Build the frame to fit the window you have: a back wall 18-24 inches tall, a front wall 12-14 inches tall, the glass facing south in the Northern Hemisphere, and a lid you can prop open 4-6 inches on a sunny day. A 3 mm sheet of glass weighs about 7.5 kilograms per square metre, so a one-square-metre window lid is light enough to lift, but a lid that falls can still injure a hand. Seal the gaps with straw or soil, but never seal the lid shut, since the frame must vent.
A salvage cold frame is a bottomless box made of reclaimed lumber, blocks or straw bales with an old window or clear sheet as its lid, set over a garden bed.
Size The Box To The Window
The carrier guide (Four-Season Growing & Season Extension) says to build a rectangular frame with a back wall 18-24 inches high and a front wall 12-14 inches high, sloping to shed rain and snow and to face the sun, face the glass south in the Northern Hemisphere, size the box to the salvaged glass, seal gaps with straw, soil or rags, and leave the lid able to prop open.
What You Need
- A window or clear sheet for the lid.
- Frame material: lumber, concrete blocks, straw bales or rammed earth.
- Hinges, or a prop stick.
- Gloves and eye protection for handling glass.
Method
Step 1: Measure the glass. The inside of the box should be a little smaller than the lid on each side, so the lid rests on the frame and overhangs.
Step 2: Choose the slope. If the box is 0.9 m deep (front to back) with a back wall of 0.55 m and a front wall of 0.33 m, the lid rises 0.55 − 0.33 = 0.22 m over 0.9 m, a tilt of arctan(0.22 ÷ 0.9) = about 13.7 degrees. A steeper tilt faces a low winter sun more squarely. At latitude 45 degrees the noon winter sun is at 90 − 45 − 23.44 = 21.6 degrees; sunlight hits a lid tilted 13.7 degrees toward the sun at an angle of 21.6 + 13.7 = 35.3 degrees to the glass, which is 54.7 degrees from the perpendicular. At a tilt of 30 degrees the ray meets the glass at 51.6 degrees, 38.4 degrees from the perpendicular, so more light is let in. Your own latitude changes the numbers.
Step 3: Build and set the box. Join the walls, dig the box into the ground slightly if the soil allows, and place it on the bed with the back to the north.
Step 4: Fit the lid. Hinge it along the back edge, or rest it loose and use a notched prop stick to hold it open at fixed heights, such as the 4-6 inches the carrier gives for a sunny day.
Step 5: Seal. Pack the outside base with soil, straw or old rags; leave the lid edge free to lift.
Step 6: Weigh the lid. A 3 mm glass pane has a mass of about 2,500 kg/m³ × 0.003 m = 7.5 kg per m², so a lid of 1.2 m × 0.9 m = 1.08 m² of single 3 mm glass is 8.1 kg before its frame. Two panes or thicker glass is heavier.
Common Mistakes
- A lid that cannot be propped — the frame overheats.
- A flat or north-facing lid — it catches little winter sun.
- A lid heavier than one person can lift — it gets left shut. Use two smaller lids.
- Treated or tarred timber in a food bed — avoid creosote-soaked posts or old railway sleepers.
Safety
Broken glass and old paint
Wear gloves and eye protection when cutting, carrying or fitting glass; edges are sharp and a dropped lid shatters. Fit a prop that cannot slip, so the lid does not fall on a hand. Paint on old window frames may contain lead: do not dry-sand or scrape it, and keep flakes out of the soil; strip it safely or seal it and keep the frame out of contact with the soil.
What’s Next
- Learn next: Ventilation and Temperature Management — how to vent it
- Combine with: Salvaged Glass — working with reclaimed panes