The goal of insulating a greenhouse for the winter is to create a controlled environment that extends the growing season, protects sensitive plants from freezing temperatures, and reduces the energy costs associated with heating the structure. This process involves minimizing heat loss through various mechanisms like conduction, convection, and air infiltration. By strategically applying insulation to both transparent and opaque surfaces, gardeners can maintain warmer soil and air temperatures, even when outdoor conditions drop well below freezing. A measured approach to winterizing a greenhouse ensures that the efforts provide maximum thermal benefit without sacrificing the natural light plants need.
Identifying Key Areas of Heat Loss
A greenhouse loses heat through four primary mechanisms: conduction, convection, radiation, and air infiltration, with conduction being the largest factor on cold nights. Conduction occurs when the warm interior air transfers heat directly through the covering material to the colder exterior surface. Single-pane glass has a high heat transfer coefficient, meaning it provides minimal resistance to heat flow. Up to 60% of the total heat loss in a greenhouse can escape through the covering alone.
Infiltration, or air leakage, is also a significant concern, accounting for up to 20% of total heat loss as cold air is drawn in through unsealed gaps and cracks. This process is worsened by the “heat head” effect, where warm air escaping through high-level gaps, such as around the roof ridge or vents, sucks colder air in through lower gaps, including those near the foundation. The ground contact area, including the foundation, often acts as a thermal bridge, siphoning heat from the soil inside the structure out into the cold surrounding ground. Without proper insulation, the soil temperature can remain too cold for optimal plant root function.
Internal Glazing Insulation Methods
One of the most effective ways to reduce heat loss through the vast surface area of the glazing is to create an additional air layer on the interior. This is often achieved using a temporary lining of horticultural bubble wrap or polyethylene sheeting. Horticultural-grade bubble wrap is preferable over standard packing material because it is more durable, features larger air bubbles for better insulation, and is manufactured to withstand UV exposure. The air trapped within the bubbles acts as a buffer, similar to double-glazing, which slows the transfer of heat from the inside.
To install this temporary lining, panels of the material should be cut slightly oversized for each pane of glass or polycarbonate. The bubble wrap is then secured to the interior frame using specialized fixings like Alliplugs for aluminum frames, or staples and pins for wooden structures. It is important to create an airtight seal by taping the edges of the bubble wrap to the frame, ensuring that the air gap between the glazing and the wrap is maintained for optimal thermal performance. Insulating the roof with this material is particularly beneficial, as hot air naturally rises and will lose a significant amount of heat through the ceiling.
A more semi-permanent and durable solution involves installing interior panels of twin-wall polycarbonate or acrylic sheets to establish a permanent double-glazing effect. Twin-wall polycarbonate has a network of internal channels that create insulating air pockets, offering an R-value nearly double that of a 3-mm single glass pane. When installing these multi-wall panels, the open ends must be sealed with special tapes: a breathable tape on the bottom to allow drainage of condensation, and a solid tape on the top edge to prevent dust and insects from entering the channels.
These interior panels are typically joined using H-channels and capped with U-channels, which help maintain structural integrity and a clean, sealed edge. The sheets should be secured to the existing greenhouse frame with specialized fasteners that allow for the material’s thermal expansion and contraction. This creates a lasting thermal barrier without the annual task of hanging and removing a temporary plastic film.
Insulating Foundations and Non-Glazing Structures
Focusing solely on the transparent surfaces ignores the substantial heat loss that occurs through the ground and structural gaps. The foundation, which acts as a major thermal bridge, must be insulated to prevent heat from escaping the soil. This is commonly achieved using closed-cell rigid foam insulation, such as extruded polystyrene (XPS) boards, because of their high compressive strength and low water absorption. XPS typically provides an R-value of 5.0 per inch of thickness, making it suitable for below-grade applications where moisture is present.
The insulation is installed as a “skirt” around the perimeter, either vertically along the foundation wall or horizontally, extending several feet away from the structure’s base. This method isolates the soil underneath the greenhouse from the cold surrounding earth, effectively keeping the retained heat within the growing area. For a vertical application, the foam is often placed 4 feet deep into the earth, with the top edge flush with the foundation.
Addressing air infiltration is equally important, as leaks around movable components like doors and vents can negate insulation efforts. Weather stripping, available in materials like foam, vinyl, or rubber, should be used to seal the gaps around the edges of doors and operable vents. This material compresses when the door or vent is closed, forming a tight seal that prevents warm air from escaping and cold air from being sucked in.
For stationary cracks and openings, such as where the frame meets the foundation or around utility penetrations, caulk or expanding foam should be applied. Caulking provides a durable seal for smaller, non-moving gaps, while low-expansion spray foam is ideal for filling larger voids. If the greenhouse includes opaque walls or gable ends, these non-glazing sections can be insulated from the interior with panels of rigid foam or reflective insulation blankets to further reduce conductive heat loss.