What a blower door test measures in a hot-humid climate

What does a blower door test actually measure, and what does it not measure?

A blower door measures one thing: the rate at which air passes through the building enclosure when the house is held at a reference pressure difference, usually 50 pascals. It produces a leakage rate and, from it, an air changes per hour figure. It does not measure indoor air quality, comfort, duct performance, or whether the house will stay dry. In Central Texas the number matters chiefly because outdoor air here is warm and carries a large amount of moisture, so uncontrolled leakage is a moisture-transport problem before it is an energy problem.

What the number is

The test depressurises (or pressurises) the house with a calibrated fan mounted in an exterior door and records the airflow required to hold a set pressure difference across the enclosure. The result is normally reported as CFM50 — cubic feet per minute at 50 pascals — and converted to ACH50 using the conditioned volume.

It is a whole-house aggregate. It tells you how much leaks, not where. Finding where requires the test to be running while someone walks the building with a smoke pencil, an infrared camera, or a hand.

• Blower door testing is a standardised, repeatable measurement of enclosure air leakage, and results from properly conducted tests are reproducible within a known tolerance.[ESTABLISHED]Established
• A single ACH50 figure does not identify leak locations, and two houses with identical ACH50 results can behave very differently depending on where the leakage is concentrated.[ESTABLISHED]Established

Why it reads differently in climate zone 2A

Most published guidance on air sealing was written from a heating-climate perspective, where the leakage penalty is heat loss and the moisture drive runs outward. The corridor from Austin to San Antonio is hot-humid climate zone 2A, where the dominant drive for most of the year is inward, and the air being pulled through the enclosure carries latent load with it.

That changes what a bad leakage number costs you. In a cold climate a leaky house is expensive. In a hot-humid climate a leaky house is expensive and it is continually importing moisture into wall cavities and interior spaces, where it has to be removed by something.

• In hot-humid climates the latent (moisture) portion of the load associated with air leakage is substantial relative to the sensible portion.[ESTABLISHED]Established
• Air leakage is a larger vapour-transport mechanism than diffusion through intact assemblies in typical construction.[ESTABLISHED]Established
• Reducing enclosure leakage below a given threshold changes indoor moisture behaviour by [figure pending verification] — we do not publish a number we have not measured on a house we built.[UNVERIFIED]Unverified

What a tight house then requires

Tightening the enclosure removes an uncontrolled path for outdoor air. It does not remove the need for outdoor air. Once the leakage rate drops, ventilation has to be designed, sized, and commissioned deliberately, and in this climate that ventilation air usually needs its moisture managed before it reaches the occupied space.

This is the point where the blower door result stops being a score and becomes an input to the mechanical design. A tight house with an undersized or uncommissioned ventilation strategy is a worse outcome than a leaky one, and we treat the two as a single scope rather than two line items.

• Mechanical ventilation is required to maintain outdoor-air exchange in enclosures tightened beyond the point where infiltration alone supplies it.[ESTABLISHED]Established
• Introducing unconditioned outdoor air into a hot-humid-climate house without moisture control can raise indoor relative humidity.[ESTABLISHED]Established

How Sage Co. uses the result

We test at least twice: once at the pre-cover stage, while the air barrier is still reachable and a defect can be fixed for the cost of an afternoon, and once at completion as a record. The pre-cover test is the one that changes the house; the final test is the one that goes into The Sage Co. Record.

The final test is performed by an independent third party. Sage Co. does not grade its own homework — an internal role can record that a test happened and what it showed, but only the independent verifier can mark the result independently verified.

A failed test is not deleted or replaced. It stays in the record alongside the remediation and the retest, so the house's history is legible to whoever owns it next.