Greenhouse shade net thickness and density: what the specs really mean

What the specs on a greenhouse shade net label really mean

Shade percentage is the only figure that tells you how much sunlight a greenhouse shade net blocks. Thickness and weight are indirect clues to durability and construction. Treat them as supporting evidence, not as the cooling rating itself.

Greenhouse shade net is a knitted or woven HDPE mesh fitted over or inside a glasshouse or polytunnel to cut solar radiation, lower air temperature and prevent leaf scorch. Retailers also sell it as greenhouse shading netting, horticultural shade cloth, garden shade netting or an agricultural shade screen. The names are largely interchangeable, and what matters is the specification behind them.

Shade percentage is the headline spec

A 50% rating means roughly half of incoming light is stopped, while the rest passes through the gaps or thin fabric. Suppliers usually test this as light transmission, but methods differ between producers, so two "50%" nets can differ by several points. UK greenhouse kits commonly include nets in the 40–60% range, and 55% is a familiar figure. For planning, think in bands: a 30% 50% 70% shade net range covers light, medium and heavy duty use.

GSM and thickness: useful, but not the same thing

GSM (grams per square metre) shows how much plastic sits in each square metre of cloth. Heavier fabric usually means more yarn and a stronger net. Yet two nets with identical weight can have different shade percentages, because the yarn is arranged differently. Thickness, quoted in microns or millimetres, is rarer on labels and normally describes the individual tape or monofilament, not the whole net. In practice, use GSM as a durability check and shade percentage as the cooling check.

Does a heavier net always cool better?

No. A heavy net and a light net with the same shade percentage block the same amount of light. Weight does change strength, wind resistance and lifespan, so a heavier greenhouse shade net is often worth it on an exposed allotment or hillside plot. Colour matters too. Dark nets absorb heat, whereas white or reflective aluminium shade screen material sends more radiation back out. Why do so many gardeners buy the darkest, densest mesh? Because it looks more protective. Yet a net that is too dense can leave tomatoes pale and leggy on cloudy British days.

Density, knit and construction: reading beyond the percentage

Density is a product of yarn width, stitch spacing and knit pattern. Two nets with the same shade rating can therefore behave very differently in wind, light quality and ageing.

Knitted shade netting versus woven

Knitted shade netting is formed from interlocking loops, so you can cut it with scissors without it unravelling, and it has some stretch. Woven nets are stiffer and can fray at cut edges unless they are heat-sealed or hemmed. That is why most greenhouse shade fabric sold for domestic use is knitted. The stretch is a double-edged feature, though. It absorbs gusts well, but the net must be tensioned evenly or it will sag onto the glass. A polytunnel shade net often needs wider sheets and reinforced edges because of the curved profile.

Tape, monofilament and mesh count

Tape yarns are flat ribbons that cover more area per thread, so fewer threads can deliver high shade and soft, diffuse light. Monofilament yarns are round and tend to give a more open mesh with better airflow at lower shade levels. Mesh count, the number of threads per centimetre, matters most for insect-proof plant protection netting. For shading it is secondary to the stated percentage. Still, a tape-heavy 70% net will pass less air than a monofilament 40% net, which is why your vents still matter.

UV stabilised shade mesh and expected lifespan

UV stabiliser additives slow the breakdown of polyethylene in sunlight. A UV stabilised shade mesh made from virgin HDPE generally outlasts one made from recycled blends, which is the main reason for price differences between similar-looking rolls. Some manufacturers, Meyabond among them, specify new 5000S HDPE with imported UV additives and bound edges, and state a service life of 3–5 years. Real life depends on sunlight exposure, wind, chemicals and handling, so treat any lifespan figure as a range rather than a promise. An untreated net can become brittle far sooner, so check that the label names UV treatment.

Choosing the right shade percentage for UK crops

For most UK greenhouses, 40–50% suits tomatoes, 50–60% suits cucumbers and seedlings, and 30–40% suits chillies and succulents. Keep 60–70% for shade-loving plants or exceptional heat. The Met Office has logged Kew Gardens at 38.1°C in a record-setting heatwave, a reminder that summer peaks can now exceed what older kits were designed for. A typical British summer still swings between grey 18°C days and bright 30°C days, so over-shading carries a real cost.

30%, 50% and 70% shade net compared

A 30% greenhouse shade net takes the edge off midsummer glare while preserving light in spring and autumn. A 50% net is the usual all-rounder for mixed growing. A 70% net is a heavy-duty choice that works for ferns, orchids and propagation, but on dull days it can cut light below what fruiting crops need. The sensible approach is to match the net to your hottest crop and your sunniest aspect, not to the hottest day of the year. Think of it as sunscreen: the highest factor is not always right for a cloudy afternoon.

Crop-by-crop recommendation table

These are horticultural guidelines, not guarantees. Variety, greenhouse orientation and ventilation all shift the optimum.

Is 50% shade net enough for a UK summer?

For most UK growers, yes. It is the most common starting point and works for tomatoes, cucumbers and general growing. If the interior still passes 30°C with vents open, move to 60% on the sunniest side or add a reflective layer on the south roof, rather than fitting 70% all year. Seasonal thinking also helps: a lighter net in May, with an extra layer added only during heatwaves, protects crops without starving them in changeable weather.

How much cooling to expect and how to measure it

Expect a reduction of a few degrees Celsius in peak air temperature, not a transformation, and only if ventilation stays open. Measure it yourself rather than trusting sales claims, because size, orientation and airflow decide the result. Heat builds because sunlight enters an enclosed space and cannot escape. The glass does not act like a magnifying lens, which is a common myth. That is why greenhouse cooling and temperature control always combines shade with airflow.

A simple logging method

1. Place a min/max thermometer or data logger at plant height, out of direct sun and away from the glass.
2. Put a second device outdoors in a ventilated, shaded spot, ideally on the north side of the greenhouse.
3. Lay a probe on the bench or soil surface to record ground-level temperature.
4. Log three to five sunny days before fitting the net, with vents and door in your normal position.
5. Fit the net, repeat the logging under similar weather and compare peak temperatures and the inside-outside difference.

Temperature thresholds that matter

Growers commonly treat roughly 25–28°C as the comfortable ceiling for tomatoes and cucumbers, and 30°C as the point to act. Sustained heat above that can reduce pollen viability and fruit set, and leaf scorch becomes likely when hot sun hits tender or wet foliage. Peppers and chillies tolerate more. Remember that surface temperature on benches and soil can sit well above air temperature, so a probe at leaf and root level tells you more than a wall thermometer.

Indicative planning ranges for ventilated greenhouses, not measured guarantees. Log your own results.

A worked example

Take a 3.0 m × 2.4 m aluminium-framed greenhouse in eastern England on a 35°C day. The following figures are illustrative, not laboratory results. Unshaded, the peak inside might reach the mid-40s°C with a bench surface hotter still. After fitting an external 50% net with the roof vent open, a peak in the high 30s is plausible, and bench-level temperatures would fall by a similar or larger margin. The lesson is that shade narrows the gap between inside and outside, while ventilation closes it.

Royal Horticultural Society guidance (paraphrased): ventilation is the first defence against greenhouse overheating, with roof vents equal to roughly one-fifth of the floor area, and shading should support rather than replace airflow.

How to calculate the size and quantity of net you need

Multiply the roof width across the slope by the length, add 10–15% for overhang and fixings, then round up to the nearest standard roll width. Ordering from floor area alone is the commonest sizing mistake, and it leaves you with a net that is too short to reach the eaves.

Measuring roof and wall areas

For a standard apex greenhouse, work out the rafter length first: rafter = √[(width ÷ 2)² + (ridge height − eaves height)²]. Roof area is then 2 × rafter × length. A wall area is simply height × length. Multiply the total by 1.10–1.15 to allow for overhang, hems and clip positions. For a reflective aluminium shade screen draped over the roof, the same formula applies, but leave extra at the eaves for tensioning.

Worked example for a 3.0 m × 2.4 m greenhouse

Suppose eaves height is 1.6 m and ridge height is 2.2 m. The table below shows each step.

Buying the roof piece plus the south wall gives roughly 15.3 m² of net. As a budgeting assumption only, at £2.50 per m² that is about £38, before clips and ties. Replace the assumed price with current listings, and always keep vents and the door free of net.

Standard UK widths, lengths and what to check

A shade net for greenhouse UK use is commonly sold in widths of 1–4 m and lengths from 1 m to 50 m, either cut to length or in full rolls. Greenhouse-model kits arrive pre-cut with clips for aluminium bars. Check that the clip profile matches your frame, because mismatched fixings are a frequent complaint. Compare offers by cost per square metre including fixings, and by whether edges are bound, since bound edges resist fraying. Some manufacturers also sew panels together for extra-wide spans, which suits large polytunnels. Always request current specifications rather than relying on old listings.

Fixing methods and wind protection

External fixing cools best because heat is stopped before it enters, while internal fixing is easier to install and safer in wind. Whichever you choose, avoid letting the net lie flat against the glass. Contact traps heat and can reduce the benefit.

External cover: weights, clips and ties

An external greenhouse sun protection layer is draped over the roof and secured with clips, bungee ties or weighted battens along the eaves. Its strengths are the best cooling and an air gap between net and glass. The downsides are exposure to wind, faster wear and the need to climb a ladder. Many people find external netting suits summer-only use, and it works well with a reflective aluminium shade screen on a south-facing roof.

Internal fixing: clips, hooks and tension lines

Internal systems hang from bar clips, screw hooks or tensioned wires inside the glazing. They are sheltered from wind, simple to adjust and gentle on the net. However, heat has already passed through the glass, so cooling is typically weaker than with external cover. Internal greenhouse blinds and shading are also easier to open on dull days, which helps with the light balance discussed earlier.

Step-by-step fitting

1. Measure the roof, then cut the net with a 10–15% allowance, using sharp scissors on knitted shade netting.
2. Check your glazing bars and match the clips or hooks to their profile.
3. Fix one long edge along the ridge or top bar first.
4. Draw the net taut over the roof and secure the opposite edge, with fixings about 50 cm apart.
5. Keep roof vents, louvres and the door clear of mesh.
6. Add ties or weights at the eaves to stop flapping.
7. Re-check tension after the first strong wind.

Wind and storm precautions

A loose net in a gale acts like a sail on a boat, hauling at every fixing point. Lower shade percentages let more wind pass through, and bound edges resist tearing. When the Met Office issues a strong wind warning, take an external net down or add extra ties. Inside the house, a net hanging loose can still foul vents, so check it before the weather turns. Gusts are the leading cause of damage to otherwise sound nets, and no fixing system is immune.

Shade net versus paint, blinds and screens: cost and ROI

A greenhouse shade net usually offers the lowest cost per season for hobby greenhouses. Shade paint is cheapest upfront but least flexible, and blinds cost most while giving the best day-to-day control. The 2026 trend is toward external reflective nets and adjustable blinds instead of one-off white paint, as heat spells become more routine.

Comparison of shading options

The ROI logic is simple: divide purchase price by seasons of use. A £38 net kept for three summers works out around £12.70 a season on the earlier assumed price, before counting your time. Paint is cheap per tub, but you repeat the cost and the scrubbing every year.

When should I put up and take down greenhouse shade net?

Fit it in late April or May, once midday temperatures regularly reach the mid-20s, and remove it from September to early October as light levels fall. Seedlings may need shade earlier on bright spring days. Dry the net, fold it loosely and store it away from sunlight.

Should I leave shade net on in winter?

Generally no. UK winter light is scarce, and a permanent net cuts it further. Leave it up only if it protects a specific crop, and then use the lightest grade. Taking it down also extends its lifespan.

Does shade net reduce yield?

Over-shading can reduce photosynthesis and yield, while correct shading can prevent heat stress and scorch that harm yield more. Results depend on crop, season and shade percentage, so no net can promise a yield gain. Aim for the lowest percentage that keeps the house in the comfortable range, and be willing to remove the net on cool, overcast spells. Netting also does not replace ventilation, because trapped hot air will still stress plants.

Bottom line for UK growers

Choose the shade percentage by crop, not by the thickest-looking roll. Check GSM and UV treatment for durability, calculate roof area before ordering, and keep the vents open. A well-matched greenhouse shading plan, built around a properly sized greenhouse shade net, is more reliable than any single spec on the label. For a historical and structural overview of the greenhouse structure itself, see Britannica.

In short, the best greenhouse shade net is the one whose shade percentage, UV treatment and size match your crops, your roof and your local summer.