Be prepared to find structural problems when an old window is removed. Rotting or missing wood, undersized, or improperly supported lintels, for example, must be corrected before the new unit is installed. Enlarging openings or cutting new ones involves structural work, and may require the rerouting of wiring and other services. The surrounding structure must be sound and dry before the window is fitted, insulated, and air-sealed. New mouldings on either side of the window will likely be required.
Doors tend to be mounted flush with the inside face for aesthetics and ease of operation.
Once the window is installed, check that it opens and closes easily, locks tightly, and is well-sealed. If condensation occurs between panes of sealed units, the seal is faulty and the unit should be replaced under warranty. Different things are covered by different kinds of warranties. Shorter periods suggest the manufacturer has used less effective sealing methods. If condensation, frosting, fogging, or discolouration appear between the layers of glazing within the warranty period, the manufacturer will replace the sealed unit.