Advanced options
The polytunnel is covered growing at a quarter of glass prices: the trade is the cover (polythene lasts 4-7 years, GBP 60-150 to re-skin, vs glass's decades), looks (the allotment committee's aesthetic debates are eternal), and heat retention (single-skin poly loses heat faster overnight than glass: it's a season-stretcher, not a tropics). Siting: door away from the prevailing wind, skin fitted TIGHT on a warm day (slack polythene flaps itself to death), and bury or batten the edges properly: the tunnel that survives storms is the one anchored like it expects them.
What The Polytunnel Calculator Works Out
- Polytunnel cost: the typical kit price for the size chosen, with the price per square metre of covered growing underneath. Polytunnels run GBP 25-40 per m2 against GBP 90-150-plus for glass, which is why commercial horticulture grows under poly, not glass.
- Same area in glass: the floor area multiplied by the greenhouse rate you pick, and how many times the polytunnel's price that is. The 12 m2 allotment classic at GBP 400 compares with GBP 1,100-1,800 glazed.
- The honest trade: polythene lasts 4-7 years by UV grade before a GBP 60-150 re-skin, against glass's decades. Single-skin poly also sheds overnight warmth faster, so it is a season-stretcher and rain-shield rather than a winter tropics.
- Custom size and price (advanced): pick the custom option in the size dropdown and enter your own floor area and quoted price, for example a 5m x 10m hobby tunnel at GBP 1,500, and every result reprices from your numbers.
- Groundwork, base rail or anchoring (advanced): trenching to bury the polythene, a timber or aluminium base rail, anti-hot-spot tape and ground anchors. These are added to the upfront cost and to the glass comparison too, since a greenhouse needs a base as well.
- Polythene cover life and re-skin cost (advanced): the simple mode assumes GBP 100 every 5 years. Budget 720-gauge polythene lasts about 4 years, premium thermal anti-fog covers 7 or more; the re-skin cost scales with the tunnel's size.
- Re-skins over the period (advanced): how many replacement covers you will buy over the years you keep the tunnel. The cover fitted at purchase counts as the first life, so 20 years at a 5-year life means 3 re-skins, not 4.
- Lifetime polytunnel cost (advanced): purchase plus groundwork plus every re-skin over the ownership period, and the same figure for glass, which needs no re-skin. The tunnel still wins comfortably over a generation.
- Cost per m2 per year (advanced): the lifetime cost divided by the floor area and the years. At the defaults it is about GBP 3 per m2 per year, which remains the cheapest weather in Britain.