Air-tightness testing measures a window's ability to block air infiltration when closed, in a lab or on site. The window is mounted on a sealed test chamber (static-pressure box) and a fan creates a stable pressure difference across it (e.g. 10 Pa, 50 Pa, 100 Pa); a high-precision flow meter then measures air infiltration through the gaps. Less infiltration means better tightness. The test follows national standards such as GB/T 7106-2008; from the measured flow, pressure difference and window area it calculates infiltration per unit area or per unit seam length, and the window is graded — higher grade, better performance. On site, simple smoke or paper tests give a rough check with lower accuracy. Air-tightness testing effectively evaluates energy savings and living comfort and ensures the product meets design requirements.