The insulation question is the most-Googled greenhouse query in the UK between October and March. The answers online range from "wrap it in bubble wrap" (genuinely useful) to "install underfloor heating" (definitely not for a hobby greenhouse). This piece is the operating manual: what's worth doing, what isn't, what the actual energy budget looks like, and how the choice of greenhouse glazing changes everything before you spend a penny on heating.
We sell wooden greenhouses with twin-wall polycarbonate glazing — so we have a starting position. But the principles below apply to any greenhouse, glass or polycarbonate, wood or aluminium.
Written by Alex Goldgewicht, founder of Waldenhaus. Reviewed by the Waldenhaus product team. Last updated 7 May 2026.
What "insulating a greenhouse" actually means
There are two distinct goals that get muddled together:
- Frost protection — keeping the interior above 2 °C when outdoor air is at -5 °C. Goal: overwinter tender perennials, citrus, geraniums, etc.
- Active growing — keeping the interior above 10–15 °C through the British winter to extend the growing season for tomatoes, peppers, salads. Goal: harvest, not just survive.
The energy budgets for these are an order of magnitude apart. Frost protection in southern England is achievable with bubble wrap and a small electric tube heater on a frost-stat (£0.50–£2 per cold night). Active winter growing in northern Scotland needs serious thermal envelope plus a heating system rated for the structure (£20+ per cold night). Don't conflate them.
Most UK hobby greenhouses target frost protection. The advice below addresses that case unless explicitly noted.
Glazing is the dominant variable
Before any add-on insulation, the glazing material sets the baseline heat loss.
| Glazing | U-value (W/m²K) | Heat loss vs glass | Light transmission |
|---|---|---|---|
| Single-glazed horticultural glass (3 mm) | ~5.8 | baseline | ~89% |
| Single-glazed toughened glass (4 mm) | ~5.8 | same as above | ~89% |
| Twin-wall polycarbonate (4 mm) | ~3.9 | 33% less heat loss | ~85% (with quality co-extruded UV layer; 90%+ on the better-quality grades) |
| Triple-wall polycarbonate (10 mm) | ~2.8 | 52% less heat loss | ~75% |
| Heat-treated horticultural glass + interior bubble wrap | ~3.6 | 38% less heat loss | ~75% (after bubble wrap) |
What this means in practice: if you start with single-glazed glass, you're losing roughly 5.8 watts per square metre per degree Celsius of temperature differential. For an 8×10 ft (~7.4 m² footprint, ~28 m² total surface area) greenhouse holding 10 °C above outdoor temperature, that's 1,624 W of continuous heat loss — equivalent to running a small electric fan heater non-stop.
Switching to twin-wall polycarbonate cuts that to roughly 1,090 W. Adding bubble wrap to polycarbonate brings it down to ~870 W.
If you're starting from polycarbonate already, your greenhouse is doing about a third of the insulation work for you before you spend anything. This is why we built NORDIC with 4 mm CrystalLight™ twin-wall as standard — not as an upgrade, but as the lowest-running-cost choice for actually using the structure year-round.
The bubble wrap question
Horticultural bubble wrap (UV-stabilised, larger 25 mm bubbles, designed not to yellow under sun) is the lowest-cost single insulation upgrade you can buy. Roughly £15 for enough to wrap an 8×10 ft greenhouse interior.
What it does well:
- Reduces heat loss by 30–40% versus bare glass; ~15% versus polycarbonate
- Cuts down condensation on the cold inner surface (water condenses on the bubble wrap, not on plant foliage)
- Trivially installable with double-sided tape or clothes pegs
What it costs:
- Cuts light transmission by 10–15% — meaningful in November when daylight is already a limiting factor
- Visually closes in the structure (matters for some growers, not others)
- Needs replacing every 2–3 seasons (UV degrades the plastic eventually)
The pragmatic advice: bubble-wrap the north-facing wall and roof slope only. North side gets minimal direct light gain anyway, so the light loss doesn't matter; you get most of the insulation benefit. South side stays clear for winter sun.
For a NORDIC owner in southern England, our standard recommendation is: north wall + north roof slope + door reveal in bubble wrap from late October to mid-March. That's enough for frost protection in everywhere from Hampshire to the Yorkshire dales without active heating. Add a tube heater on a frost-stat as backup for cold snaps below -5 °C outdoor.
Thermal mass: the under-used trick
A 200-litre water-filled drum painted black, sat in the corner of an 8×10 ft greenhouse, will absorb roughly 800 kJ of solar energy on a sunny February day and release it overnight as the air cools. Functionally, it adds 1–2 °C of overnight low-temperature buffer for free.
Variations on the theme:
- Black water drums (the classic) — 200 L oil drum, painted matt black, filled with water. £30 second-hand; lasts forever; adds significant thermal mass.
- Black-painted concrete blocks lining the back wall — useful if you can't fit a drum.
- Tomato sauce-style PET bottles filled with water — surprisingly effective in aggregate; arrange in a back-wall stack.
- Stone/brick paving inside the greenhouse — same principle on a smaller scale; absorbs and re-releases heat.
Thermal mass is more useful for evening out day-night swings than for raising overall temperature. In November–February, thermal mass alone won't keep frost away in northern England, but it materially extends how long passive solar carries you before active heating is needed.
Active heating: when and what
If bubble wrap + thermal mass + a closed structure isn't enough — usually because you're north of Manchester or actively growing tomatoes through January — you need active heat.
Realistic options for hobby greenhouses, ranked by running cost (lowest first):
1. Electric tube heater on frost-stat
- 80–135 W per tube; one tube heats an 8×10 ft to frost-free
- Frost-stat (£15) only powers the tube when ambient drops below set point (typically 4 °C)
- Running cost: ~£10–£25 per month in southern England; ~£40–£70 in northern Scotland
- Best for: frost protection, low-fuss
2. Electric fan heater on thermostat
- 1–2 kW; dual-speed; built-in thermostat
- More responsive to temperature drops than a tube heater
- Running cost: ~£20–£50 per month if used judiciously
- Best for: extending growing season into early/late winter
3. Paraffin (kerosene) heater
- Gravity-fed, no electricity needed
- 1.5–2.5 kW heat output for under £1/day at current paraffin prices
- Burning fuel produces water vapour and CO₂ — net positive for plants in moderation but raises humidity
- Best for: off-grid sites, allotments without power
4. Gas (propane) cabinet heater
- 2–4 kW, no electricity, fast response
- Like paraffin: produces water and CO₂
- Higher running cost per kWh than electricity off-peak
- Best for: occasional cold snaps in larger greenhouses
What we don't recommend:
- Underfloor heating cables — engineering overkill for a hobby greenhouse; running cost is brutal
- Wood stoves — fire risk, ventilation disaster
- Oil-filled radiators — slow response, designed for indoor air, lose efficiency in damp greenhouse air
For frost protection of the most common UK domestic case (8×10 ft greenhouse, twin-wall poly, bubble-wrapped north wall, southern England), an 80 W tube heater on a frost-stat is sufficient. Total winter running cost: £30–£60 in a typical year.
Sealing leaks: where heat actually escapes
Heat loss through glazing is the obvious one. The under-rated one is air leakage through gaps, particularly at:
- Door seals (largest single leak source on most greenhouses)
- Roof vents that don't fully close in cold weather
- Sole plate-to-base joint (especially on uneven bases)
- Joints between glazing panels (especially if W-clipped instead of screw-fixed)
A rough rule: a poorly-sealed greenhouse loses 25–40% more heat than a well-sealed one of the same construction. For an 8×10 ft running at 10 °C above ambient, that's the difference between £30 and £45 a month in heating cost.
Five-minute interventions:
- Self-adhesive draught-strip on the door reveal. £5; transformative.
- Foam sealant strip along the sole plate-to-base joint. Closed-cell foam, not the spray-foam type. £8 for enough to do an 8×10 ft footprint.
- Check the roof vent fully drops onto its frame in cold weather. If it doesn't, the spring or actuator may need adjusting (they freeze partly open occasionally).
These three together are a 30-minute job that saves 25%+ on running costs through winter.
What we built into NORDIC, specifically
Some details worth noting if you're choosing the structure itself:
- 4 mm CrystalLight™ twin-wall polycarbonate. U≈3.9 W/m²K. ~33% less heat loss than single-glazed glass before any add-on insulation.
- Single-sheet glazing wrap. No horizontal joint at the eaves — eliminates the most common air-leak point on segmented poly designs.
- Screw-fixed glazing with compressible gasket. Panels seat firmly; no air leakage at the panel edge.
- Door reveal sized for off-the-shelf draught strip. No bespoke sealing needed.
These choices make the structure roughly 30% less expensive to keep frost-free over a winter than a comparable lightweight aluminium greenhouse with W-clipped glass — without any additional insulation effort.
The full warranty: 10-year anti-rot frame warranty (conditional on re-treating timber with an approved wood preservative every 2–3 years) + 5-year polycarbonate warranty.
A specific example: an 8×10 ft NORDIC-M in mid-Wales
Real numbers from a customer in Powys, winter 2024–25 (Met Office December–February mean low: 0.4 °C):
- Glazing: 4 mm CrystalLight™ polycarbonate (factory standard)
- Add-on insulation: bubble wrap on north wall + north roof slope + door reveal (~£20 materials)
- Heating: 135 W electric tube heater + Brennan frost-stat set to 4 °C
- Result: zero plant losses to frost; minimum recorded interior temperature 3.2 °C on the night of 8 January (outdoor low: -7 °C)
- Total electricity cost for heating, December–February: £41
By comparison, a neighbour with a similar-size single-glazed aluminium greenhouse, no bubble wrap, same heater spec, ran the heater more than twice as often and spent ~£95 over the same period — and still lost three plants to frost when the heater fuse blew on the cold night.
Same outdoor conditions; structural choices made the difference.
What about for serious winter growing?
Frost protection is one thing. Actively cropping tomatoes, peppers, or chillies through December–February is another order of magnitude.
Realistic UK winter growing:
- Maintain 12–15 °C minimum — needs roughly 2 kW of continuous heat for an 8×10 ft, twin-wall poly + insulated, southern England
- Supplementary lighting — daylight in December is ~6 hours and most isn't usable for most crops; serious winter cropping needs T5 fluorescent or LED grow lights for 4–6 hours/day
- Running cost — £80–£200 per month in a typical southern English winter; double in northern Scotland
- Crop choice matters — leafy greens (chard, kale, lettuce), Asian salads, and herbs are realistic; ripening tomatoes through January is mostly not, even with heat
If "active winter growing" is the goal and budget is unlimited, what you actually want is a greenhouse with a heated, insulated propagation area inside it (a "house within a house") rather than trying to maintain 15 °C across the whole structure. That's beyond the scope of this article but the principle is: heat the smaller volume, insulate the larger volume.
Frequently asked questions
Does bubble wrap actually work, or is it a marketing trick?
It works. Horticultural bubble wrap (the larger-bubble UV-stabilised type) reduces heat loss by 30–40% versus bare glass, and 10–15% versus twin-wall polycarbonate. The trade-off is roughly 10–15% loss of light transmission. For frost protection in a UK greenhouse, the energy saving comfortably outweighs the light loss between November and February.
Can I use household bubble wrap from packaging?
Not advisable. Standard packing bubble wrap isn't UV-stabilised; it yellows and embrittles within a single season. Buy the horticultural grade — typically £8–£20 for enough to do a domestic greenhouse, lasts 2–3 winters.
Is twin-wall polycarbonate worth the extra cost over single-glazed glass?
For a structure you'll keep for 10+ years and use through winter, yes — the heating cost saving alone repays the difference within 3–4 winters. Polycarbonate also doesn't shatter (no shards if hit by a falling branch or stray football), and a damaged panel costs ~£60 to replace versus £25–£40 for glass plus the inconvenience of cleaning up shards.
Do I need to heat my greenhouse at all in southern England?
For most overwintering use cases, no — a well-sealed twin-wall polycarbonate greenhouse with bubble wrap on the north side will stay above freezing through normal southern English winters without active heating. Exceptions: tender perennials (citrus, brugmansia), tropical cuttings, or growers who want to start seeds in January–February. For those, a small tube heater on a frost-stat handles the few coldest nights cheaply.
What's the lowest-cost reliable winter heater for a small greenhouse?
A 80–135 W electric tube heater (£20–£40) plus a digital frost-stat (£15). Total cost under £55. Running cost in a typical southern English winter: £20–£40 total. Reliable, no fuel handling, no fire risk, no ventilation requirements.
How does ventilation affect heat retention in winter?
Less than people fear. You need some air movement to prevent humidity buildup and fungal disease (botrytis, downy mildew); a roof vent cracked 5 cm during the warmer part of any day is sufficient. Total air exchange this provides costs perhaps 5–10% of daily heat input — well worth it for plant health. Sealing the greenhouse completely shut for weeks invites disease, not productivity.
Will a wooden greenhouse stay warmer than an aluminium one?
Very slightly, but the difference is dwarfed by glazing material and air leakage. The wood frame itself has marginally lower thermal conductivity than aluminium (so corner posts conduct less heat outward), but framing accounts for under 5% of total heat loss in any greenhouse. Don't choose wood vs aluminium for thermal reasons; choose on construction quality, longevity, and air leakage at panel joints.
Choose your NORDIC wooden greenhouse
By size
- 8×6 ft — NORDIC-S £1,499 · patio / starter / small allotment
- 8×10 ft — NORDIC-M from £1,599 · family veg garden sweet spot
- 8×13 ft — NORDIC-L from £1,699 · serious year-round growing
- 8×16 ft — NORDIC-XL from £1,799 · multi-crop with two ground beds
- 8×20 ft — NORDIC-XXL from £1,899 · walk-in / market grower
By configuration
- Lean-to wooden greenhouse · wall-mount, space-efficient
- Polycarbonate greenhouses UK · 4 mm CrystalLight™ glazing detail
Decide before you buy
- Greenhouse Buying Guide UK 2026 · full step-by-step framework
- Wooden Greenhouses overview · the NORDIC range explained
- Shop the NORDIC range — from £1,499
After insulation, do you need heating? Greenhouse Heating UK Winter — the honest cost analysis