[shokkyy]

The temp in my tunnel went down to 3.5 degrees last night - and this morning it was at 40 degrees

Poor little plants, how on earth can they cope with it.

[wendycas]

Absolutely! A lot of ours have been moved out as they've been frying in there!

[seaside]

3.5 degrees ? Are you talking Swindon or Snowdonia ? 
Surely polytunnels / greenhouses do a better job than that. If they don't, what's the point, other than a wind break.

[mumofstig]

In the dark greenhouses will fall to within a degree of the outside temperature, after all they are only glass. Think how cold the inside of even doubleglazed windows can be.

They only gain heat from the sun - hence the greenhouse effect