Polyethylene Foam Wrap.

Polyethylene foam wrap is a packing material used much the same way as bubble wrap.
My thought is use this material instead of or in addition to greenhouse poly.
It is not transparent,  but it is translucent.
I've tried to find out what the light transmission and R value if this material is, with no success.
Resistance to UV, also unknown.
I do have a "window" I've set up as part of a greenhouse door, to test how it weathers.
This can be salvaged,  but mostly I've only found it in small pieces.
It can be bought in rolls, I've seen them offered up to 5 feet wide.

If it is viable, it might be a better shade cloth than anything, cutting light transmission and insulating at the same time.
If I aquire any significant amount of this stuff, I will probably apply it to the inside of the glass of my greenhouse, held in place with magnets.

Interesting.

Foam underlayment (for putting under flooring) may be a source of a similar material. I know I have some laying around...

Very helpful!
Looking very briefly it seems rather cheap,  plus I should be able to find it locally.

You might try looking at articles on insulating windows.  Both bubble wrap and poly wrap have been adhered to windows with a thin film of water freezing them on.  There are a number of articles on this.  So one of them might include the information you are looking for.

Also have you looked at the active foam insulation.