Early in 1964 it became evident to me that a better system was needed for covering greenhouse frames with two layers of plastic film.
Wooden frame greenhouses utilized wooden spacers and wooden nailing strips to secure and separate the two films.
Metal frame houses used a layer of plastic on the outside and a system for hanging another layer of film on the inside.
The two layers of film were necessary to reduce the ever present condensation dripping and to take advantage of a 30% fuel savings.
To solve the recovering problem I decided to try to use two layers of film on the outside of the greenhouse and separate them by forcing air between them.
Having tried this on a model successfully we constructed a 7.3 x 14.6 meter pipe frame house.
The film layers were adequately separated, the outside layer was rigid and in fact immobile except in higher winds and seemed to alter its shape to conform to the particular wind situation.
Following the success with the experimental house several of the conventional wooden frame houses on campus were covered using this technique.
Two layers of film were applied over the frame and attached at the sills and the end rafters.
The nailers and the spacers at each rafter were eliminated.
This resulted in a ten fold reduction in the time required for recovering.
A small centrifugal blower was used to force air between the layers.
The pressure normally maintained is 5 millimeters of water.
It was observed that on a 4 meter rafter the space between the layers at the mid span location was about 50 centimeters.
It has been our experience that a rafter length of 5.5 meters is the maximum allowable to maintain a reasonable space between the layers and to keep the internal stress in the film within reason.
Curved roof members of steel, wood or aluminum solve this problem because the large air spaces do not develop with that configuration.
Most if not all of the commercial houses sold in the USA use the curved rafter section and the air inflation technique for double covering the greenhouses.
There have been various locking devices developed of aluminum, which allow for secure fastening and rapid uncovering and recovering.
Present U.S. films last between 20 to 24 months so recovering occurs on an every other year basis.
Large ridge and furrow complexes offer the advantage of having the plastic on a roll in a tube form.
Covering rates observed over the years average about 0.3 to 0.5 hectares per day for large ridge and furrow complexes.
This greenhouse covering technique along with improved copolymer and UV stabilized plastic film has been thoroughly tested in all types of weather.
Greenhouse frames have been tested as never before since the film covering does not fail and the frame is expected to carry the wind and snow loads now being imposed.
A greenhouse trial conducted in Japan where a single film and double covering, air inflation, system were being tested side by side revealed that the double system was far superior.