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What is the cheapest way to heat a greenhouse?

Following on from how to keep the frost off your greenhouse, I thought I would look at the cheapest way to heat your greenhouse, and yes there are actually free methods!

Greenhouses are an essential bit of kit for anyone who is serious about gardening. Whether you have a large, glass greenhouse, a smaller plastic one, or just use a cold frame, you’ll know of the many benefits that come from using them.

If you are just planning on growing hardy veggies like Brussel sprouts and cabbages in your greenhouse through the winter, you shouldn’t need to worry about heating it too much, but if you want to expand your options, some kind of heat source, or simply knowing how to properly insulate your greenhouse, is essential.

In this article, we’ll take a look at your options and try to inform you on the cheapest way to heat a greenhouse this winter.

Best answer: How to heat your greenhouse with free energy this winter

Using an electric heater– the best way to heat your greenhouse

Without a doubt, the best and easiest way to heat your greenhouse is to use an electric heater of some sort, probably a specifically designed greenhouse heater or a space heater. There are quite a lot of options available to you when it comes to electric heaters, with the cheapest being the eco-tube type. These are very cheap to run, but they don’t really kick out a lot of heat, and so are only good for keeping things above freezing level.

On the other end of the scale, you have large fan-based heaters. The best, and usually most expensive, of these also have built in thermostats and allow you to completely, and precisely control the temperature inside your greenhouse. The main advantage of a fan based electric heater, is that the warm air is circulated all around the entirety of your greenhouse.

Unfortunately, these heaters are the most expensive to buy, and the most expensive to run too, especially if you’re turning it on all night, every night, in winter, and then there is the environment to think of on top of that. Still, I have to say that the best way to heat a greenhouse is with an electric fan heater with thermostat control.


How to heat your greenhouse with free energy this winter

Yes, you read that right. Heating your greenhouse for free is possible, well sort of anyway. Let me explain.

When I say free, I mean that you won’t have to have fuel costs or an expensive electricity bill, but you still might have to spend a few quid on some basic materials at the start.

What I’m talking about is making sure that your greenhouse is properly insulated so that you can make the most of the natural heat that you get from the sun during the day and keep it locked up inside during the night.

There are a few tricks that you can try, and I would suggest a combination of a few of them to get the absolute best results possible. Here they are.


Bubble wrap as greenhouse insulation

Use bubble wrap or foam to insulate your greenhouse. 
Bubble wrap is readily available and cheap, and it makes a great insulating material for greenhouses and is by far your biggest ally in terms of heat retention pound for pound. Try to get the bubble wrap with the big bubbles rather than the small ones as this is far more effective. The larger air gaps stop heat transfer better, be it in double glazed units, or bigger bubbles on your wrapping. It’ll take some doing, moving your plants around, but if you cut out neatly, this is something that you can put away carefully and use time and time again.

Make sure the places where the most heat is usually lost are very well wrapped up, this would be the ceiling and the north facing wall. If you’re up for it like the Quirky bid gardener you could line the whole greenhouse:

Greenhouse lined with bubble wrap to stop freezing thanks to the quirky bird gardener

Make use of thermal mass by installing a stone or concrete floor in the greenhouse, and by having black barrels of water sitting on the floor, or under the plants.

Materials such as stone and water are excellent at trapping the suns energy during the day and then releasing it slowly at night, so you should really be looking into this and taking advantage of it. Obviously this method only helps if you rise above freezing temperatures which to be fair in the UK is becoming more the normal you’ll agree, in fact, there’s not much stopping you grow in a greenhouse all year round now.

Moving your compost bin into your greenhouse is not only an excellent way of providing more warmth, it also helps to produce better compost, so it’s a win-win situation.

Of course, this will only work with a full-sized greenhouse as the compost pile needs to be quite big, but it’s worth keeping in mind.

Improve your greenhouse panels by changing out single layer plastic ones for double layered, or by backing up your glass panels with a layer of plastic on the underside.
 Consider though, double glazed units are expensive and an electric heater would probably serve better in the end.

Piling up soil against the outside of the north wall of your greenhouse can help to insulate it from the chilliest winds this winter. Not only that, but sub-surface soil is warmer than on the surface so you will get the benefit of that too.

Some people like to dig down a coupe of feet and set their greenhouses down there to take advantage of this same thing.

Doing so will greatly improve your greenhouse’s ability to retain heat at night and fight off frost.

Heating your greenhouse with a paraffin heater

An alternative to using an electric heater as your greenhouse’s main source of warmth, is to use a paraffin fuelled one instead.
This one is a good choice for the eco-friendly among you, and they are fairy cheap to run, but they do have their own issues.

For a start, there’s no way to circulate the warmth that comes from paraffin heaters, so you often have to place more than one of them around the greenhouse. Then there is the fact that they are only really good for keeping your greenhouse above freezing level and fighting off frost. One of the main problems with paraffin heaters is that they produce water vapour and if you don’t keep an eye on things, you could produce damp conditions and there are all sorts of problems that can come along with this.

Something to remember about heating a greenhouse

One thing that some people neglect when heating a greenhouse in winter is proper ventilation. They get so caught up in sealing up the greenhouse to trap heat that they forget about ventilation and this ends up leading to disease and damages their plants.

It is a fine balance to keep the warmth in while still having good ventilation, but it isn’t impossible. You can simply use any kind of fan to circulate the air, or have windows that you can open for a little while during the day, but close up at night.