A dynamic insulation system is one that works by drawing outdoor air into a building through an insulation layer which is permeable to air. Heat which would otherwise be lost by conduction is recovered in the incoming ventilation air. This paper describes what constitutes a dynamic insulation system and discusses energy savings and practicalities of building with dynamic insulation. It also discusses some of the design and buildability issues gained from practical experience. The energy savings resulting from the use of dynamic insulation are limited to the conduction loss which would be expected for the same level of conventional insulation. The major benefit would be improved comfort due to the ventilation air being preheated. Dynamic insulation is most effective in buildings which require a high and constant rate of ventilation, eg sports halls and swimming pools. Intermittent operation may allow moisture to migrate into the construction. Results of modelling studies on other building types, eg houses, are discussed briefly. 8 pages.