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Why Insulation Matters More Than Heating

If you have ever opened the greenhouse door on a frosty January morning and felt that cold slap of air, you will know the feeling. Glass is a terrible insulator. Left bare, a greenhouse loses heat almost as quickly as it gains it, and a small paraffin heater or electric fan heater will happily burn through its output trying to hold back a -6°C night. Insulation is the cheapest heat you will ever "buy" — it costs a little bubble wrap and an afternoon, and it can lift the minimum temperature inside by two or three degrees, which is often the difference between a surviving pelargonium and a blackened mush.

The principle is simple: trap a layer of still air between the glass and the growing space. Still air is a superb insulator; moving air is not. Everything below is about creating that layer, sealing the gaps where warmth escapes, and doing it without turning your greenhouse into a damp, stagnant box.

Bubble Wrap: The Workhorse of Winter Protection

Horticultural bubble wrap is the go-to for most UK gardeners. It is inexpensive, easy to cut, and lets through plenty of light, which matters enormously between November and February when daylight is already scarce. Look for the larger-bubble type sold specifically for greenhouses — the bubbles are roomier, so more trapped air, and the plastic is usually UV-stabilised so it survives a full winter without going brittle.

Fix it to the inside of the glass, never the outside. Inside, the trapped air sits between the wrap and the pane, and the wrap itself stays dry. Outside, wind and rain shred it, and any moisture trapped behind it freezes and can crack panes.

• Use plastic clips made for greenhouse glazing bars, or double-sided tape on aluminium frames. Avoid staples into timber that will rust.
• Wrap the north and east walls first — these lose the most heat and receive the least useful light. Leave the south-facing roof unlined if you can, to maximise winter sun.
• Keep the wrap clear of vents and louvres. You still need ventilation, even in January.
• Leave a small gap at the ridge if your greenhouse has one, so humid air can escape rather than condensing on cold glass and dripping onto foliage.

In an unheated 8x6 greenhouse, a full bubble-wrap lining typically keeps the inside 2–3°C warmer on a still frosty night. Add a heater and the same heater will now hold the temperature with far less running time.

Fleece, Screens and Fleece Curtains

Horticultural fleece does a different job. Bubble wrap insulates the structure; fleece insulates the plants. A double layer of 30gsm fleece draped over a tender plant creates a microclimate a degree or two warmer than the surrounding air, and it protects leaves from direct frost contact, which is what actually kills tissue.

Use fleece in three ways:

• Direct covering — drape loose over citrus, olives, fuchsias and young cuttings, using hoops or canes to keep it off the foliage. Weight the edges so wind does not lift it.
• Thermal screens — a horizontal sheet of fleece or bubble wrap pulled across the greenhouse at eaves height, drawn in the evening and opened in the morning. This traps warmth in the lower volume where your plants actually sit, and it is remarkably effective because warm air stratifies.
• Bubble-wrap curtains — a simple roll of wrap fixed above the door and dropped at dusk. Doors are the single biggest heat leak in most greenhouses.

Fleece is breathable, so it does not create the damp problems that solid plastic sheeting can. If you use polythene, always leave the top open for air exchange.

Seal the Gaps Before You Insulate

There is no point wrapping the walls if the door whistles and the vents rattle. Do a draught check on a windy evening with a hand near the frame — you will soon find the culprits.

• Replace perished foam draught strip around doors and vent frames. Self-adhesive EPDM strip lasts far longer than the cheap foam stuff.
• Fix a brush or rubber seal along the bottom of the door where it meets the threshold.
• Close any automatic vent that is set to open too early in the season; a vent cracking open at 8°C on a sunny December day loses more heat than it gains in humidity control.
• Check cracked panes and loose glazing clips. Even a 3mm gap loses a surprising amount of heat over a long night.

And do keep a little ventilation. Completely sealed greenhouses in winter are damp greenhouses, and botrytis will find every soft-leaved cutting you own. Open a vent for twenty minutes on mild, dry mornings, ideally around midday.

Managing Water, Humidity and Heat Together

Insulation changes how you garden through the winter, so adjust accordingly. With less air movement and cooler temperatures, compost stays wet for far longer. Water sparingly, in the morning, and only when the surface is genuinely dry. Overwatering in a cold greenhouse is the fastest route to root rot.

Group plants together so they share warmth and humidity, and raise pots onto slatted benches rather than leaving them on a cold concrete floor, which sucks heat out of the rootball. A max-min thermometer is worth every penny — recording overnight lows tells you whether your insulation is actually doing the job, or whether you need another layer of bubble wrap.

If you do run a heater, insulate first and heat second. A small thermostatically controlled electric heater set to 5°C, backed by a well-lined greenhouse, will cost a fraction of the same heater struggling in a bare one. For those going without heat entirely, remember that insulation buys you protection from hard frost, not from a prolonged freeze — tender exotics are still safer indoors or under a double layer of fleece.

Do the work now, while the days are still mild, and your greenhouse will carry its plants through to spring with far less fuss, less fuel and far fewer losses.