There is a lack of optimization of buildings towards energy performance in early design stages in practice. Interviews with architects and energy consultants showed that one reason is the inefficient communication between these two groups. This paper investigates how a design-integrated early-BIM tool can improve the relation between architects and energy consultants to support an optimization process in early design stages and facilitate issuing energy performance certificates. Two case studies show that the early-BIM tool provides meaningful results for the architects involved and can reduce the input time for energy consultants by 50%. Furthermore, the simple 3D model functions as boundary object between the two groups and supports the collaboration.