How To Keep A Polytunnel Warm
How warm is a polytunnel in winter? Our experience is that temperatures can drop to the same as outside (ie below freezing).
On this page we share how we were able to raise the temperature inside our polytunnel by 4oC.
How To Keep A Polytunnel Warm
There are two common approaches to keeping a polytunnel warm:
- Retaining daytime heat
- Creating a heat source inside a polytunnel
Or a combination of the above. Retaining day time heat is essentially providing insulation and draft proofing to keep warm air inside the polytunnel. Even in winter, the power of the sun through the plastic can raise daytime temperatures to 30oC or more.
There are many possible ways of adding a heat source to a polytunnel:
- Creating a hot bed of decomposing materials that naturally produces heat
- An electric heater or propane gas heater
- Using a large water butt filled with water to act like a storage heater
Our Approach To Keeping A Polytunnel Warm
The polytunnel on our allotment is several hundred metres from our house. For this reason, we did not want to use unobserved heaters or candles inside it. We had read that a hot bed or water butts can make a difference to night time temperatures, but we decided against both approaches on the basis of convenience.
Our approach was based on retaining more daytime heat during the night. The logic behind the decision was that a thermometer inside our polytunnel showed that daytime temperatures were warm – almost as high as in summer (on sunny days) . If we could prevent more of this heat escaping, this seemed the best and easiest solution to the problem.
See our video that describes the steps we took, or if you prefer, continue reading below.
How warm is a polytunnel in Winter?
We discovered that simply shutting the polytunnel door in winter was not enough to protect our autumn geranium cuttings. Our assumption was that there would be enough daytime heat to protect the plants at night. Also, that the absence of windchill would mean our plants would survive.
We were surprised to discover just how cold our polytunnel got. Put simply, our polytunnel temperatures were the same as outside temperatures. We do not know how long these minimum temperatures lasted for, or how quickly the temperatures recovered at daybreak. However, we know that the temperatures were too low for our plants.
Our first attempt at warming the polytunnel was draft proofing. At each end of our polytunnel are doors with a net panel. These are very helpful in the summer to aid air circulation and to cool the tunnel, but in winter simply let all the hot air out. Therefore we covered these with plastic.
Similarly, around the doors are large gaps between the doors and the door frames. We fitted draft excluders to help retain heat inside the tunnel.
Using A Growhouse Inside Our Polytunnel
We found that simply reducing drafts made little difference to the heat loss experienced by the polytunnel. The next attempt was to install a plastic growhouse. We decided to use our Horti Hood plastic greenhouse as this fitted comfortably inside the polytunnel.
Our thinking was that the plastic greenhouse would offer even better protection from drafts, and would have some sort of double glazing effect. In fact, the greenhouse did make a difference, but only by a degree of two. There was simply too much heat loss through the plastic.
Insulating The Growhouse
Encouraged that we were now heading in the right direction, we decided to invest in some proper gardener’s bubble wrap insulation. This insulation is UV protected to last longer than packaging bubble wrap, and has much larger bubbles. These bubbles allow more light (and therefore heat) to enter, and provide more insulation.
For all tests we compared the temperature difference between a thermometer in our garden, to the thermometer inside our polytunnel (and growhouse). With all the insulation in place, we had managed to increase the temperature inside our polytunnel by four degrees.
Nevertheless, we were still concerned about the minimum temperatures, as the thermometer showed these could still fall to below zero or particularly cold nights.
For one week we tested our set up with some daisy cuttings that had previously been nurtured on a warm sunny windowsill. These have all survived with no signs of damage. We believe this is most likely to be because the minimum temperatures experience within the polytunnel growhouse are only for short periods of time.