When is duct leakage testing required in DC? (2015 IECC)

Short answer. Duct leakage testing in DC is an energy-code requirement, not a building-code one. The source is the DC Energy Conservation Code (2017 cycle, based on the 2015 IECC), §R403.3.3 (duct testing) and §R403.3.4 (maximum leakage rates). It applies to single-family and multifamily residential where the ducts or air handlers are located outside conditioned space — for example in an unconditioned attic, crawlspace, or chase. The IMC governs how ducts are built and sealed; the test mandate is the IECC.

The test and its limits

A duct blaster pressurizes the system to 0.1 in w.g. (25 Pa). Maximum leakage:

• Rough-in: ≤ 4 CFM25 per 100 sq ft (air handler installed; ≤ 3 if not).
• Post-construction: ≤ 4 CFM25 per 100 sq ft of conditioned floor area.

The two exemptions that exempt the test (§R403.3.3)

1. Ducts and air handlers located entirely within the building thermal envelope — the main new-construction case.
2. Existing ducts extended, with less than 40 linear feet in unconditioned space — the same carve-out restated for additions (§R502.1.1.2) and alterations (§R503.1.2).

So the "is a test needed?" question turns on duct/air-handler location. Everything inside conditioned space = exemption #1.

A caution that trips people up: the insulation exception (§R403.3.1) and the sealing exception (§R403.3.2) are not test exemptions. The code does not give a blanket pass to unaltered existing ducts that aren't being extended.

Residential vs commercial — the decisive split

• Residential path (§R403.3.3): R-2/R-3/R-4 buildings three stories or fewer above grade. There is no static-pressure carve-out — any duct system with ducts or air handlers outside the envelope must be tested, even low-pressure residential ductwork. Only the two exemptions above apply.
• Commercial path (§C403.2.9.1.3): R-2 buildings of four or more stories, and all other occupancies. Here the leakage test is required only for ducts/plenums designed to operate above 3 in w.g. (747 Pa), tested per the SMACNA leakage manual. Low-pressure commercial ducts are exempt.

The common mistake is carrying the commercial "only high-pressure ducts get tested" rule over to a small low-rise apartment. It does not transfer. The determinative question for any project is simply: how many stories? Three or fewer = residential (test regardless of pressure); four or more = commercial (high-pressure only).

Who performs it

The test is performed by an approved third party and documented in a signed written report before the certificate of occupancy or final. Sampling is available only for multifamily buildings of 10 or more units (after seven consecutive passing tests); a six-unit building tests each applicable system.