A natural ventilation "calculator": The challenge of defining a representative 'performance sketch' in practice and research

Too often in architecture and engineering, the simplicity in early design of the apparent guarantees of HVAC equipment manufacturers wins out over the complexity of estimating the effectiveness of natural ventilation even in Net Zero Buildings. The reality is many people in offices and schools find themselves sitting at a fixed desk, in full direct sun, with the cold draft from the HVAC equipment guaranteeing that on average they are comfortable. The drawing of blinds 'solves' the immediate sun problem, but not the quality of the air. Experience with 200+ students designing low energy, high performance naturally ventilated spaces each year over the past decade has demonstrated that the formulae that exist in the literature for early estimation of window opening size have potential if presented in an appropriate format to facilitate sound design decisions. This paper reports the evolution of that format to a calculation dashboard that facilitates accounting for: outdoor and indoor CO2 levels; wind speeds and frequencies of occurrence; coincidence of periods of calm with high outdoor temperatures; window effectiveness coefficients; single-sided vs cross-ventilation options; building shape and orientation; code minimum ventilation rates; and even infiltration. The paper focuses on the limitations of the approach and its potential complementary role in the future as a Quality Assurance tool for critiquing the output of CFD studies of natural ventilation.