Is double glazing good for a greenhouse?

Double glazing is generally not suitable for a standard greenhouse because it traps too much heat and significantly reduces light transmission. The Royal Horticultural Society advises that most greenhouses need single glazing or specialist horticultural glass to maintain the correct temperature and light levels for plant growth (RHS, 2026). For a typical domestic greenhouse, standard double glazing will cost around £1,200–£2,500 for a 6x8ft structure, but it risks overheating plants in summer and blocking essential sunlight.

The key variable is the greenhouse type and its purpose. A standard hobby greenhouse used for growing vegetables, seedlings or flowers needs high light transmittance (above 85%) and moderate temperature control. Double glazing typically offers 70-80% light transmission, compared to 90%+ for single horticultural glass. However, for a conservatory-style structure used as a sunroom or for overwintering tender plants, double glazing can be beneficial if it uses low-iron glass to maximise light and includes adequate ventilation.

Double glazing reduces light by up to 20 percent

Ordinary double glazing blocks roughly 10–20% more light than single glazing, which can stunt plant growth. The Energy Saving Trust notes that standard double-glazed units have a light transmittance of around 70–80% (Energy Saving Trust, 2026). For comparison, horticultural single glass transmits over 90% of visible light. Plants rely on the full spectrum of sunlight for photosynthesis, so reduced light can lead to leggy growth, lower yields, and poor flowering. If you must use double glazing, choose specialist low-iron glass units, which can push light transmittance to 85%, but these cost 30–50% more than standard units.

Temperature control becomes a serious problem

Double glazing’s insulation can cause greenhouse temperatures to soar above 40°C on sunny days, even in spring. The RHS warns that temperatures above 35°C can damage many common greenhouse crops, such as tomatoes and cucumbers (RHS, 2026). Single glazing allows excess heat to escape more readily, while double glazing traps it. Without automated vents or shading, plants will wilt or die. If you insulate a greenhouse with double glazing, you must install at least 25% of the roof area as opening vents and use 50% shading netting from April to September.

Specialist horticultural glass is a better alternative

For most gardeners, single horticultural glass or twin-wall polycarbonate is more effective than double glazing. Horticultural glass costs around £30–£50 per square metre installed, versus £60–£100 for double glazing. Twin-wall polycarbonate, at £40–£70 per square metre, provides some insulation without blocking as much light, and it is shatterproof. The Building Regulations do not apply to greenhouses used solely for domestic horticulture (GOV.UK, 2026), so you are free to choose. For overwintering tender plants, consider using horticultural fleece or bubble wrap insulation on single glazing instead of installing double glazing.

A worked example

A UK homeowner with a 6x8ft hobby greenhouse in a 1930s semi-detached garden would spend roughly £1,800 on standard double glazing, but the upgrade is not recommended for growing tomatoes or seedlings. The Royal Horticultural Society states that most greenhouses need single glazing or specialist horticultural glass to maintain correct light levels for plant growth. For this scenario, the double glazing reduces light transmission to around 75%, which can stunt crops like cucumbers or peppers. The Energy Saving Trust notes that greenhouses require over 85% light transmission for healthy plant development. A better investment is single horticultural glass at £800–£1,200, with no overheating risk and 90% light levels. The double glazing option would waste about £1,000 on unnecessary insulation, and the homeowner would see no payback through plant yield improvements. For a conservatory-style structure used as a sunroom, low-iron double glazing at £2,200 might work, but the standard hobby greenhouse should avoid it entirely.

What homeowners often get wrong

The most common mistake is assuming that more insulation always benefits a greenhouse, when in fact it can kill plants by blocking light and trapping heat. Here are the three main errors homeowners make

1. Thinking double glazing saves energy in all structures A greenhouse needs high light transmission above 85%, and double glazing typically offers only 70–80%. This reduces photosynthesis, stunting growth and wasting the £1,200–£2,500 investment on a 6x8ft greenhouse.
2. Ignoring ventilation requirements for double glazing Standard double glazing can raise internal temperatures by 5–10°C on sunny days, scorching leaves and causing plant loss. Without automatic vents or shading, the greenhouse becomes unusable for three months of summer.
3. Believing all double glazing is the same for greenhouses Low-iron double glazing costs 30% more than standard units but still blocks 10–15% light versus single glass. For a hobby greenhouse, this means £300–£500 extra spent with no plant benefit, and the RHS advises against it for most growing applications.

Quick reference

• Standard double glazing reduces light transmission to 70–80%, below the 85% minimum needed for most greenhouse plants.
• Single horticultural glass offers over 90% light transmission and costs £800–£1,200 for a 6x8ft greenhouse, roughly half the price of double glazing.
• The RHS advises that double glazing is only suitable for conservatory-style structures used as sunrooms, not for growing vegetables or seedlings.
• Overheating in a double-glazed greenhouse can raise internal temperatures by 5–10°C above outside levels, damaging plants in summer.
• Low-iron double glazing still blocks 10–15% more light than single glass and costs 30% more than standard double glazing. That makes it a poor choice for most hobby greenhouses.