A small winter greenhouse is a specialized structure designed to extend the growing season by prioritizing heat retention and insulation against cold temperatures. Unlike standard summer models, a winterized design must minimize heat loss while maximizing solar gain. This guide provides the steps for building a small, highly efficient structure to keep plants thriving through the coldest months.
Planning the Winter Greenhouse Structure
The design of a winter greenhouse prioritizes efficiency, starting with size and placement. Opting for a smaller footprint, such as a 6×8 or 8×10 foot structure, makes it easier to heat and maintain a stable interior temperature with minimal supplemental energy. The lower surface-to-volume ratio of a small greenhouse slows the rate of heat escape into the surrounding cold air.
Location is crucial for passive solar heating, requiring a site that receives maximum winter sunlight, ideally six or more hours daily. The structure should be oriented with its longest side facing south to capture the low-angle winter sun. For stability, the structure requires a solid, level foundation, which can be a simple gravel base with a perimeter of pressure-treated timber or a more permanent concrete footing.
Choosing Materials for Cold Climates
Material selection is crucial for resisting extreme cold, focusing primarily on insulation properties. For the frame, pressure-treated lumber is a cost-effective and structurally sound option that resists rot. The frame must be robust enough to handle potential heavy snow loads and high winds.
The most significant decision involves the glazing material, which dictates the structure’s ability to retain heat, measured by R-value. Single-pane glass has a low R-value (approximately 0.95) and loses heat quickly, making it a poor choice for cold climates. Twin-wall polycarbonate is the industry standard for cold-climate applications, with a 6mm panel providing an R-value of around 1.54. For greater efficiency in harsh winters, a 16mm five-wall polycarbonate can achieve an R-value of 3.03, though this material has slightly reduced light transmission.
Strategies for Heat Retention
Once the frame and glazing are installed, heat retention techniques maintain warmth. One effective passive method is incorporating thermal mass: materials that absorb solar energy during the day and slowly release heat at night. Large water barrels painted black or dark-colored rocks placed in direct sunlight act as heat sinks, stabilizing the internal temperature after sunset.
Insulating the north-facing wall is essential, as this side receives the least solar gain. This wall can be made opaque and insulated with solid foam board panels or reflective materials like Mylar, which bounces light and heat back onto the plants. Sealing air leaks is necessary because drafts quickly negate insulation benefits. Use silicone caulk on frame joints and weatherstripping on doors and vents to prevent warm air from escaping.
Supplemental heating can be employed when passive methods are insufficient to prevent freezing. Options include electric fan heaters controlled by a thermostat or propane heaters. A more natural heat source is an actively managed compost pile placed inside the greenhouse, which generates warmth as organic materials decompose.
Assembly and Internal Setup
The construction phase requires careful attention to glazing installation and sealing. Glazing panels must be securely fastened to the frame using appropriate hardware, ensuring a tight fit to prevent movement in strong winds or under snow load. All seams where the glazing meets the frame must be thoroughly sealed with weather-resistant caulk or specialized tape to minimize heat loss through convection. For multi-wall polycarbonate, seal the open channels on the top edge with aluminum tape to prevent water and debris from entering, which reduces light transmission and encourages algae growth.
The internal arrangement requires installing infrastructure that maximizes vertical growing space and manages the environment. Shelving and benches should be placed to keep plants elevated off the cold ground, ensuring better air circulation. Even in winter, ventilation is required to control humidity and prevent moisture buildup that encourages fungal diseases. Establish convenient access points for water and electricity, which are essential for supplemental heating, lighting, and irrigation systems.