Which side of the glass should a coating face?

It depends on the coating’s job, and it’s worth getting right because a coating fitted the wrong way round won’t perform as intended. Surfaces in a sealed unit are numbered 1 to 4 from outside to inside: surface 1 faces the weather, surfaces 2 and 3 face each other across the cavity, and surface 4 faces the room.

• Low-E (soft coat) goes on surface 3 — the cavity-facing side of the inner pane — in a standard UK sealed unit. This is the optimum position for reflecting heat back into the room while keeping the delicate coating protected inside the cavity.
• Self-cleaning (hard coat) goes on surface 1 — the outermost face — because it needs daylight and rain to work (see below).
• Solar-control coatings are commonly positioned on surface 2 to reject heat before it enters the cavity, though the exact position varies by product.

Because the coating position is fixed by the unit’s build-up, it’s set when the unit is manufactured — so if you’re ordering a replacement sealed unit, tell the SevenDay team the glass spec and they’ll build it correctly.