Unheated greenhouses warm up by capturing the sun’s rays behind glass or plastic. Whether greenhouses are warm in winter therefore depends on how much sunlight your greenhouse receives.
A greenhouse placed in a south-facing position, during a winter with plenty of blue skies and sunshine, will warm up pleasantly even in the winter months. I have recorded temperatures upwards of 30’C / 85‘F even in February in my south-facing greenhouse here in the UK.
The greater concern is what happens on cloudy, overcast or rainy days. When there is little or no sun around there is simply nothing to warm up your greenhouse. In these situations a greenhouse won’t be much warmer than the outside air temperature.
At night, of course, where there is no sun at all, unheated greenhouses most certainly are not warm in winter. This is the reason why some gardeners opt to add supplementary greenhouse heating, helping to ensure their greenhouse is warm all winter long no matter what the weather is doing outside.
Here are the most important factors that will affect whether your greenhouse is warm in winter:
Exposure to Sunshine
As mentioned, the number one factor affecting whether greenhouses are warm in winter is the sunlight they’re exposed to. As you might imagine, stronger sunshine, and longer periods in the sun, will warm up a greenhouse far more than the same building in a shady area.
If you’re planning to use your greenhouse in the winter months then giving it pride-of-place in your garden makes sense. Choose the sunniest position possible and place your greenhouse side-on so the sun hits the greatest surface area possible.
Strong winter winds can batter greenhouses, quickly wicking any warmth away. There’s a reason weather forecasters talk about “wind chill factors”. Protecting your greenhouse from these winds can help to keep your greenhouse warmer in winter.
As an added benefit, when done right such a windbreak can also help to warm up your greenhouse yet further. Fences and walls, for example, can warm up pleasantly even on winter’s days, reflecting some of this heat at your greenhouse.
Personally speaking my south-facing greenhouse is positioned roughly a metre in front of a south-facing wooden fence which gets noticeably warm. This, combined with a number of shrubs and bushes on the most-exposed side help to keep it warmer than it otherwise would be.
Level of Insulation
You might be surprised to hear that greenhouses can vary quite a lot in terms of how well they retain warmth. This is one reason why greenhouses can be more beneficial than polytunnels.
It’s also a reason why plastic greenhouses with polycarbonate panels can get warmer than traditional glass greenhouses.
If you’re concerned about keeping your greenhouse warm in winter then it is of course possible to add further insulation. Two common materials are bubble wrap and fleece.
These are attached to the inner surfaces of the greenhouse to reduce heat loss.
If you opt for this option then consider how you’ll attach the insulation to the sides of your greenhouse – insulation clips are the most common option but won’t fit every greenhouse on the market.
The better ventilated a greenhouse is – either intentionally or not – the quicker it will lose any warmth. If you leave your greenhouse doors open during the day, for example, it stands to reason that it will get cold.
While this sounds obvious, many older greenhouses have small gaps where warm air can leach out.
If you’re concerned about keeping your greenhouse warm in winter then take the time to identify and plug any tiny holes before the temperature drops too much.
Gently snuffing out a candle so it continues to smoke, then running this around the inside of your greenhouse can help to identify draughts.
Organic Matter Breakdown
There’s a reason why some snakes choose to lay their eggs in compost heaps – rotting organic matter produces a lot of warmth. Some smart greenhouse owners make use of this fact and load up their greenhouse in the fall with organic matter.
You can do the same – set up a compost bin in your greenhouse or dig out some of the beds and distribute your vegetable peelings and so on in the channel. As it rots down, it should also raise the greenhouse temperature.
Lastly some materials tend to “store” heat better than others. Stone, brick and paving slabs are great examples of “heat stores”. They warm up slowly in the sun, but also cool down equally slowly at the end of the day. Adding some of these materials to your greenhouse – such as a paved floor – can help to keep a greenhouse warmer in winter.
As we have seen, there is no easy answer to the question “are greenhouses warm in winter?” as it can depend on so many different factors. Even the definition of “warm” can vary from one person to another. Generally a greenhouse will be warmer than the outside temperature, though how hot a greenhouse gets will depend on the amount of sunshine you receive.
Of course, for those looking for a greenhouse that is guaranteed to be warm all year round – even in the depths of winter – it is possible to add greenhouse heating. While this may slightly increase the costs of maintaining your greenhouse it can also make for a pleasant and productive growing season in all seasons.