Cover a cold frame with a salvaged window sash, a polycarbonate panel, plastic sheeting on a frame, or oiled muslin, and hinge the lid at the back so it lifts upward for ventilation. A prop stick holds the lid open at several heights, and a latch keeps wind from lifting it.
The lid of a cold frame is its glazing and its door in one: a transparent panel in a frame that rests on the box and swings open.
The Lid Decides How Much Light Gets In And How Often You Must Replace It
Four lid materials work, ranked here from best to worst:
- Old window sashes (glass) — excellent light transmission and insulation, but fragile.
- Polycarbonate panels salvaged from greenhouses or skylights — tough, good insulation, and UV-stable.
- Plastic sheeting over a frame — cheap and easy to replace, but it tears in wind and lasts one or two seasons.
- Oiled muslin or cotton — the traditional choice, with moderate light transmission, and it needs re-oiling regularly. Read the fire note in Oiled Cloth & Animal Membrane before using it.
What You Need
- The lid material and a frame to hold it
- Hinges — salvaged door hinges, strap hinges, or straps of leather or heavy cloth nailed along the back edge
- A prop stick with several notches, or a stack of blocks
- A hook or latch to hold the lid shut in wind
Glass and Hardware Salvage covers recovering sashes and hinges; Glass Recovery covers lifting a pane out whole.
Method: Build And Hang The Lid
Step 1: Measure the box top. Make the lid frame slightly larger than the box so it overlaps the walls and sheds rain. For a 120 by 180 cm box, two sashes 90 cm wide side by side fill the 180 cm length.
Step 2: Frame plastic or fabric. Stretch the sheet tight over a wooden frame and fasten its edges with battens. Tight material sheds water; slack material pools it and sags.
Step 3: Hinge the back edge. Fix the hinges to the top of the back wall and the lid frame’s back edge, so the lid lifts from its front edge. When the lid is open the hinge side stays against the back wall.
Step 4: Add the prop and latch. A notched prop stick lets you open the lid a hand’s width on a mild day or wide on a hot one. A hook on the front holds the lid shut when wind rises.
Worked Example
Weight of a glass lid. Glass is about 2,500 kg per cubic meter (a standard reference value for soda-lime glass, recalled rather than checked here). A pane 3 mm thick therefore has a mass per area of 0.003 m × 2,500 kg/m³ = 7.5 kg/m². Over the 1.2 m × 1.8 m = 2.16 m² lid that is 7.5 × 2.16 = 16.2 kg of glass, before counting the wood frame. Two people should lift a glass lid; one person can drop it.
Common Mistakes
- Hinging at the front — the lid then drops open toward the plants and cannot be propped for ventilation.
- One prop height only — you need several openings to match the weather.
- Unlatched lid in wind — a lifted lid acts as a sail and can smash glass or tear plastic.
Safety
Broken glass
A glass lid that slams shut or is lifted by wind can shatter. Prop it firmly, latch it closed in gales, and wear gloves and eye protection when clearing broken panes.
What’s Next
- Learn next: Glazing Options — the same materials at greenhouse scale
- Combine with: Ventilation & Overheating Prevention — how wide to open the lid and when