Insulating a greenhouse comes down to trapping heat where it matters most: the glazing, the north wall, the ground, and every gap where warm air escapes. Most greenhouses lose the bulk of their heat through their covering material, so that’s where your effort pays off the most. But a combination of approaches, from bubble wrap on the glazing to rigid foam around the foundation, can cut heating costs dramatically and keep temperatures stable through cold nights.
Where Greenhouses Lose Heat
Before choosing materials, it helps to understand the three ways a greenhouse bleeds warmth. Conduction moves heat directly through glass, polycarbonate, or plastic film to the cold air outside. Convection carries warm air up and out through gaps in the frame, vents, and doors. Radiation sends infrared heat straight through transparent surfaces into the night sky. A good insulation plan addresses all three, but conduction through the covering is the single biggest source of loss in most structures.
Single-pane glass has an R-value of just 0.91, meaning it’s a poor insulator on its own. For comparison, double-pane glass with a half-inch air space jumps to about R-2.0, and triple-pane glass with half-inch air spaces reaches R-3.2. Those numbers explain why even a thin layer of additional insulation on single glazing makes a noticeable difference on cold nights.
Bubble Wrap: The Simplest Upgrade
Lining the inside of your greenhouse with horticultural bubble wrap is the cheapest and most popular insulation method. The trapped air pockets act as a buffer between the cold glazing and the warm interior, roughly doubling the insulating value of single glass. It’s lightweight, easy to cut, and lets through enough light for most cool-season crops.
Use horticultural bubble wrap, not the stuff from shipping boxes. The horticultural version is UV-stabilized and made with larger bubbles that transmit more light. Standard packaging wrap degrades in direct sunlight within a single season, turning brittle and yellow. UV-stabilized greenhouse films can last three to four years or longer before needing replacement.
To install it, press the bubble side flat against the glass so the bubbles create small air pockets against the surface. On aluminum greenhouse frames, plastic clips that snap into the glazing bars hold the wrap in place without tape. On wooden frames, a staple gun works fine. Overlap sheets by a couple of inches and avoid stretching the material tight, since a slight slack helps it sit flush against the glass. Cover the roof and walls, but focus first on the north-facing side where no direct sunlight enters during winter.
Insulating the North Wall
The north wall of a greenhouse (in the Northern Hemisphere) receives almost no direct sunlight during winter, so covering it with an opaque, reflective insulation costs you very little light while delivering significant heat savings. Foil-faced bubble insulation is the go-to material here. It combines a layer of trapped air bubbles with a reflective aluminum surface that bounces light and infrared heat back into the greenhouse interior.
Cut the reflective insulation to fit your north wall panels, shiny side facing inward, and attach it directly to the frame or glazing bars. The reflective surface does double duty: it reduces heat loss through that wall and redirects light back toward your plants, partially compensating for the low winter sun angle. Some growers leave this insulation up year-round, though removing it in spring gives you more ventilation options.
Thermal Screens and Night Blankets
Internal thermal screens are retractable curtains that stretch horizontally beneath the greenhouse roof, creating a pocket of still air between the screen and the glazing. Commercial greenhouses use motorized versions, but hobby growers can achieve the same effect with a sheet of thermal fabric or even a lightweight row cover draped across the interior at gutter height. You pull it across at dusk and open it each morning.
Research on greenhouse energy efficiency found that thermal screens reduce energy consumption by 17.7% to 26.5%, depending on climate and how consistently they’re used. That’s a meaningful reduction in heating costs for anyone running a heater through winter. The key is creating a sealed air gap, so tuck the edges of the screen against the frame as tightly as you can. Any opening along the sides lets warm air rise past the screen and defeats the purpose.
For smaller greenhouses, a simpler version works well: hang sheets of horticultural fleece or thermal bubble wrap from the ridge down to bench height, creating a tent over your plants at night. This reduces the volume of air you need to keep warm and puts the insulation closest to where it matters.
Sealing Gaps and Drafts
Insulation materials won’t help much if warm air is streaming out through gaps in the frame. Walk through your greenhouse on a windy day and feel for drafts around door edges, vent hinges, where glazing panels meet the frame, and any cracked or missing panes. These small leaks add up fast.
Self-adhesive foam weatherstripping works well around doors and vents. For gaps between glazing panels and the frame, silicone sealant fills cracks permanently, while foam tape offers a removable option. If you’re sealing gaps in cold weather, keep in mind that most sealants perform best when applied between 40°F and 80°F. Silicone and polyurethane sealants can be applied at temperatures well below freezing if necessary, but they cure more slowly and need a clean, dry surface to bond properly.
Replacing any cracked or missing panes before winter is obvious but easy to put off. A single missing pane can drop the interior temperature of a small greenhouse by several degrees on a windy night.
Foundation and Perimeter Insulation
Heat doesn’t just escape upward. The ground around a greenhouse foundation conducts warmth away from the interior, especially in regions where the soil freezes. Rigid foam insulation board (typically one inch thick) installed vertically along the outside of the foundation creates a thermal break between the warm greenhouse soil and the frozen ground outside.
The recommended depth is one to two feet, or down to your local frost line, whichever is deeper. Bury the foam board against the outer face of the foundation wall, then backfill with soil to hold it in place and protect it from UV damage. Extruded polystyrene (the pink or blue rigid boards sold at hardware stores) resists moisture absorption and won’t break down underground.
If your greenhouse sits directly on the ground without a foundation, you can still gain benefit by burying rigid foam vertically around the perimeter, angled slightly outward. This diverts the frost line away from the greenhouse footprint and keeps the soil inside warmer. Even a shallow 12-inch installation makes a difference in mild climates.
Choosing a Glazing Material
If you’re building a new greenhouse or replacing worn-out panels, the covering material itself is your first and most effective insulation decision. Twin-wall polycarbonate panels are the standard choice for insulated hobby greenhouses. The two layers with an air channel between them provide roughly double the insulation of single glass, and they’re lighter, nearly unbreakable, and easier to install.
Thicker options exist. Triple-wall polycarbonate adds another air channel and gets closer to R-2.5 or higher, though it reduces light transmission noticeably. For most growers, twin-wall panels strike the best balance between insulation, light, and cost. They typically last 10 to 15 years before the UV coating degrades enough to warrant replacement.
If you already have a single-glass greenhouse and don’t want to re-glaze it, layering bubble wrap on the inside gets you into a similar insulation range as twin-wall polycarbonate at a fraction of the cost. It’s not as durable or attractive, but it’s effective.
Putting It All Together
The most effective approach layers multiple strategies. Start with the biggest sources of heat loss and work down:
- Glazing: Line the interior with horticultural bubble wrap, prioritizing the roof and north wall. If budget allows, replace single glazing with twin-wall polycarbonate.
- North wall: Cover with reflective foil-faced insulation, shiny side in, to block heat loss and bounce light back toward plants.
- Thermal screen: Install a horizontal curtain below the roof ridge to trap a warm air layer at plant level overnight.
- Gaps: Seal every draft point with weatherstripping, foam tape, or silicone. Check doors, vents, and glazing bars.
- Foundation: Bury rigid foam board one to two feet deep around the perimeter to insulate the ground.
Each of these steps contributes incrementally, but together they can transform a freezing single-glazed greenhouse into a structure that holds temperatures well above freezing on most winter nights, even with minimal supplemental heating. The goal isn’t to make a greenhouse as warm as a house. It’s to hold enough heat from the daytime sun to protect plants through the coldest hours and reduce the energy your heater needs to cover the gap.