Before diving into the specifications of the materials, it is helpful to know how to evaluate them. The best choice for your greenhouse depends on your climate and growing goals. Do you want to grow year round despite freezing winter temperatures? A multi-layer insulating material that can be well-sealed will greatly benefit the greenhouse. Or, do you live in a mild climate and only want to grow cold-hardy crops through the winter, or not grow in the winter at all? A single layer rigid plastic or polyethylene film may be the best choice. To evaluate a material, consider the following factors:
- Cost – Consider both upfront cost and lifetime cost, based on how often it is replaced
- Durability / Longevity – A number of factors play into how long the material will last. Consider the wind and snow loads at your site when weighing this factor. Also consider whether your greenhouse requires a building permit, in which case the structural loads of the material will have to meet building standards.
- Warranty – This varies by manufacturer and product; some materials come with a warranty against accidental breakage and hail damage.
- Light Transmission – How much light the material transmits is a very important factor for the performance in a year round greenhouse. But, it can be difficult to ascertain the ‘right’ amount of light transmission. It greatly depends on your climate. For most year round greenhouse growers with low light in the winter, the higher the light transmittance, the better. Greenhouses in sunny climates will likely benefit from lower light transmission, or some shading. Importantly, higher light transmission is inversely correlated with insulation. Materials that transmit more light are thinner, and usually have lower insulation ratings. This makes it a tricky balance for most growers.
- Insulation – For most climates with cold winters, higher insulation ratings are better. Additionally, greenhouses in very hot climates will benefit from more insulating glazing materials as they will block heat from entering, and keep the greenhouse cooler. Insulation is rated in R-value (higher numbers are more insulating). Sometimes, the inverse rating – the U-value — is also given.
- Transparent v translucent – Transparent materials are those you can clearly see through, like a window. Most double layer plastic glazings are translucent: they diffuse light and can’t be seen through. Translucent materials are usually better for growing; however clear view windows provide a nicer view.
- Availability – Many materials must be specialty ordered from distributors. Single-layer polycarbonate or fiberglass may be available from local hardware stores in your area. Shipping costs are another factor when choosing a material.