Greenhouse Heating and Insulation
Greenhouse Heating and Insulation
- Put up a fence or plant a hedge as a wind-break on exposed sites.
- Repair cracked panes of glass, and ill-fitting doors and vents, all of which lose heat; use draught-proofing strips if necessary.
- In winter, line the greenhouse with an insulating material to give a double glazing effect — this makes a tremendous difference to the amount of heat retained.
- Even if you want more general warmth, you do not have to heat the whole greenhouse. For example, use insulation material to screen off part of it for tender plants. Alternatively, cut off the roof space at night with a similar screen drawn over at the height of the eaves.
This site is intended to help you manage a greenhouse where artificial heating is not essential. Nevertheless, there are times when a little extra heat can be very helpful, for example, for raising plants earlier in the spring or for overwintering plants when weather conditions are extreme.
Wrap up your greenhouse for the winter
Bubble polythene is one of the best materials for greenhouse insulation. Buy the type that is designed for the purpose, not packing material as this deteriorates rapidly in the light.
- One disadvantage is that it will reduce the amount of light reaching the plants. To help counteract this effect, make sure that the greenhouse glass is clean.
- In a wooden greenhouse, secure the material with drawing or mapping pins. In an aluminium one, use special fittings which go into the channels in the framework, or use sticky pads.
- Make sure that the vents can be opened: it will be even more necessary to ventilate as the insulation will increase humidity. You may need to cut separate pieces of material and fix them independently to the vents. Cut a separate flap to hang down over sliding doors.
- In spring, the material filters the sunlight and acts as shading. However, it will do little to reduce the heat in the greenhouse in summer. It is better to replace it with a shading material or paint until the autumn This also gives you the chance to wash the greenhouse thoroughly.
Wiring up your greenhouse
Electricity can be particularly dangerous in the damp greenhouse atmosphere. To be safe, always follow these safety rules:
- Fuse the greenhouse circuit separately, and incorporate a residual current device (RCD).
- Use waterproof plugs and fittings.
- Have the wiring installed by a professional electrician.
Greenhouse Heating Equipment
- Hot benches or propagators which supply heat to the bottom of plant pots through electric -warming cables or a heated blanket use very little energy. They are a tremendous help in raising plants from seeds and cuttings, and if they are covered then you can also use them to help overwinter tender plants.
- The most common greenhouse heaters available are electric fans, electric tubes, bottled gas heaters, and paraffin heaters. A lean-to greenhouse on a house wall might have a radiator run from a central heating boiler.
- The problem with paraffin and gas heaters is that they produce water vapour, leading to high humidity and encouraging fungal diseases. Paraffin heaters may also produce small amounts of other gases which can damage some plants. To keep the humidity down, to let fumes out, and to let air in to the burners, you need to keep the greenhouse ventilated, and this wastes heat.
- Electric fan heaters, on the other hand, produce a dry heat. They circulate the warm air throughout the greenhouse and have accurate thermostats. Consequently, they only come on when absolutely necessary and are therefore less wasteful of energy than you might expect.
The heat given off by decomposing organic matter was used in the hot beds of Victorian kitchen gardens. The beds usually consisted of a layer of fresh strawy horse manure about 60cm deep, topped with about 20cm of soil, and were used, for example, to bring onand .
The main problem with having any kind of hot bed or compost heap to provide heat in a greenhouse is that the ammonia given off in the decomposition process can be harmful to plants. The Victorians had one house entirely for cucumbers, which could be ventilated well for the first couple of weeks after the beds were made, before sowing or planting. In a greenhouse with a mixture of plants, the problem can be reduced by using alternate layers of manure and soil, or by mixing leaves or garden debris with the manure to absorb the ammonia. Alternatively, you could try using materials with less available nitrogen, such as a mixture of straw and kitchen waste.
In the USA, experiments to make use of the heat from compost heaps, or animal houses next to the greenhouse, have ducted the hot air through pipes below the greenhouse borders and up through leafmould or soil to absorb harmful gases.
Read more on greenhouse ventilation, shading and watering systems