How to Install Double-Poly Inflation Step by Step

Double-poly inflation is the most cost-effective insulation upgrade for any poly-covered greenhouse. This step-by-step walkthrough covers the full installation from materials list through final testing.

Materials Needed

Two rolls of 6-mil UV-stabilized greenhouse poly (sized to cover your greenhouse with 12 inches of excess on all sides), one squirrel-cage inflation fan (20 to 50 watts, available from greenhouse suppliers for $30 to $60), wiggle wire and channel (enough linear feet to run along all base rails and purlins where film attaches), greenhouse tape for sealing overlaps and small holes, and a helper — installing poly is a two-person job.

Step 1: Prepare the Frame

Remove the old poly if present. Inspect every frame member for sharp edges, protruding screws, and rust scale that could damage the new film. File or cap all sharp points. Apply anti-hot-spot tape (foam insulation tape) to every purlin, rafter, and base rail that the poly will contact. Metal frames absorb solar heat and melt through unprotected poly over time — anti-hot-spot tape prevents this and extends film life by 1 to 2 years.

Step 2: Install the Outer Layer

Choose a calm day with temperatures above 50°F — cold poly is stiff and difficult to work with. Drape the first sheet of poly over the frame from one end, pulling it tight and smooth toward the other end. One person inside the greenhouse pushes the poly up against the frame; the other person outside presses the wiggle wire into the channel to lock the film in place. Work from the center of each rail outward toward the corners. Pull the film taut before locking — wrinkles trap condensation and reduce light transmission.

Trim excess poly 2 inches beyond the wiggle wire channel. Fold the trimmed edge under and tape it to the frame for a clean, weathertight seal. Pay special attention to corners and door openings, where wind can work poly loose.

Step 3: Install the Inner Layer

Attach a second set of wiggle wire channels to the inside faces of the purlins and base rails, spaced 2 to 4 inches from the outer channels. This creates the gap that the inflation fan will pressurize. Drape the inner poly sheet and lock it in place using the same technique as the outer layer. Ensure the inner sheet is smooth and wrinkle-free. Seal all edges with greenhouse tape to prevent humid greenhouse air from entering the inter-layer space.

Step 4: Mount and Connect the Inflation Fan

Cut a hole in the inner poly sheet at one end of the greenhouse (typically the end wall), sized to match the fan's outlet diameter. Mount the fan with its intake drawing outdoor air and its outlet exhausting into the space between the two poly layers. Use duct tape and a short section of duct to connect the fan to the hole in the inner layer. Seal around the connection with greenhouse tape.

The fan should run continuously, maintaining a slight positive pressure between the layers that keeps them visibly separated. If the inner layer balloons excessively into the growing space, partially cover the fan intake to reduce pressure. If the layers are not separating, check for air leaks in the inner poly where pressurized air is escaping into the greenhouse.

Step 5: Test and Adjust

With the fan running, walk through the greenhouse and check for areas where the two layers are touching — contact points eliminate the insulating air gap and reduce performance. Adjust film tension or add spacers where contact occurs. Check all edges for air leaks — use a lighter or incense stick near seams to detect drafts (flame flickers or smoke moves toward leaks).

Install a thermometer inside the greenhouse and compare overnight lows before and after installation. You should see an immediate improvement of 5 to 15 degrees in overnight minimum temperatures, depending on your starting insulation and climate. This translates to 30 to 40 percent lower heating costs over the winter season.

Troubleshooting Common Problems

Condensation between layers is normal in small amounts — moisture from the outdoor air intake condenses on the cooler inner surface. Excessive condensation that pools and drips indicates either a failing seal that allows humid greenhouse air to enter the inter-layer space, or an improperly sized fan that is not maintaining enough positive pressure to keep the layers separate. Check all inner-layer seals and increase fan airflow if needed. A small amount of condensation actually improves insulation slightly by adding a moisture film that blocks some infrared radiation, so do not be alarmed by light fogging.