Your builder says the house is energy efficient. How to read blower door and duct leakage numbers

A blower door holds the house at a 50-pascal pressure difference with the outdoors and measures the airflow, CFM50. Multiply that by 60 and divide by the house volume to get ACH50, according to the Energy Department's Building America Solution Center. A duct test seals the registers and pressurizes the ducts to 25 pascals. That fan flow is CFM25, compared per 100 square feet of conditioned floor area. A total test counts every leak; a leakage-to-outside test counts only air lost to attics, crawlspaces, garages or outdoors.

The International Energy Conservation Code has capped total duct leakage at 4.0 CFM25 per 100 square feet since the 2012 edition, and the 2021 edition added an 8.0 limit for systems that sit entirely inside the thermal envelope, the Solution Center says. ENERGY STAR allows 8 at final. A total result of 8.9 is above all three. The Solution Center puts the cost in cooling terms: a 2.5-ton system with 20 percent duct leakage loses about half a ton.

On air tightness, the 2012 through 2018 codes required 3 ACH50 in climate zones 3 to 8. The 2021 code keeps 3.0 on the prescriptive path but caps every path at 5.0, the Journal of Light Construction reported. A house near 5 ACH50 in those zones therefore passes only on a performance or energy rating path, and which edition applies depends on what the state and town adopted.

Ask the builder which code edition and compliance path the permit used, whether the duct figure is total or to outside and was taken at rough-in or final, where the ducts run, and for the rater's full report. Raters test under RESNET protocols, and the limits apply to each duct system, not to the house as a whole.