Window boards are attached to the wall with angle irons every 400 – 600 mm. The board is 20 – 30 mm thick. The board can also be attached by pins that are sunk into the edge of the board, but in this case the board must be at least 30 mm thick. Window boards are usually 150 mm – 250 mm wide. A 10 mm gap must be left between the wall and the window board to ensure the circulation of warm air. Alternatively, the board must be perforated.