How can I keep my greenhouse warm without electricity?

Keeping a greenhouse warm during cold weather can be challenging, especially if you don’t have access to electricity or other conventional heating sources. However, with some clever DIY solutions, it is possible to keep your greenhouse warm enough for plants to survive and continue growing during winter. In this article, we will explore various methods for passive solar heating, thermal mass heat storage, and other techniques to keep a greenhouse warm without electricity.

Understand the basics of passive solar greenhouse design

Passive solar greenhouses are designed to collect and retain heat from sunlight during the day, and release that warmth at night. Here are some key factors to consider when passively heating a greenhouse:

• Orientation – Position the greenhouse to receive maximum sun exposure, typically facing south or southwest in the Northern Hemisphere.
• Glazing material – Use materials like polycarbonate panels or plastic sheeting that allow sunlight to enter and heat up interior surfaces.
• Thermal mass – Incorporate materials like concrete, stone, or water containers that absorb heat during the day and radiate it back out at night.
• Insulation – Add insulation to reduce heat loss, especially by insulating north wall and foundation.
• Venting – Use roof vents with automatic openers to release excess heat and provide ventilation on sunny days.
• Sealing – Caulk and weatherstrip all cracks and gaps to minimize air leaks that allow heat to escape.

Following these basic principles of passive solar design will help maximize heat gain and retention in your greenhouse.

Take advantage of thermal mass

Thermal mass refers to materials that absorb and store heat energy. Incorporating thermal mass into a passive solar greenhouse design helps regulate internal temperatures. During the day, the mass absorbs solar energy and warms up. At night, the stored heat is gradually released back into the greenhouse. This evens out temperature swings to keep plants warmer overnight.

Some good options for thermal mass include:

• Concrete, brick or tile floors and walls – Absorb heat during the day, radiating it back overnight.
• Barrels or containers of water – Water has high heat capacity and can store large amounts of heat.
• Rock beds – Use beneath floor or line south-facing wall with rocks or stone.
• Phase change materials (PCMs) – Special paraffin-based products designed for heat storage.

For maximum effect, expose thermal mass materials directly to sunlight so they warm up efficiently during the day. The goal is to store enough heat to help maintain acceptable temperatures for 8-12 hours overnight in the greenhouse.

Insulate extensively

Insulation is key for reducing heat loss in a passive solar greenhouse. Recommended insulation steps include:

• Add R-10 to R-30 insulation under the foundation and perimeter of greenhouse.
• Insulate north wall and any non-solar exposed walls with rigid foam board or straw bale walls.
• Use double-poly films or double pane glass for glazing to reduce heat loss.
• Install an insulated shade or curtain to cover glazing at night.
• Seal all edges, cracks and gaps between panels or window frames with caulk.

Also consider adding insulation above the greenhouse, under the roofing. This will retain more heat in the air mass inside the structure. Proper insulation drastically cuts heat loss so the passive solar gain and thermal storage can keep the greenhouse significantly warmer.

Add south-facing windows or skylights

To maximize solar heat gain, add more glazed surfaces that face south or southwest. Ideas include:

• Attaching a new lean-to sunspace onto the south wall.
• Using solar window boxes or small cold frames along the south facade.
• Installing a passive solar skylight or roof window on the south slope of the roof.

Just be sure any added glazing has proper insulation or shading to prevent overheating on sunny midwinter days. More south-facing windows allow the low winter sunlight to penetrate deeper into the greenhouse, boosting heat absorption and retention, especially if thermal mass is situated appropriately.

Incorporate a solar collector on exterior

Attaching an enclosed solar collector outside the greenhouse provides additional solar heat gain. Hot air from the collector can be blown into the greenhouse via fans or ducts. Use empty aluminum cans painted black or layers of screened black mesh inside the collector box to absorb heat.

Face collector due south, tilted for optimal winter sun angles. Insulate well and seal up any edges. Install adjustable vents on the collector to regulate temperature – if it overheats, vent excess hot air outside. A correctly sized collector can further boost greenhouse warmth on sunny winter days.

Add supplemental solar heating inside

Besides passive solar gain through glazing, some additional solar components can be incorporated inside the greenhouse:

• Solar hydronic piping – Coils carry water heated during day to radiate warmth overnight.
• Solar air heaters – Small units with powered fans to channel warm air into greenhouse.
• Portable solar mats or lamps – Roll out mats or hang lamps over plants needing direct warmth.

While not completely electricity-free, using a small amount of power with these items during daylight may be feasible. Focus solar heat on the plants themselves at night rather than heating the entire greenhouse space.

Use heat storage tanks

For greenhouse spaces without much thermal mass, use tanks or drums of water to store solar heat. Place black painted tanks/barrels in sunny spots in the greenhouse during the day. At night, move them beside vulnerable plants to radiate warmth. This also allows heat to be directed only where needed, rather than heating the entire greenhouse.

Alternatively, tanks can store hot water from a simple solar collector. The water later circulates through pipes/tubes to deliver warmth when needed. This solar heat storage method lets you actively direct warmth to plants overnight.

Weatherstrip doors, vents and edges

Sealing up cracks, gaps and leaks around doors, vents, wall panels and edges is a simple way to reduce heat loss. Use adhesive weatherstripping, caulk and sealants to draft-proof your greenhouse. Prioritize edges that receive direct wind or cold. Keeping out cold drafts will help retain more warmth from solar gain and thermal storage methods.

Add heat-retaining shutters or shades

Install shutters, insulating shades or thermal curtains on glazing to help retain heat overnight. Close up the greenhouse at sunset to lock in warmth from the daytime. Some materials like bubble wrap, quilts/blankets, or rigid foam boards can be used as effective do-it-yourself shutters and shades. Make sure to open covers during the day to allow solar gain.

Use a small backup heater

For periods of prolonged overcast weather, have a backup heating source ready. Small propane or kerosene greenhouse heaters can boost temperatures just enough to protect plants. Keep a thermometer inside the greenhouse and only use backup heat as needed to maintain ideal plant growth conditions.

Grow cold hardy plants

Choose greenhouse plant varieties suitable for colder temperatures. Many herbs, greens, root crops, and cool weather vegetables will tolerate chillier nights and shorter winter days. Growing plants naturally acclimated to lower temperatures reduces the amount of heating the greenhouse needs.

Conclusion

With smart passive solar design, thermal mass, and proper insulation, it is possible to keep a greenhouse warmer throughout winter without electricity. Several supplemental heating techniques like small backup heaters, heat storage tanks, and heat-retaining shutters can also help maintain temperatures. Monitor conditions daily and use cold-hardy plants to make an electricity-free heated greenhouse feasible. The key is capturing solar gain during the day, storing that heat, and minimizing loss overnight. With some creativity and commitment to management, you can keep your greenhouse productive all winter long.