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Insulation & Frame

4 min read

Insulation behind and around a collector absorber cuts heat loss from the back and sides, but about 5 cm is enough: the glass front loses far more heat than the back does. Because the absorber reaches 120-220°C when no water flows, use insulation that tolerates that heat, such as mineral wool, and keep dry straw, paper and foam board away from the hot plate.

Part of Flat Plate Collectors

The collector frame is a shallow insulated box that holds the absorber and glazing and slows heat loss everywhere except the glass face.

The Back Matters Less Than The Front, But It Matters

Heat flows through a flat layer at Q = U × A × ΔT, with U = k ÷ thickness (k is the insulation’s conductivity). Inputs, assumed for the example: 5 cm of insulation with k = 0.041 W/m·K (the corpus’s figure for wool, Natural Insulation Materials); absorber at 80°C, ambient 20°C, so ΔT = 60 K; a single-glazed front with an assumed U of 6 W/m²·K.

  1. Back: U = 0.041 ÷ 0.05 = 0.82 W/m²·K, so loss = 0.82 × 60 = 49 W/m².
  2. Front: 6 × 60 = 360 W/m².
  3. The back loss is 49 ÷ 360 = 14% of the front loss, and 49 W against a peak absorbed 765-855 W/m² (Absorption vs Reflection) is about 6%.

So a thicker back layer buys very little. Spend the effort on a tight frame with no air leaks, a dry interior, and good glass instead.

What You Need

  • A shallow box — wood, 5-10 cm deep (parent page), with a back sheet
  • Insulation that survives the stagnation temperature
  • A reflective or foil layer over the insulation (optional)
  • Weep holes at the bottom for condensation
  • Fixings to hold the frame against wind

Method: Build The Box

Step 1: Make the box rigid with joints that keep rain out. Line the bottom and sides with insulation.

Step 2: Put a heat-tolerant layer or an air gap between the insulation and the hot absorber. Mineral wool and glass fibre survive heat; dry straw and paper chars slowly at the temperatures a stagnating collector reaches; rigid foam softens or melts. The parent page lists fibreglass batts, rigid foam or packed straw; the safest of those at the hot face is fibreglass.

Step 3: Set the absorber on spacers so it floats clear of the insulation, with the 2-3 cm gap below the glass.

Step 4: Drill weep holes at the low edge. Water that gets in must be able to get out, because wet insulation loses most of its value.

Step 5: Fix the frame to the roof or stand against wind lifting the glazed face (Wind Loads).

Common Mistakes

  • Foam or straw touching the absorber. Both fail at stagnation temperatures.
  • Thick back insulation and a leaky front. Gaps and a poorly bedded pane lose more than the extra thickness saves.
  • No weep holes. Condensation soaks the insulation.

Safety

Combustible insulation next to a plate that can pass 120°C

A collector with no water flowing reaches 120-220°C (The Greenhouse Effect in Collectors). Dry straw, wood and paper in contact with a plate at that temperature can char and, over weeks of sun, ignite; rigid foam softens, gives off fumes and can burn. Keep the hot plate separate from combustibles by an air gap or a mineral layer, and do not leave a collector with damaged insulation in use. The corpus’s treatment of stove clearances applies to the same logic (Chimney & Stove Safety).

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