Polytunnel vs Greenhouse: Which One Should You Buy in Scotland?

A polytunnel is the cheaper structure to buy and a glass greenhouse the longer-lasting one to own. In Scotland the choice also turns on where you garden: wind decides it on the west coast and islands, frost matters more inland and light more in the north.

Scotland's weather, station by station

Met Office 1991-2020 averages for six stations, used for the wind, light, frost and rainfall points below:

The west and north (Tiree, Stornoway, Lerwick) are windier and wetter. Inland sites such as Aviemore are calm but cold, with far more frost nights. Sunshine is lowest at Lerwick, Aviemore and Stornoway, and every Scottish station here gets less of it than Camborne.

The short version

Cost: cheaper to buy, not necessarily cheaper to own

A polytunnel wins on the price tag: it needs a significantly smaller investment than a comparable greenhouse, and the RHS calls tunnels a highly cost-effective way of growing summer crops. Prices vary by size, brand and retailer, so check live listings. A 6x8ft kit is the usual starter size for comparing the two.

The saving is partly deferred. The polythene cover is a wear part, and sunlight breaks it down over time. It's usually replaced every 7 to 10 years, and a storm-torn cover can force a replacement earlier whatever its UV rating says. Anchoring and tensioning are what prevent that: see securing a greenhouse or polytunnel against wind. Glass has no scheduled replacement, but "absent breakage" matters on an exposed coastal or island site, where wind-driven panel loading can make cracked or blown-out panes a recurring cost. The greenhouse's advantage is narrower there than the lack of a replacement schedule suggests.

So the comparison is an upfront saving plus a new cover roughly once a decade, against a higher upfront cost and little scheduled spend afterwards. How long you keep the structure and how well the cover survives your local wind decide which comes out cheaper.

Lifespan: frames outlast covers, glass outlasts everything

• Polytunnel frame (galvanised steel): a manufacturer quotes 20 years and more.
• Polytunnel cover (polythene): usually replaced every 7 to 10 years. UV and wind stress wear it faster than age alone.
• Greenhouse frame (aluminium): needs no upkeep, and is often the lifetime of the structure.
• Greenhouse glazing (glass): can potentially last a lifetime, with breakage the usual limit. Polycarbonate is durable in adverse weather and good at retaining heat, but is generally shorter-lived than glass.

Expect to re-skin a polytunnel at least once over a couple of decades. A well-built greenhouse is closer to a one-off purchase. Neither is wrong: it comes down to paying less now and budgeting for a cover swap, or paying more now and mostly forgetting about it.

Light transmission: where the gap matters most in Scotland

Glass lets about 90% of light through, and clear polycarbonate about 83%. Polythene film loses light as it ages: UV breaks it down and dust and algae build up on it. Commercial grower guidance puts the loss at around 2 to 5% a year and replaces film every 4 to 5 years, earlier than the 7 to 10 years a garden manufacturer quotes. The decline is slow enough that growers rarely notice it. Cloudiness and yellowing come late, so a cover that still looks acceptable may already be costing you light. Glass doesn't age optically, though it needs washing.

Light is the limiting factor in winter and early spring, and Scotland starts with less of it. Edinburgh gets about 13% fewer sunshine hours than Camborne, Aviemore about 28% fewer and Lerwick about 31% fewer. A weathering cover stacks on top of that, and the effect on late-winter seedlings and overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → crops is not marginal. It doesn't make a greenhouse mandatory. It does mean replacing the cover on schedule rather than running it past its life, and it counts in glass's favour if your plot is already shady or northerly. Washing the cover helps too.

Heat retention: a baseline difference, not the whole story

Polytunnels do not retain their heat as well as a glasshouse, and overwintering plants in one need additional heating and protection. Tunnels are not usually economical to heat and keep frost-free over winter. That gap costs most where frost nights are most frequent. Edinburgh logs about 43 air-frost days a year against Camborne's 7, and Aviemore about 84. The island stations log far fewer (Tiree 9, Stornoway 22, Lerwick 29), so heat retention matters most at the Aviemore and Edinburgh end of the range.

It is a baseline, not a rule. Both structures can be insulated, heated and managed to change it. Bubble wrap cuts light by about 10% but is generally considered worth it to keep heating bills down, and it works in either structure. One polytunnel manufacturer says a tunnel covered in thermal polythene, without a greenhouse's draughts, can help with overwintering crops. For the detail on bubble wrap, thermal mass and heaters, see Insulating a Greenhouse for Winter. Most of it adapts to a tunnel.

Wind resistance: where Scotland's geography changes the answer

The two fail differently. Wind gets under or across a polytunnel's whole taut skin and can lift, balloon or tear the cover, and in severe cases lift the structure itself if the anchoring is inadequate. A greenhouse takes wind as direct panel loading: individual panes or polycarbonate sheets crack or blow out, but the frame is usually more rigid, so the whole structure rarely goes at once. Neither is the safer choice in high wind. For anchoring, tensioning and storm-proofing either one, see Securing a Greenhouse or Polytunnel Against Wind.

The exposure varies a lot across Scotland. Lerwick's mean wind is about 2.5 times Aviemore's and Stornoway's about double. Lerwick and Tiree run roughly 35 to 40% above Camborne, while Stornoway is only about 13% above it. A tunnel on a sheltered inland plot such as Aviemore's faces a different wind regime from one on a Hebridean or Shetland croft, and storm gusts sit well above an annual mean. On a Tiree- or Lerwick-type site a well-anchored greenhouse may be the more manageable risk, or at least the case for a heavier-gauge cover and serious anchoring rather than a budget kit. On a sheltered inland site the tunnel's saving comes with much less of that trade-off.

A calm site is not automatically a low-risk one. A tunnel's curved hoops tend to shed snow better than a greenhouse's pitched roof, but either can be damaged by wet, heavy snow left to build up. Cold inland sites such as Speyside are where snow showers are likelier, so clear the roof as snow settles, whichever structure you choose.

Salt-laden coastal air also tends to corrode a polytunnel's galvanised hoops and a greenhouse's metal glazing clips faster than inland air. The rate depends on exposure, so inspect fittings more often on a west-coast or island structure than an inland one.

Rainfall and condensation: not a uniform "Scotland is wetter" story

Rainfall depends on which coast you're on, and Edinburgh is far drier than the west. Tiree, Stornoway and Lerwick all take 1,235mm or more a year, wetter than Camborne's 1,076mm, while Edinburgh gets 728mm and Aviemore 985mm. The west and north take the Atlantic weather.

Neither structure has an advantage against rain falling on the outside. The problem is the humid air inside, from watering and plant transpiration, which condenses on the coldest surfaces of polythene and glass alike. Ventilation controls it, not the choice of covering. The RHS guidance for glasshouses, plastic greenhouses and polytunnels alike is about one square metre of ridge ventilation for each five square metres of floor area (20%), and small structures ideally need more. The door, roof vents and side vents all count. A wet site doesn't need a different structure, only the same ventilation either one needs.

Ease of DIY installation

A polytunnel is generally the more forgiving self-build. It is usually built straight onto a soil base, a small one can be assembled relatively quickly (a weekend is plausible, ideally with a second pair of hands to pull the cover taut), and it's easier to dismantle, move and reassemble. There's no glass to handle and more tolerance for a base that isn't perfectly level, though a level, well-drained base still helps door alignment and drainage.

A greenhouse asks for more precision. The base needs to be level and square before the frame goes up, panes or sheets need careful handling and sealing to avoid leaks and heat loss at the joints, and glass adds breakage risk during the build. Greenhouses are time-consuming to construct and hard to dismantle once erected. Aluminium kits are designed for self-assembly and many gardeners manage them solo, but handling large panels on a windy exposed site is a real hazard, so weigh the site's wind before you buy.

Slugs and ground-level pests

A polytunnel's beds are usually open ground with no sill or threshold, while many greenhouses sit on a low wall or base with staging that puts some of the growing a step above the soil. Slugs and snails reach either, and the same controls work in both: encourage predators, protect seedlings and plant sturdier transplants. If slugs already trouble your plot, ground-level crops are the ones most exposed in a tunnel.

Which suits which use

Vegetables, salad crops and season extension: Both work; this is the classic polytunnel strength. The larger floor area per pound spent and the chance to grow straight in the ground suit big, productive beds, and the RHS rates tunnels highly for summer crops.

Tender ornamentals, alpines and overwintering pot plants: A greenhouse's more consistent light and better heat retention suit plants that need steadier conditions, and glass looks better close to the house.

Allotment plots: Space, budget and tenancy matter. A tunnel is lighter to move and cheaper to replace if you lose the plot. Check your site's own rules first, as some restrict the height or footprint of structures.

Small town gardens: A greenhouse's smaller footprint, permanence and looks often suit a garden that doubles as a place to sit. Check the planning rules before building anything of size. The Scottish Government's guidance allows a shed, garage or greenhouse without planning permission at the back of the house if it is no higher than 4 metres (2.5 metres within 1 metre of a boundary, with eaves no higher than 3 metres) and, with other buildings, doesn't take up half or more of the ground behind the home. It doesn't name polytunnels, so ask your local council before putting up a large one.

Exposed rural, coastal or island sites: Budget for wind-proofing on top of the purchase price: a heavier-gauge cover with serious anchoring, or a well-secured greenhouse. Inspect corrosion-prone fittings (glazing clips, hoop joints) often.

Sheltered inland sites (Speyside, Perthshire and similar): Wind is less of a deciding factor, but plan for winter cold and snow-clearing instead.

A few more practical differences

Rainwater harvesting. A greenhouse's rigid roofline takes guttering easily, so a downpipe can run straight into a water butt. A polytunnel's curved roof doesn't take standard guttering; dedicated polytunnel gutter kits exist but are an extra purchase. That counts for the greenhouse if you want to collect rainwater.

Wildlife getting in. A greenhouse door seals shut. A tunnel's door and end panels are more often a flap or a loosely zipped gap left ajar for ventilation, which lets rabbits, birds and (on a rural plot) deer in more easily. Neither is wildlife-proof, but it is worth weighing if rabbits or deer are already a problem.

Secondhand and reclaimed options. Reclaimed timber-and-glass greenhouses (a common find on allotments and in classified listings) and secondhand tunnel frames are a real budget route into either structure, so check before assuming new is the only option.

Common questions

Sources