Polyethylene VS Polycarbonate, which is the most cost effective?
It would seem that polycarbonate, a strong, rigid greenhouse plastic, would offer the best insulating factor for your greenhouse. It is the standard greenhouse covering, here in the United States. Science, however, can prove how thick polyethylene plastic engineered with enclosed bubbles of air insulates better than polycarbonate.
Here are some things to remember, just to keep things clear:
- Conduction is heating by contact.
- Convection is heat transfer by the movement of a liquid or gas.
- Hot air or hot liquid rises to the top of the space it is in.
Air conducts heat very poorly, however air transfers heat or moves heat through the space convection wise, very well. Air will hold heat in the space it occupies. If this is a large space, the hot air will rise and remain at the top of the space. If the air is in a small space, there is less of a distance between the hot and cold spaces, this will not allow the air to separate very well.
The lighter weight a material is, the better insulating properties it has. The denser the material is, the faster it transmits heat and the faster it loses heat, as well. Glass, being denser than polycarbonate, loses heat faster than any other greenhouse covering in comparison.
In Ianto Evans’ book “Rocket Mass Heaters: Super-efficient Wood stoves YOU Can Build” he states: “To delay heat moving by conduction, a good rule of thumb is to choose the most lightweight material available. When a greenhouse plastic is lightweight, it usually means there is air trapped in it. A down parka, sheep’s wool socks or a fiber-filled bed quilt all contribute to our comfort by trapping air, one of the most effective insulating possibilities known.”
Conduction always flows from hotter to colder.
Polycarbonate is a rigid, dense plastic. It quickly conducts heat from inside a greenhouse to the outside. Polyethylene is much more lightweight so it slows the heat transfer. When the polyethylene has air bubbles engineered into its design, the heat transfer is slowed down even more, making it thermally more resistant to temperature changes.
Polycarbonate may seem to be able to insulate better. However, due to the properties of heat, and the way it behaves when transferring though different materials, we can see that polycarbonate will insulate less and lose heat faster than polyethylene greenhouse film. Polyethylene, or PE, designed, as a greenhouse plastic constructed with air bubbles is an energy saver.
The best-known polyethylene greenhouse sheeting on the market is known as Solawrap ™ Polykeder ™ Polydess® LP Keder. The distinctive double-layered system encloses air burls or pockets of air, which act as light ray diffusers.
SolaWrap™ a new kid on the block here in the US, is a uniquely engineered greenhouse plastic allows 83% of sunlight to both pass through and scatter through this plastic’s polyethylene multiple layers and air burls. This thick durable greenhouse plastic allows sunlight energy to reach the lower parts of plants.
Diffused sunlight scatters light rays more evenly, which helps to eliminate burn spots and shadowing within the structure. This is actually better than direct sunlight and more effective than when it passes through traditional greenhouse films and coverings.
Solarwrap’s™ design resembles nature’s honeycomb structure which gives the greenhouse plastic film more impact-resistant durability. The Multi-layer air burl composition makes impacts less damaging to the whole sheet of material by localizing the impact. If the outer 8.6 mil wall is punctured, the hole is localized to the air burls, which were compromised, and only that piece will require patching.
SolaWrap™ is easily installed and remains extremely durable, lasting for many, many years. So far reports show a sheet of Solawrap Polydress ® LP Keder greenhouse plastic lasting 30 years in a 4-season climate.
SolaWrap ™ is currently manufactured in Germany, where their measurement of R-Value is called “Thermal Resistivity” and is different than US values.
However, the closest estimation of The Thermal Resistivity of Polydress ®LP Keder aka SolaWrap ™ North America, is 3 to 5 times greater than other products on the market. However the exact R-Value, is yet to be determined, as the scientists are currently performing tests to determine the true US R-Value of the product.
If you are planning to build a greenhouse kit to grow food year round, applying a greenhouse plastic that is durable, has 83% diffusion and a higher R- Value would be the most cost effective choice. Contact us if you have questions at 1-781-816-7811 or firstname.lastname@example.org.