The annual performance of a refurbishment or new build project incorporating thermal mass depends on several important factors. This Information Paper looks at the impact on thermal and energy performance of: building envelope, internal heat gains, operating period, ventilation, thermal mass and control. It highlights the importance of reducing infiltration and conduction losses when refurbishing a building to include thermal mass, the difference between winter mid-season and summer operation in terms of energy use at different levels of thermal mass, and the impact of the night cooling strategy selected on energy use. This IP considers the effect on acoustics and lighting of the introduction of exposed ceilings. It also gives designers best practice guidelines to be used when modelling thermal mass performance. An associated two-part BRE Digest 454 gives guidance on the incorporation of thermal mass into office buildings. 12 pages.