Airtightness / Blower Door test

Definition

The airtightness of a building envelope is verified with a Blower Door test, a pressure-difference method according to DIN EN ISO 9972. A calibrated fan is installed in an external door and generates a pressure difference of 50 pascal between inside and outside — both under negative and positive pressure. The test measures the air flow per hour leaking through the envelope. The result is expressed as the n50 value (air change rate at 50 Pa). The Buildings Energy Act (GEG) specifies limits: 3.0 per hour for residential buildings without mechanical ventilation, 1.5 per hour with a ventilation system. Higher-tier Efficiency House standards such as Effizienzhaus 40 or Passive House often require values below 0.6 per hour. Accompanying leakage detection with fog or anemometer helps identify concrete weak spots.

When is the term used?

The Blower Door test is a mandatory component of KfW 261 renovations to Effizienzhaus 55 or 40 and of new buildings with a ventilation system. In older buildings, weak points are often found at roof connections, uninsulated knee-wall areas, window and door installation joints, sockets in external walls, roller-shutter boxes and service penetrations. Anyone aiming to improve airtightness should plan for it during the renovation design — retrofitting sealing is much more demanding than clean detail engineering before construction. The test is often carried out in two stages: a preliminary test during construction (to identify and fix leaks) and a final confirmation test after completion.

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