Salvaged window glass is the best collector cover: it passes 85-90% of sunlight, blocks the infrared the hot absorber gives off, and keeps its strength at stagnation temperatures. Greenhouse polycarbonate is the next choice and clear plastic film is a one-or-two-season stopgap, so test any plastic against the collector’s no-flow temperature before relying on it. Leave a 2-3 cm air gap between the cover and the absorber.
Part of Flat Plate Collectors
Collector glazing is the transparent cover over the absorber; it lets sunlight in and slows heat loss, and it must survive both hail and heat.
The Cover Must Survive The Hottest Day, Not The Average One
A collector passes boiling when water stops flowing, and a glazing that works at 60°C can sag or crack at 150°C (The Greenhouse Effect in Collectors). Test a plastic by leaving a sample sheet in an empty, sealed collector box on a clear day. If it warps, yellows or sags, it will fail when the collector is not flowing.
| Cover | Transmission and life (parent page) | Notes |
|---|---|---|
| Window glass | 85-90% of visible light; durable | Blocks infrared; best choice; heavy and breakable |
| Greenhouse polycarbonate | Nearly as good as glass | Lighter, shatter-resistant; check its heat limit |
| Clear plastic sheeting | Works for 1-2 seasons | Degrades in UV; emergency option only |
| Shower and sliding-door glass | Large pieces | Ideal for big collectors; tempered, so it cannot be cut |
A second layer reduces heat loss in cold weather but also reduces the light that gets through (parent page). The wiki’s glazing guide for greenhouses gives similar comparisons for lifespan and sourcing (Glazing Options); it is about greenhouse covers, not the ceramic glaze in the materials section.
What You Need
- Glass panes — measure them all and design the box around the size you have (Salvaged Glass, Glass Salvage & Handling)
- Putty or silicone for bedding, and battens to hold the pane
- Gloves and eye protection
Method: Fit The Cover
Step 1: Size the box to the pane. Measure first; a cut tempered pane shatters.
Step 2: Set the air gap of 2-3 cm above the absorber (parent page).
Step 3: Bed and seal the pane so rain cannot enter, but cut weep holes at the bottom edge so condensation drains out (parent page).
Step 4: Check the first sunny day for fogging inside the glass. A little is normal; a lot means a leak or damp insulation.
Worked Example: Weight On The Frame
Glass of 4 mm thickness at about 2,500 kg/m³ (typical soda-lime density, recalled) has mass 2,500 × 0.004 = 10 kg per square metre, and weight 10 × 9.81 = 98 N per square metre. A 1.5 m² pane weighs 15 kg, or 147 N. The frame and mounts must carry that, plus wind.
Common Mistakes
- Plastic film against a hot absorber. It touches, softens and sags.
- Sealing the pane airtight with no weep holes. Condensation pools and clouds the glass.
- Cold water on a hot pane. Rain or a hose on a stagnating collector can crack ordinary glass.
Safety
Broken glass over a hot collector, and heat that can melt the cover
A cracked pane drops sharp pieces onto a roof or a person; wear gloves and eye protection when fitting, and mount away from walkways. A collector without flowing water is hot enough to burn skin on the frame and to soften some plastics. Never touch the frame in the afternoon sun without checking its temperature.
What’s Next
- Learn next: Insulation & Frame — the box that holds the glass
- Combine with: Wind Loads — how hard wind pulls at a tilted pane