Heating a greenhouse can be expensive over a long, cold winter, and not just at night. Winter sunshine doesn’t provide a huge amount of heat, and the cold air outside lowers greenhouse temperatures even more. Inside my greenhouses here in New England, summer temperatures outside can easily soar to over 100˚F, but during winter the typical temperature on a sunny day barely reaches 75˚F, and it’s far lower when the sky is overcast. That’s why it’s important to make sure you keep whatever greenhouse heat you have – both natural and artificial – inside the greenhouse where you want it to be. This can be done in several ways, including eliminating air leaks, installing insulation, and using heatsinks.
Be a Leak Freak
Simply filling cracks and replacing worn or broken window seals is a relatively inexpensive job that can have a surprisingly large payback when it comes to heating a greenhouse. All you need to do is check your greenhouse carefully for air leakage, which can produce quite a major heat loss. Some estimates are that up to 30% of heat is lost this way, and that significantly impacts heating costs. So look for doors and window frames that have warped or shrunk, and install a seal or caulking. And don’t forget leaks as small as keyholes or bolt holes for screens. They can be plugged up quite easily to prevent warm air from escaping.
During winter, some people install bubble-wrap on the inside of their greenhouse to help keep the heat inside. If you decide to do this, buy the widest bubble-wrap available, up to 4 feet wide. Then face the bubbled surface toward the glass and tape the seams. This effort will not only give you an extra layer of insulation; it will also help eliminate any air leaks you overlooked.
Other people choose to insulate the outside of a greenhouse. Some use plastic sheeting, such as heavy-duty polyethylene from a hardware store, which may be able to last two winters. For greater strength (and a somewhat higher cost), you can buy a roll of clear Shrinkwrap®. This product, which is often used to cover boats for the winter, usually comes in an opaque white, but a clear version is also available. After wrapping it around the object to be covered, you can apply heat to shrink the material and create a tight seal or you can leave the material unheated and use it several times. Unfortunately, if you heat it, you can use it for only one winter because it can be shrunk only once. A third option for exterior insulation is greenhouse film. This material has tiny threads running through it, and it’s treated to prevent drips from condensing on the inside. Usually available from commercial greenhouse suppliers, it can typically be reused for several seasons.
Although you may not think about it, greenhouse lighting is affected when you cover your greenhouse for the winter, even when using a transparent material. A single pane of glass allows up to 96% of light through, but its insulating ability (called its R-value) is only about 1. A double-pane window allows about 84-88% of light through, while its R-value rises to about 4. If you install bubble-wrap on the interior of your greenhouse, you cut the lighting by about 15-20%, but you increase the insulating capacity to as much as R-10. These effects of light are important to keep in mind because they may affect the plants you grow. Lower light levels suit some plants, such as orchids and Cape primroses, but other plants may need supplemental LED lighting when winter insulation is installed.
A heatsink is a device for absorbing heat, with the intention that the stored heat will be used later when needed. In a greenhouse, it is heat from daytime sunlight that gets absorbed and stored, with the goal of helping to heat the greenhouse at night when the temperature drops. Among the things that can serve as greenhouse heatsinks are water barrels and a Trombe wall.
Water barrels painted black are among the simplest of greenhouse heatsinks. The water inside the barrels absorbs solar energy during the day and becomes a source of warmth after the sun sets. Be aware, however, that water heats up quite slowly because of the way its molecules are connected. In fact, as water absorbs energy, its temperature rises significantly slower than that of most other common substances. So don’t expect the water in your barrels to get extremely warm from winter sunlight alone. Given the limited number of daylight hours during winter, the barreled water will probably heat up no more than 15˚s even on the sunniest days. For this reason, some people put fish-tank heaters in the barrels to boost the water’s daytime temperature and continue heating it at night and cloudy days. But just as water’s temperature rises slowly, it also drops slowly as heat energy is lost. This is what makes water an excellent heatsink. It means that a barrel of relatively warm water won’t go cold very quickly at night. It will remain relatively warm for a long period of time and keep emanating some warmth to the surrounding air.
A Trombe Wall
Another heatsink option is a Trombe wall, which is a dark-colored stone or concrete wall located where sunlight can heat it during daytime. In some applications, the wall is positioned within inches of glass panels on a building’s south side. Solar energy then heats the wall as well as the air in front of it, which is typically vented to a room that lies behind the wall. Such an arrangement is not appropriate for a greenhouse, of course, where it’s essential that sunlight coming through the glazing reaches plants. In a greenhouse, therefore, a Trombe wall is positioned on the interior’s north side, with insulation placed behind it. The sun then heats the wall during the day, and at night this surface radiates that heat back into the air inside the greenhouse.
Usually, a Trombe wall is painted black to absorb as much solar energy as possible, but even stone of a dark natural color will still soak up heat. I’ve often thought that a beautiful stone wall at the back of a greenhouse, with built-in pockets for plants, would be a very interesting feature. Add some water trickling down the wall and a small pond at the bottom, and you’d have an even more dramatic heatsink for a high-end greenhouse.
A Heat Reservoir
Another suggested method of keeping a building or greenhouse warm is to have a large pile of stones under the structure through which air, warmed in the greenhouse during the day, is blown. At night the fans blowing the air into the heat sink are reversed to bring warm air into the greenhouse. The biggest problem with this method is that the stone reservoir needs to be almost as large as the greenhouse to be effective and the fans large enough to move a lot of air through the stone. My thinking is, if the fans have to be that large, why not simply use an electrically heated fan and dispense with the stone altogether.