Why ACH50 alone did not explain this home’s modeled heat-loss change 

In the home behind our recent EnerGuide case, ACH50 increased by about 3.4%. Equivalent leakage area at 10 Pa increased by 7.6%. The model’s air-leakage and natural-ventilation heat loss increased by about 9.3%. Why did three related numbers move by different amounts?

They describe different parts of the problem. ACH50 summarizes flow at a test pressure relative to the home’s volume. Leakage area expresses the test result at another reference pressure. Annual heat loss is a model output. Reading them together gives an advisor more useful information than treating one number as the whole airtightness story.

The changes in this pair

The percentage changes were calculated from the unrounded file values; the table displays rounded leakage-area and heat-loss values. The dwelling volume was effectively unchanged. The two files used the same HOT2000 version, ruleset and weather location, and each retained seven blower-door test points.

The paired EnerGuide case explains the solar side of the rating improvement. Here the question is narrower: what does the blower-door evidence tell us about pressure-dependent leakage?

A blower-door test is more than the 50 Pa result

A multipoint test records flow at several pressure differences. The fitted relationship is commonly written Q = C × Pⁿ: flow depends on pressure, a coefficient and the fitted exponent, n. The exponent describes how quickly flow changes with pressure. Retrotec’s measurement guidance explains why the exponent matters when converting a measured flow to another pressure.

The original test reports for this pair show rounded exponents of 0.700 and 0.660. Those are test-report values; this article does not claim that they are the exact exponents stored or used internally by HOT2000.

At a pressure below 50 Pa, the lower exponent produces a larger fraction of Q50. For example, at 10 Pa these normalized curves give about 32.4% and 34.6% of Q50. That illustrates why a relatively small change in ACH50 need not produce the same percentage change in a lower-pressure quantity.

Keep the reference pressure in the name

A leakage-area number needs both its definition and reference pressure. The HOT2000 values discussed here are equivalent leakage area at 10 Pa. Retrotec distinguishes equivalent leakage area from effective leakage area; the terms are not interchangeable merely because both are sometimes abbreviated ELA.

In this pair, 10 Pa is below the lowest recorded test pressures, so that result involves extrapolation of the fitted relationship. It is not a direct measurement of leakage on a mild day. Low pressure across the envelope can occur under various weather and operating conditions; a description such as “mild weather” does not specify that pressure.

The test reports do not establish the wind conditions needed to judge their influence here. Keep that uncertainty attached to any physical interpretation of the difference. The figures support a closer look at the leakage behaviour, but do not identify a new hole or prove that installation work caused one.

What deserves attention on the next job

Where work crosses or disturbs the air barrier, inspect unintended openings at service penetrations and joints and preserve continuity. Keep required ventilation and combustion-air provisions intact. NRCan’s air-leakage guidance describes the connections worth checking.

ACH50 is one reference point in the leakage picture. Here, ELA10 and the pressure-flow relationship help make sense of the larger change in modeled infiltration heat loss. The practical takeaway for renovation work is to protect air-barrier continuity and seal unintended openings at penetrations and joints, while preserving required ventilation and combustion air. These results support that attention to detail without identifying renovation work as the cause of this home’s change.

Source and scope

This selected 2026 single-home comparison uses the paired HOT2000 files, Homeowner Information Sheets and original blower-door reports. HOT2000 provides the reported ACH50, 10 Pa leakage area and annual heat-loss output. The original test reports provide the rounded fitted exponents used in the explanatory chart. No controlled model re-run, leak-location survey or attribution to solar work is claimed. Property identifiers and underlying private records are omitted.