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Bubble wrap. Buy it in sheets off a roller in a good garden centre or DIY store. If you have an aluminium frame, you can also buy the little clips that slot into the frame to attach it.
I've had mine for several years and it's still in fine fettle. I'm going to buy some more to make a double layer to insulate the lower walls better.
Yes, I agree with obelixx. Bubble wrap is the most effective. If the cost seems prohibitive, you can use it for several years. If still too much, you can always use it and only insulate part of your greenhouse, creating a wall across with the bubble wrap. It depends how much you need to put into the greenhouse for winter protection.
There is an article written about insulating your greenhouse by Matt Biggs here I remembered reading it last year when I was looking at insulating my glasshouse. In the end I went for the clip option as mine has an aluminium frame.
You may find it's cheaper if you buy from a packaging company instead of a GC. I got mine a couple of years back from Staples.
There are 4 choices in insulating a greenhouse.
1. Bubble Wrap - This is the cheapest and cost effective option to insulate your greenhouse but the downside is, it only lasts around 3-4 seasons.
2. Thermal Screens- These screens are made of clear translucent materials which prevents heat from rising above and traps the warm air below the greenhouse where the plants are located.
3. Base Cladding- This traps heat with the use of polystyrene panels installed at the base of the greenhouse. This is placed in the space between the greenhouse and the ground and prevents heat from going out
4. Double Glazing- The most efficient and expensive way to insulate your greenhouse that is built at the same time the greenhouse is built.
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.