To obtain a high thermal resistance in windows there are several steps of improvement that can be undertaken. Clearly double glazing is superior to single glazing in terms of preventing heat loss, and triple glazing is, by and large superior to double glazing. Performance is also affected by the gas used to fill the sealed units and the material used to manufacture the surrounds. For some products there are also other features, such as self-cleaning or solar control properties which add to its sustainable credentials.