Hoop house/Polytunnel insulation vs light loss what's optimal?

Hi Just planning a 12m x 7m polytunnel

I'd love a Keder but can't afford one

So looking to do a budget version by getting a normal polytunnel and putting a 'fake' roof made out of Bubble wrap to create a heat barrier and cavity at the top.



Also putting a layer of bubble wrap on top of the cloches. I'll stick in 6000litres of water too, to buffer temps a bit.

My question is

1) Is it worth having the three layers of plastic (main tunnel plus the two bubble wrap) or will the reduction of light mean it's not worth it?

2) If three is good, why not four, five what's the limit here?

I'm trying to grow lettuces and oriental salads in the UK, so average winter temp of 5c or so, dropping to -5c on exceptionally cold nights.

I'll be using Autovents on the main polytunnel body and also the cloches to manage hot temps as i've found they work really well.

Watering will be via  Edyn moisture sensing water valve.

With those temperatures I think your limiting factor is going to be sunlight, not temperature.

Assuming you are not trying to grow tomatoes in January I'd think a single layer would be quite sufficient. Here in the US the double layer of plastic (not the bubble stuff) is common and is inflated to insulate. My winter temps are about the same as you or a bit colder and I only have a single layer of plastic. There's maybe 1500 litres of water as thermal mass.

Actually come to think of it I do start some tomatoes in January.

Your comment made me think of adding a reflective sheet to the north side of the tunnel. So I might give that a go..