Posted: 16/01/2013 at 12:15
My way is to not bother hanging sheets of bubble wrap across the greenhouse windows, use just enough to cover the seed boxes plants and pots.
Make a frame or put sticks in the corner of the boxes then cover with a piece of bubble wrap, on a very cold night drape some fleece over that I also have a frost guard fan heater.
All insulators cut off some light and light is very important to growing plants so remember to lift the cover on warmer days and let some air in at the same time.
My other option is because the green house is a lean to on the garage wall a heated sand box with thermostat and shelving above on which I can fit quite a few boxes and pots, cover only that section with a curtain of bubble wrap , the rising warmth keeps the plants comfortable.
Over many years having tried all kinds of heating and insulating none of them will ever pay you back in produce, use only enough to save valuable plants or early seeds and on the first day it looks a bit bright and warmer then sow your seed, your plants will flower, produce food just a couple of days later than early sown seeds. The way to go is keep on sowing a little and often way past when the packet tells you and have late season flowers and food.