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Choosing the right glazing for a cold frame depends on whether you prioritise maximum light transmission, thermal insulation, or physical durability. Glass offers good light transmission, while twin-wall polycarbonate combines light weight with insulation and resistance to breakage. Acrylic is another clear plastic option, but it is more brittle than polycarbonate. Compare the exact sheet specification: thickness, wall structure and coatings affect performance.
Table of Contents
Light transmission and plant growth
The primary purpose of a cold frame is to create a protected environment that extends the growing season. Light levels are critical for seedlings, cuttings, and plants requiring high light intensity. The RHS gives around 90% light transmission for greenhouse glass, which makes it a useful reference when comparing glazing.
Plastic alternatives reduce the amount of light reaching the plants. The RHS quotes about 83% for twin-wall polycarbonate and 85% for twin-wall acrylic; these are examples, not ratings for every plastic sheet. While these differences may seem small, they can limit the effectiveness of the cold frame when raising seedlings or rooting cuttings that are sensitive to light levels. If your primary goal is propagation in the depths of winter or early spring, good light transmission is particularly valuable.
Thermal performance and heat retention
Cold frames are typically unheated, relying on solar gain to warm the soil and air. The way a material handles heat affects how well it protects plants from frost and how quickly it warms the interior in spring.
- Glass: Glass shelters plants and limits heat loss, but a single glazed lid still loses heat overnight. Do not assume it will keep tender plants frost-free.
- Polycarbonate: Twin-walled polycarbonate is specifically designed to hold in heat well. Its structure provides an insulating layer of air, which can be beneficial for overwintering tender plants.
- Acrylic: While providing some insulation, the insulation depends on whether the acrylic sheet is solid or multiwall.
Durability and physical resilience
The environment of a UK garden exposes cold frames to wind, hail, and accidental impact. The choice of glazing significantly affects the lifespan of the structure.
Polycarbonate is the most durable of the three. It is lightweight and highly resistant to breakage, making it an ideal choice for areas prone to adverse weather or for gardeners who want a low-maintenance, breakage-resistant lid. Acrylic is also lightweight but is notably more brittle than polycarbonate and has a tendency to crack during the installation process.
Glass is the most fragile option. It is susceptible to breaking under the impact of hail or if an object falls on the lid. To mitigate this risk, toughened glass should be used for any areas where there is a higher risk of impact. Additionally, lightweight frames must be pegged down to prevent gusts of wind from shifting the structure and causing the glass to crack.
Comparing glazing materials
| Feature | Glass | Polycarbonate | Acrylic |
|---|---|---|---|
| Light Transmission | Example: ~90% | Example twin-wall: ~83% | Example twin-wall: ~85% |
| Impact Resistance | Low (unless toughened) | High | Moderate (brittle) |
| Heat Retention | Limited with single glazing | High (insulates) | Moderate |
| Durability | Degrades slowly | Durable in weather | Can crack during fit |
Practical application: Hardening off seedlings
Cold frames are essential for ‘hardening off’—the process of acclimatising plants from a stable greenhouse or indoor environment to the harsher outdoors. This typically takes two to three weeks.
To illustrate the management of a cold frame during this process, assume a seedling is moved from a heated propagator to a cold frame in late spring. The goal is to gradually increase exposure to outdoor air and temperature fluctuations:
- Week 1: Adjust night protection to the forecast; bring frost-sensitive plants indoors if necessary. During the day, open the lid slightly to introduce fresh air.
- Week 2: Gradually increase the angle of the lid opening each day to increase ventilation and lower the internal humidity.
- Week 3: Raise the lid further, eventually removing it entirely before planting the seedlings into the garden.
The aim is gradual exposure to outdoor conditions, whatever the lid material. Increase ventilation only when weather and the plants’ response allow it.
Installation and maintenance considerations
The choice of glazing also affects the construction and long-term care of the cold frame. Glass requires precise fitting and secure glazing bars to prevent rattling and leaks. It must be cleaned regularly to maintain light transmission, as dirt and algae can quickly reduce efficiency.
Polycarbonate and acrylic sheets are easier to cut to size and install. However, gardeners should be aware that some plastic covers on mini-greenhouses or cold frames can be difficult to replace once they degrade or become clouded. When building a DIY cold frame, using rigid plastic sheets provides a balance of ease and longevity.
For those using cold frames in winter to shield alpines or overwinter annuals, additional insulation can be added. A layer of bubble plastic can be installed on the inside of any of these glazing types to provide extra draught-proofing and frost protection, though this will further reduce the light transmission and ventilation.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Toughened glass sheets: Recommended for cold frame lids to reduce the risk and consequences of breakage from hail or accidental impact. Ensure the thickness is appropriate for the frame depth and that it is specifically rated as toughened safety glass. Search Amazon for Toughened glass sheets.
- Twin-wall polycarbonate sheeting: A durable, heat-retaining alternative to glass for cold frames and mini-greenhouses. Check the wall thickness (e.g., 4mm or 6mm) to ensure it fits the glazing bars of the frame. Search Amazon for Twin-wall polycarbonate sheeting.
Related garden guides
- Small Greenhouses UK: Grow All Year Round – 2026 Guide
- Growing winter salad in a cold frame
- More greenhouse & growing guides
Sources and further guidance
- Greenhouses: Choosing the Right One | RHS Advice
- Coldframes and Mini-Greenhouses | RHS Advice
- Hardening off tender plants | RHS Advice
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.