Air-density correction and the RESNET one-point 10% adjustment are separate calculations—and the order matters. A blower door does not measure building leakage directly. It measures the fan airflow needed to maintain a pressure difference across the enclosure. Because air density changes with temperature and elevation, the airflow displayed by the gauge may need to be corrected to standard conditions. That correction is commonly called the Air Density Correction Factor (ADCF). ANSI/RESNET/ICC 380 requires corrected CFM50 to include adjustments for density and viscosity in accordance with Section 9 of ASTM E779. For a single-point test, after corrected CFM50 has been determined, Section 4.5.1 requires a separate 10% adjustment when the result will be used for an energy rating or to assess compliance with an enclosure-leakage limit. Use the calculations in this order Determine nominal CFM50. Record the airflow at 50 Pa. If 50 Pa cannot be achieved, first convert the measured airflow to nominal CFM50 as permitted by Section 4.4.1.2 of Standard 380. Apply the air density correction. Use the method supplied for the specific blower door system, including the correct test direction, indoor and outdoor temperatures, and site elevation. This produces corrected CFM50. Apply the RESNET adjustment. For a HERS rating or leakage-limit compliance, multiply corrected CFM50 by 1.10. Use the adjusted value when calculating ACH50 or other derived metrics. Correct sequence: Nominal (tested) CFM50 × applicable ADCF(s) = Corrected CFM50; then Corrected CFM50 × 1.10 = Adjusted CFM50 Manufacturer-specific guidance The Energy Conservatory (TEC) states that its indicated fan flows are already referenced to a standard air density of 0.075 lb/ft³. Its Model 3 manual provides separate correction tables for depressurization and pressurization, plus an altitude correction. Select the table that matches the fan direction – do not interchange the two. For example, let’s assume you are conducting a single-point test with an indoor temperature of 70°F and an outside temperature of 40°F at an altitude of 4,000 feet. The test result is 3,200 CFM50. TEC’s depressurization table gives a factor of 0.971. A nominal result of 3,200 CFM50 x 0.971 becomes 3,107 corrected CFM50. Now the altitude adjustment factor is calculated. Altitude Adjustment = (1+(.000006 x 4,000 ft)) x 3,107 CFM50 = 3,182 CFM50 The 10% adjustment is then applied: 3,182 × 1.10 = 3,500 adjusted CFM50, after rounding. Retrotec likewise explains that temperature differences change the relationship between measured fan flow and airflow through the enclosure; under extreme conditions, the effect can approach 10%. Retrotec’s current blower door manual directs users to enter the test data into FanTestic, which manages the required corrections. Its DM32 manual also contains direction-specific tables for manually correcting flow for temperature differences. Use the current Retrotec workflow and inputs for the Retrotec system being operated. Both TEC Autotest and Retrotec Fantestic will automatically calculate the ADCF when performing an automated multipoint blower door test. The practical rule is simple: record indoor temperature, outdoor temperature, elevation, test direction, equipment model, and the correction method used. Do not borrow a table from another manufacturer or treat the ADCF as part of RESNET’s 10% adjustment. They address different sources of uncertainty and must remain separate in the calculation record. Resources ANSI/RESNET/ICC 380-2019 — see Sections 4.4.1 and 4.5.1 The Energy Conservatory Model 3 Minneapolis Blower Door System Manual – See Chapter 4 Retrotec Blower Door Operation Manual — current manual for 300, 5000, 6000, and 7000 systems
