 in a hit and miss battening style, so there is still ventilation through but no one below can see anyone above?
Because I also want to stop water dripping through from above. The balcony deck is slightly sloped towards a gutter on the wall so the liner will also carry any rain water/liquid spills to that.
Yes, that would be nice but expensive as it'd need to be in an oak woodgrain to look right. I also think that rainwater will get into the T&G joints and drip onto the deck below for a while after it's rained unless I also have a DPM above it.
So I spoke to the company that manufactures 'Antinox', one of the twin wall floor protection sheets. They said their boards are not UV resistant and would not stand up to prolonged outdoor use and weathering. They also doubt that similar sign boards would have the longevity I'd need as the material is only really use for temporary signage. Just when I thought I was onto something...
Outdoor signage board is often up for years in full sun and all weathers. I used to work with it. It’s designed for that environment so with respect they don’t know what they’re talking about. It’s in no way a similar product to flooring protection sheets. You’re using it largely covered over anyway. Equally you’re covering your proplex. They’re not going to stick their neck out and recommend their product for something it’s not designed for of course. Anyway. Your call I think you have more than enough solutions suggested to decide how to go ahead.
You may well be right and I'm hoping so as it seems like the most cost effective solution. I'll try and get hold of a sign board supplier and sound them out on durability.
Just make sure you’re talking to them about the right stuff. You want high density PVC foam board, not foamex or the twin-walled plastic stuff. You can also buy it with an aluminium sheet either side but it gets spendy and its designed to be covered so unless you want to paint it, vinyl cover it, or give your neighbour below 3rd degree burns just stick with the plain stuff.
A bit late to the party but these guys use no-compostable joists and a sub frame on their balcony should anyone encounter a similar issue - https://www.360gss.co.uk/blog-wargrave-rd