Concrete, pebbles, rocks, bricks, slate, natural stones. These elements are fantastic at stabilizing temperatures. They absorb the sun's energy all day, then slowly emit the heat all night enough to keep many plants cozy.

Hi Val...The fleece cover goes on the inside,and if you have any big polystyrene flat pieces from packaging,I put these on the shelving,to stand pots on. I try not to use the top shelf,if possible..so all the others have more protection with the insulated shelving above them..good luck :o)

Thanks Sandra, Hope you and your family are well.Val x
Thanks Bathgate for your comments.

In 2013 there was some discussion about leaving a night light or candle under a terracotta plant pot. The pot heats up and radiates the warmth. I don't suppose it would last all night but with good insulation it might make enough difference. Worth a try on a cold night?
http://www.growsonyou.com/question/show/175236.

Condensation will happen on the bubble wrap as well as on the cover of the grow house - they are both plastic. So you'll still have the problem.
It would be a good idea to open the cover on dry days in order to provide some ventilation.
Don't forget to close it again in the night.

Personally I wouldn't even try to use a plastic grow house over winter, to me it is a recipe for disaster for the plants. By the time you've put up the fleece and the bubble wrap just how much light t is going to get through and plants need light. A cool windowsill or two will do far better in my view, Bulba bring our tender cyclamen in an over winters in the two upstair bedrooms on the windowsills. Plants that basically just need protecting from wet go into our polytunnel which is open at both ends to let air through.

The warmer you keep it inside the grow house, the more condensation will form. Is it possible to channel the condensation to keep it away from the plants?

I have never used bubble wrap in mine,for the reasons given above,and air does get in and circulate from the bottom..I can see the cover moving in the wind..The Fleece is also breathable..However,I only put in plants that are frost tender in,such as certain species of Fuchsia's and similar,that die down anyway..so light is not essential,till you can see them starting up again in spring..I just keep a check on them,and by all means open the door on decent days,but leave the fleece in place ..I haven't lost any plants yet..

I only put cuttings of shrubs and perennials in mine. The zips have broken so they get very cold but they can survive that.
I've never used fleece or bubble wrap with them but sometimes I cover them with newspaper on frosty nights.
I always put frost tender plants in a shed.

Where is the growhouse sited? Ours is sited against the west-facing wall of our house, and we've never had a condensation problem.

Mine is in my unheated greenhouse, just to give more tender things some extra protection.

Mine has died of old age and been replaced with a wooden one with better ventilation.