Plastic film is the most practical greenhouse covering for large structures: 6-mil greenhouse-grade polyethylene lasts 3-4 seasons, ordinary hardware-store poly about 1, and a double layer with air blown between the sheets insulates dramatically better than a single layer. Salvaged polycarbonate panels are tougher and last longer, and suit smaller or more permanent houses.
Part of Glazing Options
Plastic film and rigid plastic panels are lightweight greenhouse coverings that cost less than glass and wear out faster.
Film Trades Lifespan For Weight, Price And Size
Film is light enough for a thin frame, comes in wide rolls that cover a large house in few pieces, and is cheap to replace. It degrades in sunlight and tears in wind, so a film greenhouse is a recurring job. The salvage supply matters: greenhouse-grade film lasts longer than builders’ sheeting, and once existing stocks run out, film in a rebuilding world is a finite resource. Polycarbonate panels, salvaged from greenhouses and skylights, are tougher and UV-stable.
6-mil means 6 thousandths of an inch. One mil is 0.0254 mm, so 6 mil is 0.152 mm, about 0.15 mm.
What You Need
- Film — 6-mil greenhouse grade if you have it; hardware-store poly works but degrades faster in UV (about 1 season against 3-4 seasons)
- Battens and fasteners — thin wooden strips and nails or screws to pin the film without cutting holes in it
- A smooth frame — rounded edges and no protruding nails, or the film snags and tears
- For double-layer inflation: a small fan or wind catcher, and a way to keep the air gap sealed
Method: Cover A Frame
Step 1: Prepare the frame. Sand or pad every corner and head that touches the film. Cover sharp points with cloth or tape.
Step 2: Lay the film on a calm day. Wind turns a roll of film into a sail. Unroll it across the frame, overlapping the ends by a hand’s width, and pull it evenly tight.
Step 3: Pin with battens. Sandwich the film edge between the frame and a batten, and fasten through both. Do not drive nails straight through unprotected film; the holes tear in wind.
Step 4: Add a second layer if you can. Lay a second sheet over the first and blow air between the two with a small fan or let a wind catcher inflate the gap. The trapped air layer improves insulation dramatically and also stiffens the surface so it flaps less.
Step 5: Inspect after every storm. Patch small tears quickly with tape or a glued patch; a small hole in film becomes a large one in the next gust.
Common Mistakes
- Putting film on a rough frame — it wears through at every corner.
- Slack film — it flaps, chafes and fails sooner; also ponds rainwater or snow that can pull the frame down.
- Sealing a double layer with no vents elsewhere — a double-layer house still needs ventilation. See Ventilation & Overheating Prevention.
Safety
Film burns and a closed house cooks
Polyethylene film is flammable: keep stoves, lamps and hot work away from it. Film also traps heat; close the house and a sunny afternoon can exceed 50 °C inside.
What’s Next
- Learn next: Oiled Cloth & Animal Membrane — when there is no plastic left
- Combine with: Frame Materials: Wood, Bamboo, Salvage — building a frame film will not snag on