Research and practice agree that decisions taken early in a project have a higher impact and are less costly. Current building performance assessment methods are not suited to accommodate the responsiveness required for early design processes and are often used for validation in the later stages where the feedback has little design impact. Tools developed specifically for early-stage Design Decision Support (DDS) are either too simplistic, provide no solution to addressing combined indoor environmental quality (IEQ), or risk worsening the overall IEQ by optimizing performance indicators in isolation. Most comprehensive building assessment methods evaluate several topics but follow a linear approach which fails to support holistic performance feedback and fails to meet the demand for assessment speed. This paper presents application examples of a holistic IEQ assessment tool (IEQCompass) in design processes. Design experiments demonstrate that the applied approach can meet the challenges of early-stage DDS pointed out in existing literature. Findings from the experiments indicate that the IEQCompass can provide: (1) seamless early-stage assessments through rapid-feedback on changing designs, (2) timely decision support by guiding design teams with criteria overviews, design comparisons and holistic assessment results and (3) dialogue and communication support between architects, engineers and clients.