Double-poly inflation is the single most cost-effective insulation upgrade for any greenhouse covered with polyethylene film. By adding a second layer of poly and inflating the gap between them with a small squirrel-cage blower, you create an insulating air pillow that reduces heat loss by 30 to 40 percent — the equivalent of upgrading from single-pane glass to double-pane without replacing the glazing. The materials cost $100 to $300 for a hobby greenhouse, and the payback in heating savings is typically one winter.
How Inflation Works
The blower pushes air between the two poly layers, creating a gap of 2 to 4 inches. This trapped air acts as insulation (R-1.5 to R-2.0 for the assembly, compared to R-0.8 for a single layer of 6-mil poly). The slight positive pressure between layers also prevents wind from fluttering the outer layer, reducing mechanical fatigue that shortens poly life.
The blower runs continuously and consumes minimal electricity — a typical squirrel-cage inflation fan draws 20 to 50 watts, costing $2 to $5 per month. The energy savings from reduced heating dwarf this operating cost.
Choosing Poly Film
Use 6-mil greenhouse-grade polyethylene with UV stabilization. Standard hardware-store poly (the clear or black sheeting sold for painting and landscaping) is not UV-stabilized and degrades in 3 to 6 months of sun exposure. Greenhouse-grade poly is treated with UV inhibitors and anti-condensate coatings that extend its life to 4 years and prevent interior dripping.
For the inner layer, anti-condensate (AC) coating is especially valuable — it causes moisture to sheet down the film in a thin layer rather than forming droplets that drip onto plants and promote disease. For the outer layer, infrared-blocking (IR) additives reduce nighttime heat loss through radiation. Many greenhouse poly products combine UV, AC, and IR treatments in a single film.
Installation Overview
Attach the outer layer first using wiggle wire (spring lock channel) or poly lock track along the base rails and purlins. Pull the film tight and smooth before locking it in — wrinkles trap water and reduce light transmission. Then attach the inner layer to a separate set of channels or directly to the inside of the purlins, leaving a 2 to 4 inch gap between layers.
Mount the inflation fan in the end wall, drawing air from outside (to prevent humidity from accumulating between layers) and exhausting it into the inter-layer gap. A simple duct connection between the fan and the gap is all that is needed. The fan should provide enough pressure to keep the layers visibly separated but not so much that the inner layer balloons into the growing space — adjust by partially covering the fan intake if overpressure is an issue.
Seal all edges of both layers to prevent air leakage. Gaps in the inner layer allow humid greenhouse air into the inter-layer space, where it condenses and blocks light. Gaps in the outer layer allow wind to penetrate and reduce the insulation value. Greenhouse tape and UV-rated caulk are the standard sealing materials.
Performance and Maintenance
Expect a 30 to 40 percent reduction in heating costs compared to a single-layer poly greenhouse. In practical terms, a grower spending $200 per month on propane to heat a single-poly high tunnel in zone 5 would spend $120 to $140 per month after inflation. The materials investment ($100 to $300) pays back within the first winter.
Light transmission drops by approximately 10 to 15 percent with the addition of a second layer. This is generally acceptable for most crops, but growers pushing DLI-dependent crops like tomatoes in winter should account for the reduction when sizing supplemental lighting. The tradeoff — better insulation versus slightly less light — favors double-poly in all but the sunniest, mildest climates.
Replace poly every 3 to 4 years as UV degradation yellows the film and reduces light transmission. Plan replacement for late summer or early fall when weather is cooperative and the growing season allows a brief disruption. When replacing, inspect the wiggle wire channels and frame for corrosion or damage and repair before installing fresh film.
Frequently Asked Questions
Troubleshooting Common Inflation Issues
The most common problem with double-poly systems is uneven inflation — one area of the greenhouse balloons while another stays flat. This usually indicates a gap or tear in one of the poly layers that is allowing air to escape unevenly. Inspect both layers for holes, paying special attention to areas near wiggle wire channels where the film is under tension. Repair holes with UV-rated greenhouse tape — standard duct tape degrades in sunlight within weeks.
Condensation between layers reduces light transmission and can promote algae growth on the inner surfaces. This occurs when humid greenhouse air leaks into the inter-layer space through gaps in the inner poly. The solution is to source the inflation air from outside the greenhouse (dry air) rather than from inside (humid air). Connect the fan intake to an exterior vent or duct that draws outdoor air. In very humid climates, a small inline desiccant filter on the intake can further reduce moisture accumulation.
Fan failure is the other vulnerability. Without continuous inflation, the two poly layers collapse together, losing their insulating air gap and fluttering in wind, which accelerates mechanical fatigue. Keep a spare inflation fan on hand, and consider wiring the fan to a dedicated circuit with a low-wattage indicator light or alarm that signals if the fan stops running. A $10 outlet timer with an audible alert provides adequate monitoring for most hobby setups.
Alternatives to Full Double-Poly
If full double-poly inflation is impractical for your greenhouse (perhaps it already has rigid polycarbonate walls and a poly roof), you can achieve partial benefits with bubble wrap insulation. Horticultural bubble wrap — larger bubbles and UV-stabilized film — attaches to the inside of the glazing with clips or double-sided tape and provides R-1.0 to R-1.3 insulation. It is not as effective as inflated double-poly (R-1.5 to R-2.0) but is far better than nothing. Apply bubble wrap to the north wall and roof panels during winter and remove it in spring when light levels increase. This approach is especially practical for greenhouses with mixed glazing — poly roof, polycarbonate sidewalls — where full double-poly is difficult to implement uniformly.
How much does double-poly inflation cost?
Materials for a 200 to 400 square-foot greenhouse typically cost $100 to $300: two rolls of 6-mil greenhouse poly ($40 to $80 each), an inflation fan ($30 to $60), wiggle wire and channel ($30 to $50), and greenhouse tape for sealing. Installation is a DIY job that takes a few hours with a helper.
Does double poly reduce light?
Yes, by approximately 10 to 15 percent compared to a single layer. Each layer of clean 6-mil poly transmits about 88 to 90 percent of light, so two layers transmit roughly 77 to 81 percent. This reduction is generally acceptable for most crops but may require supplemental lighting adjustments for light-hungry fruiting crops in winter.
Can I inflate existing greenhouse poly?
If your greenhouse already has one layer of poly attached with wiggle wire, you can add a second layer to the inside of the frame and inflate the gap. The key is maintaining a consistent air gap — if the layers touch, insulation value drops to nearly zero at that contact point.
How long does greenhouse poly film last?
UV-stabilized greenhouse-grade poly typically lasts 3 to 4 years. Standard hardware-store poly degrades in 3 to 6 months. Always use greenhouse-grade film with UV inhibitors. Anti-condensate and IR coatings add cost but improve performance and may extend useful life.