24-Bld-A5 Building Science, May 2016, Q1

Question 1 (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (A) — required characteristics of an air-barrier material. An air barrier's job is to stop uncontrolled, pressure-driven bulk airflow through the enclosure, which is a far larger energy and moisture-damage risk than vapour diffusion, so the material chosen for it must satisfy several requirements simultaneously. It must be air-impermeable in its own right: the accepted material-level acceptance criterion (ASTM E2178, adopted by NBCC and ASHRAE 90.1) is an air leakage rate no greater than 0.02 L/(s·m²) at a 75 Pa test pressure, roughly two orders of magnitude tighter than the 0.2 L/(s·m²) allowed for a completed air barrier ASSEMBLY once seams, fasteners and transitions are included. It must be structurally adequate: the layer has to carry the full range of wind, stack and mechanical-pressurization loads expected over the building's life without rupturing, delaminating, or deflecting enough to open a path at its fasteners or laps — this is why flexible sheet air barriers are rated to a design wind pressure (CAN/CSA A440 or the NBCC's own wind-load provisions) rather than assumed adequate by default. It must be continuous, or capable of being made continuous: the material only performs as a SYSTEM once every seam, penetration, window/door rough opening, and transition to roof or foundation is sealed with a compatible tape, sealant, or transition membrane — a perfect sheet material with unsealed laps is not an air barrier. It must be durable for the service life of the assembly (resist embrittlement, UV exposure during the construction sequence, and loss of adhesion over decades) and compatible with the layers on either side of it. Finally, its vapour permeance must be chosen deliberately for the climate and assembly — an air barrier is not automatically also a vapour barrier, and combining or separating the two functions is a design decision, not a given.

Part (B).

Given. 8 mm plywood sheathing, total area 30 m², pressure difference across it ΔP = 24 kPa (24 000 Pa) per the question as printed.

Find. The air flow rate through the sheathing at this pressure difference, and whether plywood is adequate as an air barrier material.

Check: a pressure difference of 24 kPa (about 3.5 psi) across an above-grade wall assembly is not physically realistic — stack effect and wind loads on real buildings run from a few Pa up to a few hundred Pa, and a differential this large would fail the wall structurally long before any air-barrier question mattered. The value is used literally as printed (most likely a units slip for 24 Pa), and the result is cross-checked against the standard 75 Pa test condition below so the adequacy conclusion does not depend on the anomaly.

Approach. Treat the sheathing as a sheet material with a rated air permeance at the standard ASTM E2178 test pressure of 75 Pa, then extrapolate to the stated pressure using the linear (Darcian) scaling appropriate to flow through a continuous solid sheet rather than through a discrete crack.

Adopt the reference air permeance of plywood sheathing. Uncut, unpunctured plywood sheathing is one of the most airtight common sheathing materials; its measured permeance sits at or below the NBCC/ASHRAE air-barrier MATERIAL acceptance limit, so take C₁ = 0.02 L/(s·m²) at ΔP₁ = 75 Pa.
Scale to the stated pressure difference. For laminar flow through a homogeneous sheet, permeance scales linearly with pressure: q = C₁·(ΔP/ΔP₁). With ΔP = 24 000 Pa, $$q = 0.02\times\frac{24\,000}{75} = 0.02\times 320 = 6.40\ \text{L/(s.m}^2\text{)}$$
Scale to the full sheathing area. Multiplying by the 30 m² surface area, $$\boxed{Q = 6.40\times 30 = 192\ \text{L/s} = 0.192\ \text{m}^3\text{/s}}$$
Cross-check at the realistic 75 Pa test condition. At the standard test pressure the same sheet gives Qₛ₅ = C₁×A = 0.02×30 = 0.6 L/s — negligible for a 30 m² wall, confirming the material itself is airtight; the large total at 24 kPa is a consequence of the (implausible) pressure magnitude, not of the material being leaky.

Problem 1B — final results
Quantity	Value
Reference permeance, plywood @ 75 Pa	0.02 L/(s·m²)
Permeance @ 24 kPa (linear scaling)	6.40 L/(s·m²)
Total flow @ 24 kPa, 30 m²	192 L/s (0.192 m³/s)
Total flow @ realistic 75 Pa, 30 m²	0.6 L/s

At the pressures a building envelope actually experiences, plywood's own air permeance is at or below the code threshold for a certified air barrier material, so it comments favourably: plywood sheathing is adequate as the air-barrier COMPONENT, provided its seams and penetrations are sealed to bring the assembly (not just the sheet) under the 0.2 L/(s·m²) @ 75 Pa system limit.

Concept & Theory

Topic: Air permeance of sheet air-barrier materials and its pressure dependence.

Key relations: q = C·(ΔP/ΔP₁)ⁿ, with n ≈ 1 for flow through a continuous porous sheet (laminar/Darcian) versus n ≈ 0.65 for flow through a discrete crack (turbulent, per the power-law crack model used for whole-building blower-door results).

Why this works: Material-level air leakage testing (ASTM E2178) and assembly-level testing (ASTM E2357) both report a leakage rate at a single reference pressure of 75 Pa because that pressure is representative of design wind/stack loads; extrapolating to any other pressure requires an assumed flow exponent, and for flow driven through the body of a sheet material (not around it) the linear assumption is standard.

Common pitfall: using the whole-building n ≈ 0.65 crack exponent (from blower-door analysis) for a homogeneous sheet material, which overstates flow at high pressure and understates it at low pressure relative to the material's real behaviour.

Source: ASHRAE Handbook — Fundamentals, Ch. 26 (air leakage and air barrier requirements); NBCC air barrier system provisions.

Question 1 of 6

Question 1 (20 marks)