Is Passive Practical?
One of the significant concerns for many preppers and survivalists, is moderate- to long-term loss of municipal services, such as electricity. Many people invest in generators for that reason, but in highly destructive disasters like Hurricanes Sandy, Maria, and Katrina, or the Mississippi floods in 2011, destruction of basic infrastructure like roads and bridges may make it impossible to obtain or replenish fuel supplies to operate those generators. It’s a smart move to immunize yourself and your family from these kinds of dependencies.
On the other hand, you’ll likely hear a lot of discussion about greenhouses and how expensive they are to heat. You may hear that an unheated greenhouse isn’t good for much beyond starting seeds, hardening off young plants, or overwintering a few perennials until they’re ready to come out again as the weather warms up. With all those restrictions, you may begin to wonder how a greenhouse fits into a survivalist lifestyle.
Fortunately, there are some excellent strategies to keep your greenhouse thriving on entirely passive principles. Even better, they are low-tech, require little-to-no maintenance and nothing more than some muscle power to operate. Heating is a major concern for most greenhouse gardeners, so let’s look at some options.
Geothermal
Passive geothermal heat systems operate on the principle that the earth itself produces heat, which radiates outward, warming the soil, the oceans, etc. Temperature extremes, such as sub-freezing winters, can override the soil’s heat, but only for a few inches or feet in most cases. This means that if you dig several feet (generally 6’ - 8’) below ground, you’ll reach a depth where the temperature of the soil remains relatively constant year-round, regardless of the ambient air temperature above. This is the operating principle behind passive geothermal heat.
In a typical passive geothermal system, an array of simple plastic tubes is connected and buried at a depth of approximately 8 feet, or whatever is appropriate for that geographical region (at least a few feet below the frost line), where the soil temperature remains at about 55°F to 65°F all the time. Air in the underground tubes will naturally equalize with the temperature of the soil through the thermal flywheel effect. When the weather is cold, air warmed by passing through the underground pipes will keep the internal temperature of the greenhouse well above freezing.
Geothermal heat is most effective in a well-insulated greenhouse and is sufficient on its own in most regions of the US to maintain cold-weather crops. You’ll need to plan for how the underground pipes connect to your greenhouse and design a system where the air is pulled naturally through the pipes for passive circulation throughout the greenhouse.
Thermal Mass
Using thermal mass (thermal batteries, natural batteries) to keep your greenhouse warm during cold winter nights is the lowest cost, lowest effort method available, and it should probably be employed no matter what other warming strategies you use. Thermal mass refers to the ability of a material to absorb, store, and release heat over time. Essentially, thermal mass produces a kind of inertia against temperature change.
When thermal mass is used for supplemental heating, the mass (often a set of large water barrels) should be located where it is fully exposed to the sun. During daytime hours, the sun will gradually heat up the material (think of hot pavement during the summer) and then in turn, will gradually release it as the air temperatures cool overnight. The inertial effect of the stored heat essentially flattens the temperature extremes inside the greenhouse.
Perhaps surprisingly, thermal mass can also be used for cooling. In this case the mass should be placed where it’s protected from direct sunlight but exposed to the air in the greenhouse. During daylight hours as the temperature rises, the thermal mass absorbs warmth from within the greenhouse and stores it for release in the cooler nighttime temperatures.
Composting
As a survivalist, composting should already be a way of life, as a safe way to handle organic waste and a renewable source of soil nutrients for crops. It’s well known that compost heaps can produce intense heat as biological processes break down organic materials. This heat production is 100% natural and inevitable, so it only makes sense to enlist it in keeping our greenhouses warm.
Your composting system can be as simple as setting up small compost bins every few feet in your greenhouse and allowing natural air circulation to keep the heated air well mixed. Another option is to employ trench composting, where your planting rows alternate with trenches partially filled with organic matter. The composting process not only keeps your plants warm but improves the soil so that the following year, you can alternate rows for planting or simply mix the soil well before seeding and digging new trenches. A less formal combination of both these methods is to dig holes about 1’ deep throughout your greenhouse and fill them with composting scraps. This is a great strategy for supplying distributed heat with waste organics.
Maximizing Passive Principles
Greenhouses are fundamentally powered by passive solar energy. Your plants need sunlight, so greenhouses are always oriented for maximum exposure to the sun. The transparent glazing, or greenhouse cover material allows sunlight to penetrate the interior of the greenhouse as thermal energy, where it is collected by the greenhouse walls. Your job as a greenhouse operator is to collect and use that energy effectively and efficiently.
- No home is going to stay warm if there are holes in the walls, open windows, or an absence of insulation. Greenhouses also need to be well insulated, especially in colder climates where nighttime temperatures dip into single digits. All that hard-won heat should stay inside, where it’s needed. Keep this goal in mind as you design your greenhouse and select materials.
- Framework and covers should be securely fitted, and any vents should be well sealed when they’re closed.
- Any damage to the greenhouse cover should be promptly repaired. Tears or holes will release a lot of heat on cold winter nights, and they’re almost certain to grow larger over time.
- Choose a double layer greenhouse cover. The fundamental idea behind a double layer cover is the layer of “dead air” between the layers. This is like the concept of double paned windows, where the air layer itself acts as an insulator.
- Choose a greenhouse cover with high natural light transmission. Light that can’t make it into the greenhouse is wasted energy.
- In very extreme temperatures, even a well-insulated greenhouse may not be able to maintain temperatures in the 45°F to 55°F range, considering all those huge clear panels exposed to the cold, especially if winds are a factor. In these cases, it’s a good idea to have some extra blankets on hand to provide that extra level of insulation to your greenhouse. You can purchase greenhouse blankets or quilts online to drape over the roof and sides of your greenhouse; you can wedge temporary insulating panels between the ribs of the frame, you can attach hooks to the peak of your greenhouse roof and make panels from rolls of reflective insulation (radiant barrier) that drape to the ground. Make sure the reflective side faces the interior of your greenhouse!
