Close the back of a solar panel with an insulating backing sheet, seal the whole edge with a sealant that stays firm above 60°C, and run the cable out through a sealed junction box with strain relief and a drip loop. Then prove the seal: the resistance from the cell wiring to the frame should read above 1 megohm, and a reading that falls after rain means water has got in.
Part of Solar Panel Construction
The back and edges of a solar panel: backing sheet, edge seal, cable exit and a simple moisture test.
Moisture Gets In At The Edges And The Cable
The carrier’s closing steps are to add a backing sheet (thin plywood, sheet metal or another glass pane), frame with aluminium angle or hardwood, and seal all edges with silicone caulk. Its summary: moisture kills panels. Three weak points matter in practice: the edge seal, the backing material and the cable entry.
Backing. Sheet metal conducts, so it must be kept apart from the cell wiring by sealant or an insulating layer, and it can rust. Plywood absorbs water unless sealed on every face. A second glass pane is moisture-proof and heavier. Whatever you choose, it should sit in the sealant so no air gap remains.
Edge seal. The sealant must hold above 60°C (Panel Encapsulation and Framing). For an edge that does not touch the cells, a hard pitch is a candidate: Pitch and Waterproofing says petroleum pitch is exceptionally water-resistant and suits static uses, though harder and more brittle in the cold, and pine pitch is softer (Tar and Pitch: pitch softens in heat and cracks in cold). Beeswax melts at 62–64°C and is unsuitable.
Breathing. General knowledge, not from the other articles: a panel that is not completely filled warms by day and cools by night, and the air inside expands and contracts through any leak, drawing in humid air. So fill voids with sealant, and leave no cavity.
What You Need
- A backing sheet — sealed plywood, insulated sheet metal or glass
- Edge sealant — silicone or a pitch tested for heat
- A junction box with a cable gland — Junction Boxes
- Two-core cable with weatherproof insulation, size from Wire Selection
- A multimeter with a high-resistance range — Testing Faults
Method: Seal And Test
Step 1: Fit the backing into sealant, pressing from the centre out to remove air.
Step 2: Fill gaps between the cells and the frame so there are no cavities.
Step 3: Fit the junction box over the cable exit, with the cable passing through a gland or grommet so it cannot pull on the ribbon, and bend the cable down in a loop before the box so water runs off it.
Step 4: Seal the full perimeter with a continuous bead, then a second bead after the first has set.
Step 5: Test insulation. With the string disconnected from everything and covered, measure resistance from the positive and from the negative wire to the frame. Above 1 MΩ is normal; below 100 kΩ shows that moisture or damage has already got in (Testing Faults).
Step 6: Repeat after the first rain and again after a month.
Worked Example: Sealant And Leakage
A panel 379 × 794 mm (the size worked out in the encapsulation article) has a perimeter of 2 × (0.379 + 0.794) = 2.346 m. A bead of 6 mm × 6 mm (an assumed section) is 36 mm² = 36 × 10⁻⁶ m², so the volume is 2.346 × 36 × 10⁻⁶ = 8.4 × 10⁻⁵ m³ = 84 cm³ per panel, enough to plan tubes of sealant. If the panel makes 18 V, a leak path of 1 MΩ draws 18 ÷ 1,000,000 = 18 µA and one of 100 kΩ draws 18 ÷ 100,000 = 180 µA, ten times as much: the ten-fold ratio between the two thresholds shows up directly as leakage current to the frame.
Common Mistakes
- Unsealed plywood backing — it swells and delaminates
- A bare metal backing against the wiring — it shorts or corrodes
- No drip loop — water follows the cable into the box
- A single thin bead — one pinhole lets water in; use two coats
- Skipping the follow-up test — seals fail after weeks, not on the first day
Safety
Leakage to a metal frame can make the frame live
A panel is live whenever light falls on it, and a wet panel can pass current to the frame; a series string reaches a voltage that is dangerous even though a cell is not (three 18 V panels make 54 V, six make 108 V). Test the frame before touching it, ground metal frames, work with the string covered or disconnected at the array, and treat it as live regardless (Lethal Current). Sealant solvents need ventilation and keep flames away.
What’s Next
- Learn next: Charge Controllers — connecting the finished panel to the battery
- Combine with: Mounting and Orientation — putting the panel in the sun