Waterbutt in the Polytunnel – A final conclusion

As I’ve said more than once (here and other places) I’ve been intrigued by the question of whether putting waterbutts filled with water into the greenhouse/polytunnel can beneficially affect/improve the temperature in the polytunnel. To try and come to a conclusion, I built a Raspberry Pi temperature sensor with four sensors (see here). I put one outside the polytunnel, one inside the polytunnel, one in a small watering can of water and one in a waterbutt. For security I then copied the resulting file to a centralised Raspberry Pi over the network (like this) and finally copied the file to Dropbox so that I could read it elsewhere (like this).

All of this was done through a series cronjobs on the various machines.

These sensors have given me the ability to accurately measure all the temperatures and make some observations which go like this:

  1. The temperature in the polytunnel is most affected by sun. When the sun is shining on the tunnel, the temperature rises rapidly. If there’s no sun, the temperature in the polytunnel is the same or perhaps up to 0.5C warmer than the temperature outside the tunnel;
  2. The rate of water temperature rise is more affected by sunshine than the temperature in the polytunnel (this is a bit more difficult to prove as the main cause of rises in the polytunnel temperature is sunshine);
  3. The temperature of the water rises and falls more slowly than the temperature in the polytunnel. The water temperature changes proportionally to the difference between the temperature in the polytunnel and the temperature of the water.
  4. The temperature of the water becomes static at -0.3C, regardless of how much colder the polytunnel gets;
  5. The watering can (5L changes temperature more quickly than the waterbutt (200L).
  6. The temperature of the water is (in general) higher than the temperature in the polytunnel from 16:00 to 08:00 (when the sun is not shining)


  1. Having a waterbutt of water in the sun in the polytunnel must make a difference. The water will give out heat when the polytunnel is colder than the water;
  2. However, the effect is primarily driven by the number of hours of sun.

So, overall, the conclusion is (at least for the months of December, January & February, putting a waterbutt in the polytunnel has no appreciable of beneficial effect. The days are too short and there are to many sunless days.

This doesn’t mean that March and April won’t benefit as the number of sunny days increase and the nights get shorter. Let’s see.