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6 Natural Ways To Make An Unheated Greenhouse Warm

6 Natural Ways To Make An Unheated Greenhouse Warm

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For most people, gardening is limited to the summer months, when the weather is warm. However, commercial growers don’t always depend on the weather being cooperative. While most farmers are limited by the weather as well, there are some who manage to grow their produce most of the year, if not all of the year.

Having extra growing seasons or even growing year-round has some distinct advantages. More than anything, it can ensure a year-round food supply, rather than having to depend on the food that one has canned in harvest time. It also evens out the workload, rather than working a huge garden in the summertime.

The key to this is a greenhouse. Greenhouses were invented by the ancient Roman Empire for the purpose of growing vegetables.

Although professionally built greenhouses are very expensive, made with aluminum framework and glass windows, you can build a homemade greenhouse quite cheap. A quick search online shows countless examples of homemade greenhouses, mostly made out of PVC pipe or 1 inch x 4 inch dimensional construction lumber and visqueen plastic sheeting. Of the two, building out of PVC is the easiest, although PVC pipe will become brittle after a few years if you have a lot of sunlight.

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The key to using your greenhouse year-round is to maximize the heat produced inside it. Your goal has to be to keep the temperature inside the greenhouse above freezing. If you can keep it even warmer, that will be better for the plants. There are a number of things you can do to help accomplish this; not all of them need to be done, and you’ll need to pick a combination that works for your situation.

1. Use a double layer of plastic for the “windows”

Insulation helps, but most insulation blocks the light. So, instead of insulating the south and west sides of the greenhouse, use a double layer of plastic for the windows. That will double the R-value of the greenhouse. It may not seem like much, but it will help tremendously.

2. Use compost

The natural breakdown of organic material to make compost generates a lot of heat. Specifically, it is the bacteria that is breaking down the material which generate that heat. So, topping your garden beds with fresh compost before the cold weather hits will help to keep your plants, especially the critical roots, warm. Make sure that you add a good layer, two to three inches thick, as the bacteria like a warm environment. The thicker layer helps the bacteria create that warm environment.

3. Use black wood mulch for the walkways

6 Natural Ways To Make An Unheated Greenhouse Warm

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The space between your planting beds is prime real-estate for absorbing sunlight and converting it to heat. This can either be left as bare dirt or covered with black wood mulch. Whatever you do, make sure that it is black or at least dark colored, as dark colors absorb more sunlight. Black-colored cement would be even better, although it would add a lot of cost to your greenhouse.

4. Add heat-absorbing barrels

One of the best things you can do is to place black plastic barrels, filled with water, inside the greenhouse, where the sun can strike them. The sunlight entering the greenhouse will be absorbed by the black plastic and converted to heat, warming the water inside. That water will act as a thermal mass, holding the heat like a battery, until it is needed. Then, usually after the sun goes down, that heat can be radiated into the air.

You must be careful about the placement of these barrels, so that they do not block the sunlight from reaching any of your plants. Remember that the sun will be lower down on the horizon, so sunlight will be blocked easier. The best place to locate these barrels is along the north wall of your greenhouse. For that matter, you can make the north wall out of them and then cover them up with white fabric in the summertime, so that they don’t create extra heat in your greenhouse.

5. Insulate the north side

6 Natural Ways To Make An Unheated Greenhouse Warm

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The north side of your greenhouse isn’t going to generate any heat for you. That’s because we live in the northern hemisphere and the sun is always south of us. So, there’s really nothing to be gained by having the north wall be clear plastic, like the other walls and roof. You’re better off insulating it with Styrofoam sheets, helping to hold in the heat and blocking any wind.

6. Build your greenhouse partially underground

Probably the hardest, but one of the most effective strategies is to build the greenhouse partially underground. The deepest you’ll want to go is about 4 feet, with another four feet of roof sticking up above the ground. By building it underground, the earth around the greenhouse will act as an insulator from the cold outside air. The lowest that the ground temperature can reach is 32 degrees Fahrenheit, so if you have colder air in the winter, the ground will actually be warmer.

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This works extremely well if you can build your greenhouse into a south-facing hillside. The sun coming into the greenhouse can warm the earthen wall on the north side, just the same as it warms the ground. Between the two, it will produce more heat.

Add a heater, if you must

When all else fails and these ideas don’t keep your greenhouse warm enough, you might have to add a heater. This becomes more likely the farther north you go. A small space heater inside your greenhouse may be just enough to break the chill, especially at night. Don’t worry about producing too much carbon dioxide, as your plants will consume that, converting it to oxygen.

Finally, grow cold-weather plants. All plants are assigned a “growing zone” in which they grow best. These zones come from a map produced by the USDA and equate to the temperature encountered in those areas of the country. During the winter, pick out plants that grow best in the northern part of the country. This will be indicated by a lower growing zone on the seed packet.

Do you have a greenhouse? What tips would you add to the list? Share them in the section below:

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  1. Actually, ground temperatur may be below 32 degrees Fahrenheit. Depends upon air temperature, and if that is let’s say -50 degrees Celcius, the earth will be frozen up to 1-2 meters below surface, and ground temperature may be well below 32 F many inches below ground level. In extreme conditions you have to make it deep, so only the roof is above ground level. All the walls below ground level, and rather have a big surface roof area to get as much sunlight as possible. In extreme conditions you need a heating source, like a wood furnace, heating water which is then going in tubes around the perimeter. Heavy insulation around the submerged walls etc. You can also use a heat-pump to suck heat from the ground, the deeper you go with water pipes for this, the better.

    • Good point about the frost line. Also, Rome may be 41degN like Chicago, but Rome is warmed by the south winds from Africa and a shallow nearby sea. Average January low temp there is 40deg. You can grow lettuce outside. In Chicago, ave. lo is 20degF. Why do you think Caesar’s legions could get by wearing those leather mini-skirts all year?

      A good article over all, but success for an unheated green house depends on frequent sunny days. It won’t warm at all when it’s cloudy.

  2. I see two sensible “active” heat options, one with more labor-requirements than the other.

    1. Annualized Geo-Solar.
    Cheap, mostly hands-off. Just a fan, air piping to mass, thermostat, and some insulation. Go to, there’s a video about this.
    2. Rocket mass heater. More “hot” heat. Slightly more labor-intensive. See for gobs of info.

  3. I seen a hot bow made up of 4×sheet of ply wood and 4×8 sheet lexan …the core was 275 aluminum cans thats were drilledout on the bottoms and caulked together to make long rows …..they were painted black to absorb the heat……little magnet fan provides extra lift .as heat in cans rises. ….you tube video

  4. I saw that YouTube video too. If I can get enough cans together I want to try it this fall here in Maine

  5. Dear,
    I need your help. I have a huge old aluminum storage building and I’d like to convert 1/2 or 1/3 of the building into greenhouse. That means I will need to bring in more sunlight by doing a lot of renovation. My main concern is keeping it from freezing in winters. There is a pot belly wood stove but it won’t keep the building warm by itself. I read that double plastic sheets or bubble wrap will do wonder. I guess I’ll have to create a room in there and cover it with that and then put in windows on east, south and west sides. Would that work? Do I include that wood stove in the room? Or leave it outside in the other part of the building? Maybe don’t need wood stove if have the double plastic sheets or bubble wrap covered up? I need help. I hope you can help me. I’ll be looking forward to hearing from you.
    Thank you.

    • The heating requirement depends partly on where (what latitude) you live, Amy. It would also help to have an idea of the building dimensions and where you’re thinking of partitioning it off for the greenhouse. Along the S wall, I’m guessing. Consider insulating the north wall and roof with fiberglass batting. If the ceiling is very high you’ll have a lot of dead space to heat, but you’ll circulate that air with fans. If you don’t put the wood stove in the greenhouse space you’ll have to lead the heat in somehow, and no matter where you put it you’ll have to vent the smoke through a chimney.

      Double plastic sheeting slows convective and conductive heat transfer effectively only if the layers are kept apart with a low-pressure air pump. But as you’re starting with a solid-walled building and not with a hoop house (high tunnel), I wouldn’t use sheeting; I’d install polycarbonate panels if you can bear the expense. But if you live in a cold climate, don’t count on solar gain alone to keep your plants from freezing. Unless you use some of the thermal storage methods partially covered here, without supplementary heating your greenhouse temp will often descend to near the outside air temp overnight or in a prolonged period of overcast and cold weather.
      Hope this helps.

  6. Has anyone tried this in say Canada or Alaska? I’m at 53deg N in Alberta.. was thinking about digging 2 feet and doing most of the rest of these tricks. It gets really cold here… -35 celsius on occasion I assume everything would die without a heater?

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