Can Two Airtightness Tests Show Building Deterioration?
Two airtightness tests support a conclusion that a building changed only when they measure the same boundary under comparable conditions and the difference is meaningful against the uncertainty of the comparison. A higher n50 is a clue, not proof of deterioration: n50 relates air leakage at 50 Pa to the internal volume used in the calculation. Compare the reports and their uncertainty, then look for a physical change that could explain the result. [1] [2] [3]
- n50 is air leakage per internal building volume at a 50 Pa pressure difference, reported in h⁻¹; a change in the volume used can alter n50 even if the airflow is unchanged. [1]
- Repeatability tests use the same building, operator, equipment and short time interval; they do not capture systematic errors or differences in building preparation and pressure sequence. [3]
- In one reproducibility study, independently prepared tests deviated from a reference by −4% to +55%, compared with −6% to +7% when one person prepared the building in advance. [3]
- In a 25-year case, n50 rose from 0.21 (3) to 0.26 (3) h⁻¹ as window lip seals lost elasticity, then returned to 0.21 (3) h⁻¹ after renewal; the reported ranges overlap. [4]
What does a change in n50 measure?
The n50 value is air leakage per internal building volume at a pressure difference of 50 Pa. It is reported in h⁻¹. Air leakage and volume are separate inputs, so either can change the result. For example, if measured airflow stayed the same but the volume entered in the second report were 5% smaller, the calculated n50 would be about 5.3% higher. This is an illustrative calculation, not evidence about a particular building. If the tested boundary or volume calculation differs between reports, the apparent change may come from the calculation rather than the envelope. [1]
Match the test boundary and preparation
A repeat comparison starts by confirming that both tests cover the same building spaces and treat openings and services consistently. Check which doors, windows, hatches and ventilation ducts were open, closed or temporarily sealed, and whether later alterations changed the tested envelope. Building preparation is a protocol variable: a REHVA Journal review reports that independently prepared operator tests deviated from a reference by −4% to +55%, while tests on a building prepared in advance by one person ranged from −6% to +7%. The study demonstrates how preparation can affect reproducibility; those ranges are not uncertainty limits for every building. [3]
Compare the procedure and test conditions
Keep the pressure sequence and other test procedures comparable, and record changes in equipment, operator and weather where possible. Repeatability describes tests on the same building by the same operator with the same equipment over a short interval. It does not capture systematic errors or variations in preparation and pressure sequence. The review also reports that high wind speeds made standard deviation an unreliable indicator of repeatability in some studies. A changed setup does not automatically invalidate a result, but it makes the comparison harder to interpret. [3]
Ask whether the difference exceeds uncertainty
Every measurement has uncertainty, and the National Physical Laboratory says a result is incomplete without an uncertainty statement. Request the uncertainty budget for each test and ask the tester to assess the uncertainty of the difference. A point estimate alone cannot establish a change. The REHVA review gives empirical repeatability and reproducibility findings, but cautions that repeatability does not include systematic errors or protocol differences. Even a difference beyond stated uncertainty supports a change in measured performance only if the boundary, volume and procedures are comparable. [2] [3]
Connect a measured difference to a physical cause
A comparable difference that exceeds uncertainty supports a change in measured performance; identifying deterioration also needs evidence of a plausible physical cause. Inspect for later penetrations, failed seals, movement at junctions or damage from work after the earlier test. A published study of a Passive House built in Darmstadt in 1990/1991 reports that a repeat test after 25 years found n50 had risen from 0.21 (3) to 0.26 (3) h⁻¹ as window lip seals lost elasticity. The reported ranges overlap: 0.18 to 0.24 and 0.23 to 0.29 h⁻¹. After the seals were renewed, the original 0.21 (3) h⁻¹ value was attained again. The investigators also found critical connection points remained airtight. This case supports a physical explanation for that building; it does not establish an ageing rate for other buildings. [4]
What should the reports contain?
Place the original reports side by side before attributing a higher n50 to ageing. Confirm the tested boundary and volume calculation, then compare preparation, equipment, operator, pressure sequence, weather conditions and each result’s uncertainty statement. If records are missing, the defensible conclusion is that the n50 values differ while the reason remains unresolved. Request the original reports and a tester’s written comparison before treating the difference as evidence of deterioration. [1] [2] [3]