I am currently planning on building some grow-houses/cold frames. Actually mostly raised beds 18" (45cm) high, with the ability to fit panels over it in winter turning them into cold frames.
The rear wall will be north facing and against the neighbours 6 foot fence. This means that the sunlight will never come from this direction.
I will be using twinwall polycarbonate for the window panels. I have thought that the rear panel may be permanent and use a thicker, and therefore more insulating panel for this. The rear panel being around 30% of the surface and I can reduce heat loss through this section without effecting the light reaching plants.
Would it be better to leave the rear panel clear, make it white to aid light reflecting on to plants or make it black to aid heat capture?
I am in East Lothian, Scotland. During summer it will receive 11 hours of direct sunlight, during Autumn around 4 hours. In Winter there will likely be no direct sunlight but it will be light for 8 hours.
My intention is to extend the growing season, maximising crop production as well as protecting from frost.
I mentioned raised beds which could have panels over the top, I was referring to some thing like this, to work with the square foot/meter garden. An alternative mentioned by @bamboo which I had not thought of would be 2 panels leaning on each other to make a cloche type arrangement.