Narrow cavity insulated glazing units are often favoured in listed buildings or conservation areas, as they provide a compromise between energy-efficiency and authenticity. Where possible, it is always preferable to use standard cavity IGUs when replacing windows. They are less expensive,have a longer life and are more energy-efficient. They can provide modern standards of comfort in a timber frame that has correct, narrow glazing bars and authentic detailing.  However, where narrow cavity insulated glazing units are required, it is important to check they have the correct test evidence in place to ensure they are not subject to premature loss of performance or failure.