Warehouse Air Barrier: Wrong Area, No Extrapolation, Undocumented Seals
A 150 m × 80 m single-storey distribution warehouse has a 12,000 m² floor on slab-on-grade and a 9 m clear height. The specification requires air leakage no greater than 0.60 L/(s·m²) at 75 Pa over the air barrier area, tested per ASTM E779. The 24 dock doors are not yet installed; the contractor seals the openings with 6-mil polyethylene and construction tape, a condition the specification does not address. Four fan units are staged at the main entrance. On test day the fans cannot hold 75 Pa; the maximum stable pressure is 62 Pa, where the measured flow is 8,800 L/s. The technician divides 8,800 L/s by the 12,000 m² floor area, gets 0.73 L/(s·m²), declares a failure, and leaves. No temperature or barometric pressure data were recorded. The general contractor calls you for a second opinion before scheduling remedial work on the roof membrane.
1. Which area normalizes the flow, and what is the correct number?
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The specification says 'air barrier area,' so use the area of the surface that is actually the air barrier - not the floor. On a slab-on-grade building with a roof, the boundary is walls plus roof.
Perimeter = 2 × (150 + 80) = 460 m. Wall area = 460 × 9 = 4,140 m². Roof = 12,000 m². Total = 16,140 m².
Using the floor area understates the reference area and inflates the result:
- 8,800 ÷ 12,000 = 0.73 L/(s·m²) - the technician's number
- 8,800 ÷ 16,140 = 0.545 L/(s·m²) - the correct normalization of the same 62 Pa reading
Neither figure is the answer to the specification, because the specification is written at 75 Pa and the fans only reached 62 Pa. But note how much work the area choice does: on this data set it is the difference between an apparent clear failure and an apparent pass. Before quoting any number, read the specification for the defined reference area and state the definition in the report. If the spec instead says floor area, use floor area - but say so.
2. The fans cannot reach 75 Pa. What should have been done, and what is the defensible way to reach a 75 Pa result?
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A single point at 62 Pa cannot be reported as a 75 Pa result. Extrapolating from one point also cannot separate the envelope's flow behavior from fan and gauge error, which is exactly why the standard's multi-point procedure exists.
What should have been done: collect a series of points across the attainable range (roughly from 10 Pa up to the maximum stable pressure), fit q = C·Δp^n, and report n and the correlation coefficient alongside the flow at the target pressure. Those points also tell you whether a single fan reading is trustworthy.
Defensible estimate now: with a typical envelope exponent n ≈ 0.65 (use the fitted value when you have the series), q75 = 8,800 × (75/62)^0.65 = 8,800 × 1.132 ≈ 9,960 L/s, and 9,960 ÷ 16,140 ≈ 0.617 L/(s·m²).
That is over 0.60 - but only just, and inside the combined uncertainty of an assumed exponent, the flow measurement and the extrapolation. The defensible outcome is not 'fail.' It is: 'approximately 0.62 L/(s·m²) at 75 Pa by extrapolation from 62 Pa, within the uncertainty band of the limit; the result cannot be adjudicated and must be resolved by a test reaching at least 75 Pa.'
To reach 75 Pa: add fan capacity, or find and seal the large leaks first - a handful of big holes will dominate this envelope and may be cheaper to fix than to out-blow.
3. How do the temporary dock seals and the missing environmental data affect the verdict and the report?
Read the worked answer
Temporary seals. Twenty-four dock openings at roughly 3 m × 3.5 m is on the order of 250 m² of taped 6-mil poly. Poly taped at the edges is not an air barrier: the taped perimeter leaks, and the assembly does not represent anything that will exist in the completed building. The specification is silent on phasing, which makes this a legitimate 'as-built, incomplete' test - but it must be reported as such, with the seals described, located, quantified and photographed, and with an explicit statement that the result does not accept or reject the finished envelope. Otherwise the owner will one day believe they hold a whole-building compliance result they do not have.
Missing environmental data. Without indoor and outdoor temperature and barometric pressure you cannot apply the density correction, cannot show the test conditions, and cannot reproduce the result. The report is not defensible, and neither the pass nor the fail survives scrutiny.
Recommendation to the general contractor: do not touch the roof membrane. Run a multi-point test with conditions recorded, density correction applied, temporary seals documented, and the reference area defined per the specification. If the envelope then sits near 0.60, locate the major leaks before deciding on remediation - the number as it stands was produced by the wrong area, an undocumented extrapolation and an uncorrected flow.