Winter Insulation for a Small Greenhouse

Why a Small Greenhouse Chills So Quickly

A six-by-eight lean-to or a little four-by-six at the end of the plot has one great advantage: it costs far less to heat than a full-sized glasshouse. It also has one stubborn disadvantage. Because it has a small internal volume compared with the area of glass around it, it loses heat almost as fast as you put it in. On a still, clear February night the temperature inside can drop two or three degrees below the outside air, because the ground and the plants radiate heat up through the glass to the sky.

The answer is not to fight that with a big heater. It is to slow the loss, block the draughts and give the structure something warm to hold on to. Do those three things well and a small greenhouse will ride out most UK frosts with only a whisper of extra heat on the very coldest nights.

Line It With Bubble Wrap

Bubble wrap is the single most effective cheap fix. The air trapped between the bubbles is a poor conductor, and lining the inside of the glass creates a still layer of air that stops warmth escaping. You can expect it to lift night temperatures by roughly two to four degrees Celsius, which is often the difference between a surviving fuchsia and a blackened stump.

• Use horticultural bubble wrap if you can. Ordinary packing wrap goes brittle and yellows in ultraviolet light within a season.
• Aim for 20mm to 30mm bubbles. Smaller bubbles give a tidier finish; larger ones insulate slightly better.
• Fix it with plastic clips pushed onto the glazing bars, or with aluminium tape. Washing-up liquid smeared on the frame helps clips grip in cold weather.
• Leave the roof vents and any louvre clear, or cut flaps you can fold back. Insulation you cannot ventilate through is worse than none.
• Do not seal the gap at the eaves. Air needs somewhere to move, or you will trade frost damage for grey mould.

Line the side walls and the roof, but leave the floor bare. A sheet of bubble wrap laid across the floor only traps damp and rots the staging legs.

Draught-Proof the Gaps You Cannot See

Most heat in a small greenhouse is lost through gaps, not through the glass itself. A door with a two-millimetre gap all round is like leaving a window ajar all night. Go round the structure on a still morning and feel for cold air with the back of your hand.

• Fit brush or foam draught excluder around the door frame, and a rubber seal along the bottom of the door.
• Check the hinges. A door that has dropped will never seal until you adjust it.
• Run a bead of silicone or mastic along cracked panes, and replace missing glazing clips before the wind finds them.
• Slide a strip of foam pipe lagging under the bottom rail if the frame sits on a wooden or concrete base with gaps.
• On an allotment, screw a strip of timber across the base of the door to stop the wind driving in underneath.

Whatever you use, keep the vents free. Draught-proofing is about stopping the wind, not about sealing the building airtight.

Build a Thermal Store

Every kilogram of water, brick or stone you put in the greenhouse acts as a battery. It soaks up warmth during the day and releases it slowly after dark. In a small space you have to be choosy, so concentrate the mass low down where the cold air collects.

• Filled two-litre plastic bottles, painted matt black, stood along the back wall and under the staging.
• A dark-coloured water butt, if you have room, will hold a surprising amount of heat and double as a water source.
• Gravel trays or a layer of sand on the staging hold warmth and, just as usefully, soak up surplus moisture.
• Terracotta pots stacked in a corner. Unglazed clay is dense and cheap.

Cover the store during the day in really cold spells, then uncover it at dusk so it can give its heat back when you need it. A simple sheet of bubble wrap draped over a water butt is enough.

Use Heat Sparingly and Accurately

If you do add heat, add it only when it is needed. A tube heater or a small thermostatically controlled fan heater set to come on at about 2 to 4 degrees Celsius will protect tender plants through the worst of a British winter for a fraction of the cost of running it continuously. A maximum and minimum thermometer tells you exactly what your greenhouse did overnight, so you can stop guessing.

Paraffin heaters work, but they produce a great deal of water vapour as they burn. In a small, sealed structure that moisture settles on leaves and encourages botrytis and damping off. If you use one, ventilate a little during the day and keep a tray of gravel underneath to catch drips.

A Simple Winter Routine

Insulation is a set of small habits, not one job. Open the door and vents on mild, bright mornings, even in January, and close them again by mid-afternoon while there is still warmth to trap. Water sparingly, in the morning, and never leave saucers standing full. Move the most tender plants to the middle of the staging, away from the glass, and wrap them in a layer of horticultural fleece for the coldest nights.

Keep an eye on the forecast for clear, still nights, which are colder than windy ones. On those evenings, push a sheet of bubble wrap or fleece across the inside of the roof and over the staging frame, then lift it again in the morning. It is an extra five minutes of work, and it is often the difference between a greenhouse that merely survives the winter and one that comes into spring with healthy, ready plants.