The standard and best material for insulating your greenhouse is simply bubble plastic. Bubble plastic can be obtained with various sizes of bubble, the 20 mm large bubbles are best as they allow more light through and are better at retaining heat. Each bubble holds air which means the sheeting works in a similar way to double glazing.
Correctly applied bubble wrap will also cut down on draughts which suck out the warm air.
Horticultural Bubble Wrap Insulation
You are best buying purpose made horticultural bubble wrap rather than packaging wrap. The horticultural bubble wrap is tougher and UV stabilised and will last longer, at least three years.
Decide how you are going to insulate and measure up so you know how much insulation to buy. Allow some extra for overlaps and wastage, 10% extra should be about right.
How to Install the Insulation
If you, for example, decide you need half the house insulated drape a curtain of bubble plastic from side to side. You are best to insulate the half furthest from the door so a cold blast of air won't enter the warm area when you enter the greenhouse on a winter's day. Use two pieces of wrap, one to the left and one to the right with an overlap in the centre to form a doorway to the warm area.
For an aluminium greenhouse you can buy special clips to hold the bubble wrap. These clip into the channels on the glazing bars and then onto the bubble wrap. For wooden greenhouses drawing pins will suffice.
If you have a large greenhouse with excess space above head height, drape the bubble wrap from side to side to form a roof at a lower level. This again reduces the volume you have to heat.
Otherwise, start from the ridge apex and work down to the vertical glazing bars. Then fix the insulation to the vertical faces.
To buy bubble wrap go too sites like, www.packaginggb.co.uk, where you can buy large size rolls