The performance gap is the difference between the original design intent, and that achieved in practice.  An independent £7M monitoring programme funded by Innovate (,uk) covering some 400 buildings has identified an energy and carbon performance gap of around a factor of x2.  In other words energy and associated carbon emissions from the buildings studied, was around twice that the occupiers could reasonably have expected.  Performance gaps were also found with indoor air quality and overheating risk.