Insulating a Greenhouse for Winter: What Works in a Scottish Garden

A greenhouse keeps rain and wind off, but it does little to keep cold out. The main loss of heat from a greenhouse is through draughts and the structure itself, and on a still, clear winter night an unheated single-glazed greenhouse can end up barely warmer than the garden outside. That is why insulation and heating matter as much as the structure itself. What differs in Scotland is how often, and how hard, the outside air gets cold enough for it to matter.

How much colder is a Scottish winter, really?

Met Office 1991-2020 station averages show every mainland Scottish lowland station logging several times more air-frost days than Camborne in Cornwall, one of the mildest stations in mainland Britain:

Most of lowland, inhabited Scotland sits in a fairly tight band of roughly 38 to 49 air-frost days, all clearly colder than Cornwall but broadly comparable to each other. Inland upland sites such as Aviemore are in a different category, and the coldest nights get far colder there than in the lowlands: the UK record of -27.2°C was set at Braemar in 1895 and 1982 and equalled at Altnaharra in 1995, and Carnwath in South Lanarkshire reached -24.6°C in 1979, so a hard extreme isn't confined to the Highlands. See Scottish frost dates by region for all eight regions, and RHS hardiness zones for Scotland for the hardiness framework these figures map onto.

These are regional averages, not a prediction for your plot. A greenhouse in a hollow where cold air drains and pools, or in an open spot with no wall or hedge, will run colder on a still night than the station figures suggest, sometimes by several degrees. One tucked against a south-facing house wall picks up stored heat from the brickwork and is milder than the open-ground figures imply.

What insulation is worth doing?

The data splits Scotland into two cases. For most lowland gardens (Edinburgh, Dumfries, Dyce, Charterhall, Inverness), the frost-day count and typical frosts of a few degrees below zero mean the standard approach is proportionate: a well-fitted layer of bubble wrap, sensible plant siting and a max/min thermometer to catch the exceptional cold snap. Insulation and vigilance matter more here than a Cornwall-written guide implies, but the gap is one of degree, not category.

For upland and Highland-glen gardens, with frost most months and minimums far below the lowland range, the case changes. Bubble wrap buys a modest few degrees by trapping an insulating air layer against the glass. It does not create heat, only slows its escape. Against a typical lowland frost of a few degrees below zero that is often enough to keep an unheated space above freezing, but against a deeply sub-zero upland night it won't keep a tender (RHS H2The RHS's H1a–H7 scale for how much cold a plant can survive outdoors, H1a being tender indoor-only and H7 hardy anywhere in the UK. Full definition → or tenderer) plant alive. Pelargoniums, citrus, tree ferns and young alpine seedlings are typical examples. There the proportionate response for anything not fully hardy is frost-free heat from a thermostatically controlled heater (the RHS defines frost-free as 4C, enough for H2-rated plants), with bubble wrap reducing how hard that heater has to work. It's also worth planning for a power cut on the coldest night of the year: a paraffin heater kept as backup, or at least a max/min thermometer with an alarm, is cheap insurance.

Most Scottish gardeners are not gardening in Braemar. The real dividingPropagating a clump-forming perennial by digging it up and splitting it into several smaller pieces, each with roots and shoots attached. Full definition → line for what's worth doing is lowland versus upland and Highland glen, not Scotland versus England as a whole.

Insulation methods compared

For fitting, large-bubble horticultural bubble wrap (the bubbles roughly 25mm across, not the small-bubble packaging kind, which insulates less well) can be fixed to the frame with plastic clips pushed into the glazing bars, or drawing pins on a wooden frame. Both are quicker and less fiddly than tape, which tends to fail once it's damp.

Heating is relevant from autumn to spring. Put bubble wrap up as the first hard frosts start (typically October in lowland Scotland; check the frost dates for your area) and take it down once night frosts are behind you, typically April or May, since it also blocks light your plants need when the growing season restarts. A heater's running cost depends on its wattage, the greenhouse's size and insulation, and your tariff, so check the wattage against your own tariff before buying. A thermostat-controlled heater does most of its work overnight, so an off-peak tariff is worth checking.

For a large, lightly stocked greenhouse, you can grow everything at one end and screen off a smaller heated area with bubble wrap and battens. Many gardeners also insulate only the north-facing wall and roof, where little useful winter light comes from, and leave the south-facing glazing clear, which matters because short, low-angle winter daylight this far north means anything grown through winter is already short of light. Fit enough to do the job, not more.

Paraffin and gas heaters produce combustion products that can harm plants, so the greenhouse has to be ventilated, which partly undermines the heat being generated, and they are the least environmentally friendly option. That's a case for electric as the everyday heater and paraffin only as outage backup. For an exposed or west-coast site, rigid panels (cellular polycarbonate or Correx cut to fit the frame) are a sturdier alternative to bubble wrap on the north wall at a higher one-off cost. If the job is raising seedlings rather than keeping mature plants alive, a soil-warming cable or heated propagator mat warms the compost directly, which is far more efficient than heating the whole greenhouse's air.

Getting ventilation and heat loss in balance

The instinct in a cold Scottish winter is to seal everything up, which is a mistake for a different reason: ventilation is essential in winter to prevent mould and dampness. A sealed, bubble-wrapped greenhouse traps the moisture plants transpire and compost gives off, and that moisture condenses on the coldest surfaces overnight. Grey mould (botrytis) thrives in still, humid, poorly ventilated conditions.

• Vent briefly on frost-free or milder days, even for twenty minutes around midday, then close up before the temperature drops in the afternoon.
• Space plants apart so leaves don't touch, and stand small pots on wood or slate to lift them off cold, damp staging.
• Cut back watering through the darkest, coldest months: cold, wet roots are more vulnerable to rot than to cold itself.
• Keep bubble wrap and fleece off foliage where possible, since damp trapped between plastic and leaf is a common place for rot and mould to start.

Too much ventilation on a hard frost night undoes the insulation you've fitted. Too little breeds the mould and rot that kill overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → plants as reliably as frost does.

Islands and exposed coasts: wind, not cold, is the bigger problem

Everything above assumes cold is the main threat, which holds for most of the mainland but not for the islands and exposed coasts. Stornoway averages 22 air-frost days a year and Tiree only 9, with Stornoway's worst months (December and January) averaging about 5 frost days each. On ground that mild, insulating against cold matters far less than keeping the structure standing through a winter gale, so a west-coast or island gardener is better off starting with securing the structure than warming it. Heavy or wet snow is worth a line too, especially on the Grampian and Highland side: clear it off a greenhouse or polytunnel roof before its weight cracks glazing bars or collapses the frame.

Common questions

Sources