Film vs Glass for Geothermal Greenhouses

There’s a common misconception that you’ll need either a completely buried or glass greenhouse to take advantage of geothermal heating, especially passive modes. This isn’t true anymore thanks to many different methods for retaining and maximizing heat. Furnaces and space heaters can be scaled up as necessary to keep up with dropping temperatures, even when it means installing new power lines or fuel tanks. But geothermal systems must be sized right from the start since it’s not that easy to pop in and drill a whole new well or run more buried loops. Poly film greenhouses are just as capable of making the most of geothermal heating as glass and offer a wide range of benefits over the other options.


Poly films, especially woven materials like ArmorClear, are more durable than glass despite their obvious thinness. These films stretch and bend with wind forces and falling debris. In contrast, the brittleness of glass makes it a liability in an open field. Hailstorms, falling tree limbs, and even just high winds can leave panels shattered and in immediate need of replacement. Waiting even a few hours for glass or panel delivery can result in a temperature drop that threatens fruit or flower loss on delicate plants.

Maintenance and Repairs

Poly film costs only a fraction of the investment required for glass panels, even when you need a full replacement to renovate an aging structure. Glass is expensive, tricky to patch and tends to continue losing heat. Glass needs cleaning inside and out seasonally in many climates to keep algae and dust from reducing light transmission. Poly films also need rinsing, but dust and dirt tends to accumulate less on the surface of plastic than glass. The films are also very strong and not easily damaged, reducing routine repair costs.

Infrared Coatings

Glass greenhouses typically feature infrared coatings as a standard feature, while it’s an optional feature on film covered structures. This kind of heat trapping coating is highly valuable for the geothermal greenhouse since it slows loss through radiation at night. In cold climates, it’s often easy to keep a greenhouse warm all day but nearly impossible to stabilize it at night. There’s no need to choose glass just to get access to IR coatings since they’re widely available for all types of poly film now. If you don’t want to replace existing film with a product that has IR reflection built in, you can simply add a layer of film or spray to your existing greenhouse cover.

Film Selection Tips

The term, poly film, is a large umbrella that covers many different types of flexible film. The term poly is short for polymer, so all these greenhouse films really have in common is being formed from a durable plastic. You can easily find polyvinyl chloride (PVC), low density and high-density polyethylene (LDPE and HDPE), vinylacetate ethylene copolymer (EVA), and products that aren’t labeled to the content. For maximum durability, stick with LDPE and HDPE. These materials offer the best UV resistance and stretch rather than tear, even after years of constant sun exposure.

In addition to choosing the right material for your film, you should also consider how it’s made. Most sheets are poured or stretched from raw materials, resulting in thin and smooth sheets that tear relatively easily. In order to increase the durability of your poly cover and reduce repairs, look for a product with a reinforced layer of material. Reinforced poly films feature a woven core of fibers that add strength and durability to the plastic without making it too thick or inflexible to use on curved structures. Finally, make sure you choose a product designed for multiple years of use. Many affordable greenhouse films are only designed for a single season or year and break down rapidly after that. If you’re going through the trouble of building a geothermal system to heat your structures, you don’t want to add the work of installing new covers every year or multiple times a year just to save money on film. Instead, choose a quality cover like BTL Liners ArmorClear, that will last for decades to come.

Double Walled vs Single Walled Systems

Both glass and film can be installed in a single layer or doubled with air trapped between them. Double-layered walls are obviously preferred for geothermal greenhouse designs thanks to the insulating effects of just an inch or so of air. Plastic film is far less expensive to install in this way than glass. Even a single layer of glass costs hundreds of dollars per covered square foot of greenhouse growing space, while poly film costs around one tenth of that. Since you have to essentially double your costs for cover by adding the second layer, most greenhouse owners stick with poly film to achieve the air gap. It’s true that double-paned glass doesn’t require a constant air source to keep the materials apart. However, the small amount of electricity needed for the fan doesn’t equal the significantly higher cost of the glass.

Don’t fall for the misconception that you need glass or polycarbonate panels to heat your greenhouse with a geothermal heat pump. If you install a reliable product like ArmorClear from BTL Liners in a double layer configuration, you’ll get all the insulation you need at a much more affordable price. Installation will also go much faster and require a less specialized team of installers to complete the job.

Covers by BTL


Using a two-color technology, ArmorClear is formulated for your greenhouse to maximize your plant growth.

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