Check each window for signs of damage: rot, mould and/or staining on or around the window, the condition of the glass, putty and paint, weatherstripping and the operation and condition of the hardware. Some windows may need only minor air sealing work, while others require major upgrading or even replacement. Check for air leakage around the frame and at all movable joints. Combine a visual inspection with a test using a leak detector as described in "How your house works."