Winter greenhouses are a specialized form of greenhouse used to start seedlings in the late winter and early spring, and for growing food during the winter. Winter greenhouses are designed to capture as much light and heat from the sun during the daytime as possible, storing heat during the day, and radiating stored heat at nighttime.
Greenhouses are best oriented in an east-west direction, with the longer, glazed side facing south if you live in the Northern Hemisphere, or facing north if you live in the Southern Hemisphere. This maximizes the amount of sunlight entering the greenhouse during the shorter days of winter, thus maximizing heat gain.
The back wall on the surface of the greenhouse, or the wall opposite the main face of entry for sunlight, is best left unglazed, and can be made of a heat-absorbing material such as rocks, concrete, or mud. The back wall can also be insulated and conjoined with your house, a garden building, or a wall.
Maximum solar heat gain is achieved when the glazing is nearly perpendicular to the angle at which the sun’s light enters on the winter solstice, and when the greenhouse’s length-to-width ratio is at least two to one. A double-door vestibule at each end is ideal to prevent heat loss during entry and exit.
Heat storage can be utilized using black-colored 55-gallon drums filled with water and placed along the back wall. When the air temperature drops in the evening, the stored heat is then radiated back out into the greenhouse, thus helping keep the inside warmer for longer.
The smart money for winter greenhouse design also utilizes a geothermal heat pump coupled with a heat exchange system to reduce cold weather heating costs. This could utilize the same system as your house, or have its own dedicated system. Specially designed greenhouse heat exchangers are called air-to-soil heat exchange systems, earth-air tunnel systems, or ground tube heat exchangers. The cold interior greenhouse air is slowly pumped underground through large-diameter tubes where the air is warmed, and then circulated back into the greenhouse to impart its warmth.
Winter greenhouses can also be covered at night with insulating blankets, rigid foam sheets, or plastic sheets, to reduce heat loss and keep plants a few degrees warmer than outside. Renewable energy sources like electric heaters, biogas burners, or wood pellet biomass heaters can also be used to warm the greenhouse interior.
Another cold climate design option is an underground greenhouse, or walipini. This is dug six feet or more into the ground, where the average ambient earth temperature remains constant throughout the year, and is much warmer than the ground surface temperature. Underground greenhouses utilize a heat exchange with the earth on five of its surfaces, whereas with a conventional greenhouse heat exchange happens only at ground level.
During the day, the walls and floor of an underground greenhouse absorb heat, acting like a heat storage battery, with the aforementioned five surfaces providing radiant heat during the night to warm your food crop. You can also place an insulating cover over the roof at night to reduce heat loss.
These greenhouse design principles have been used by Siberian, Russian, and Canadian farmers and others who must endure cold winter and spring climates. Their methods are well proven for growing winter food.
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