How do I insulate a greenhouse with bubble wrap?

Lining a greenhouse cuts heat loss substantially and can halve the cost of keeping it frost-free. It also cuts light in the months when light is already the limiting factor, and it raises humidity. Both trade-offs are manageable — but only if the vents stay working, which is the detail people get wrong.

What you'll need

• Horticultural bubble insulation with large bubbles, true
• Greenhouse insulation clips for your glazing bar profile, or drawing pins for timber, true
• Scissors or a sharp knife
• Tape measure

Step by step

1. Clean the glass first.Once the lining is up you cannot clean the inside of the glass until it comes down in spring. Do the autumn clean, then line.
2. Buy horticultural grade with large bubbles.Horticultural bubble insulation is UV-stabilised and has large bubbles, which transmit noticeably more light than the small bubbles of packaging wrap. Packaging bubble wrap works for one season and blocks more light.
3. Fix it to the glazing bars, leaving an air gap.Proprietary clips push into the aluminium glazing bar channel; on a timber house use drawing pins or thin battens. The still air trapped between the sheet and the pane is what insulates — a sheet taped flat against the glass does far less.
4. Do the priority faces first.If you are only lining part of it: the north-facing side and both gable ends first, because they contribute the least light and lose the most heat. Then the roof. The south-facing side last, or leave it clear to keep the winter light.
5. Cut around every vent and the door.Each roof vent and louvre must open fully and each opening must stay clear. Cut the sheet to the opening and clip the edges back. A lined greenhouse with sealed vents becomes a damp box and loses more plants to grey mould than the cold would have.
6. Consider partitioning instead of lining the whole house.A curtain of the same material across part of the length creates a small heated zone and leaves the rest cold. Heating a third of the volume costs a third as much, and most people only need frost protection for a limited number of plants.
7. Overlap the sheets and seal the corners.Overlap adjacent sheets by 100mm or so and clip through both. Draughts through gaps at the corners and the eaves undo a lot of the benefit.
8. Take it down early in spring.Up in late autumn, before the first frosts. Down as early as the weather allows in spring — March in a mild area — because light is the limiting factor for everything you are about to sow.

Tips

• Lining raises humidity because it reduces air changes. Ventilate on every mild day through the winter, and water sparingly and only in the morning.
• Keep the sheets, labelled by which face they came from, and reuse them for several seasons. Horticultural grade lasts far longer than packaging wrap.
• Line the gable ends and the north side permanently in a house you use mainly for propagation. Those faces contribute very little useful light at any time of year.

Common mistakes

• Taping the sheet flat against the glass — It removes the air gap that does the insulating. Fixing to the glazing bars, with the sheet standing off the pane, is what makes the difference.
• Covering the vents — A sealed greenhouse in winter is a damp, still box. Botrytis will take more plants than the frost would have, and the plants that survive will be soft and drawn.
• Using packaging bubble wrap — Small bubbles scatter and absorb far more light, and non-UV-stabilised film goes brittle and yellow within a season.
• Leaving it up into late spring — It costs light at exactly the point when seedlings need the most, producing drawn, weak plants.

Questions people ask