A single layer of polyethylene film is a great solution to cover cold frames or starting seeds.
Polyethylene film is available in a wide variety of thicknesses to cover a variety of gardening needs.
This type of cover maintains optimal growing conditions better than glass and can help reduce the cost of heating. It can be used effectively in concert with glass if you insist in using glass (there are reasons glass is a good idea too!).
Using a polyethylene or greenhouse film cover is a cheaper alternative to glass as well.
While this is a viable option for greenhouse covers, polyethylene greenhouse film does have a limited lifespan. Directly related to the thickness of the film, one will have to replace it every three to five years still (but it's still cheap).
The benefits of low cost, heat retention and light transference out weighs the negatives for most nursery owners.
In addition to large nursery uses, polyethylene greenhouse film can also be a great option for the hobbyist. Many portable and smaller scaled greenhouses are composed of this type of coverage. If you are using this type of film, you will need to replace the film about every four years.
As you begin the task of trying to find replacement film, make sure that you are purchasing a polyethylene film intended for use in greenhouses.
Another great reason to upgrade and look for a better film is that technology is advancing incredibly fast and in 4 years they may have created a longer lasting or simply better concept.
Many less expensive versions are intended to be used as vapor barriers in construction and do not offer the necessary UV protection.
If your not using the right film you could actually damage your plants instead of protect them. If you use construction plastic, for example, too much sun could come in while blocking the nutrients (good stuff blocked bad stuff let in).
Do your homework when shopping around for this film, it's easy to find and a powerful ally in your greenhouse building.