Blower door test
A calibrated fan seals into an exterior door and depressurizes the house, measuring exactly how much air leaks in — and letting you feel where.
What it establishes: Your airtightness number, where the big leaks are, and whether air sealing should be step one. The single measurement that separates a real audit from a sales walkthrough.
Full guide: what's involved, how to get it done, the fixes →