Should I use twin skins if building a polythene greenhouse?

I am considering building a single span poly house to grow speciality cucumbers. I have a friend who says twin skins are essential if I plan to heat the greenhouse, but others warn that the twin skin loses light. What do you recommend?


Twin skin greenhouse

It is important to find a balance between energy saving and maximum light distribution.

Twin skin greenhouses
The main reason for using twin layers of flexible plastic covering in a greenhouse is to reduce the heat loss through the greenhouse cover. Two sheets of plastic are installed plus a fan to pump air to inflate the space between the two sheets. This gives a layer of air, which acts as an insulating blanket. It performs better if thin. If too thick a layer of air, convection currents will ensue and this air movement will increase heat loss.

The fan needs to use outside air because the air inside the greenhouse usually has a higher absolute humidity than outside air because of plant transpiration. This means that the inside air has a higher dew point and consequently, fogging and condensation could occur within the enclosed envelope. Fan pressure should be low at between five and 15mm of water. Higher pressures can lead to the plastic stretching and weakening, and lower pressures lead to the skins flapping and potentially being damaged.

It is possible to use double skins on the roof of the greenhouse and/or walls. The best type of structure would also have roof vents, and depending upon the design, these would also be double skin, but may be single skin. The full job would be to twin skin everything, that is, the roof, the roof vents and all the sidewalls.

The downside to twin skins is the reduction of light coming into the greenhouse. The common rule of thumb is that 1% loss of light = 1% loss of yield. Say the plastic in your greenhouse has a high light transmission of 90%. In a twin layer this is reduced to 90% x 90% = 81% transmission. That is, the loss of an additional 9% light compared to single skin. The result is a significant loss in photosynthesis and hence, a 9% loss in yield.

Thermal screens

Against the number of advantages of twin skin covers, must be weighed the extra reduction in light.

Thermal screens
Another alternative to twin skins is to keep a single skin, but to add a movable thermal screen, which is closed at night. This has the advantage that the screen remains retracted during the day and hence, doesn’t reduce light transmission (other than the extra shading of a few per cent because of the space taken up by the retracted screen). Its other advantage is that the screen reduces the volume of greenhouse air that needs to be heated.

Note that there are a wide range of screen types, some of which can operate as both a shade and a thermal screen.

Single skin greenhouse

This single skin greenhouse features ridge venting.

Energy requirement single and twin skin
We need to calculate the difference in heat energy required for each of the three major options. These are:

  • Single skin polythene only
  • Single skin polythene + thermal screen
  • Twin skin polythene.

The annual heat loss from a greenhouse can be calculated from the climatic data for the location of the greenhouse.

The measure used is ‘degree hours’, that is, the total differences between the minimum required greenhouse temperature and all lower outside temperatures each hour for the whole year. These have been converted to give the energy required for the year for 14 climate zones within Australia. I have used graphs of this data published in Greenhouses edited by Dr Keith Garzoli, AGPS 1988. Available from Casper Publications.

As examples, I have taken the locations as Brisbane and Hobart (latitudes 270 S and 430 S, respectively). I have also taken the minimum greenhouse temperature to be 190 C.

Brisbane (warmer climate) annual heating requirement:

  • Single skin = 240 MJ/m2
  • Single skin + thermal screen = 150 MJ/m2
  • Twin skin = 150 MJ/m2.

Hobart (colder climate) annual heating requirement:

  • Single skin = 1250 MJ/m2
  • Single skin + thermal screen = 800 MJ/m2
  • Twin skin = 850 MJ/m2.

Financial evaluation
Notice that there is virtually no difference between the heat losses between twin skin and the single skin plus thermal screen.

A simple financial evaluation comes down to evaluating the additional costs associated with each of the different options. That is:

Single skin
The cost of the additional heat energy required.
Single screen + thermal screen:
The cost of installing and maintaining the thermal screen.

Twin skin
The extra cost of installing the second skin plus the associated 9% loss in yield.

Even if using single skins on the roof of the greenhouse, with or without thermal screens, the use of twin skins on sidewalls can be effective. This is because they reduce heat loss without significantly reducing the light input into the greenhouse. This especially applies to smaller greenhouses where the sidewall area is a significant proportion of the heat loss surface.

It is important to find a balance between energy saving and maximum light distribution. Ω

PH&G September 2016 / Issue 171