Historically there have been problems with controlling the quality of glazing on building sites leading to failures in insulating glass units. Glazing of units in factories offers the potential for higher quality work than for those glazed on site where poor weather and poor site conditions can produce damaged or defective units. In the absence of existing guidelines, BRE has developed - under a Partners in Innovation project funded by the DTI and the glazing and building industries - good practice guidance to maximise the benefits of factory glazing. The guidance, prepared as a two-part Digest, is based on research and good practice. It is not prescriptive but sets out the general principles, supported by drawings, important for factory glazing. Part 1 of this Digest introduces the subject and describes the types of insulating glazing units and the preparation of a suitable factory workspace. It includes guidance for fully bedded systems and for drained and ventilated glazing. Part 1 - 12 pages, including 2 pages listing relevant standards and other references. Part 2 deals with transport, storage, installation, maintenance and performance of factory glazed windows.