Question 1 of 6

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 system exists to stop uncontrolled, pressure-driven bulk airflow through the building enclosure — a far larger source of energy loss and moisture damage than vapour diffusion alone — so the material chosen for the role must satisfy several requirements together. First, it must be air-impermeable in its own right: the material-level acceptance criterion recognised by NBCC and ASHRAE 90.1 (per ASTM E2178) is an air leakage rate no greater than 0.02 L/(s·m²) at a 75 Pa reference 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 service penetrations are included. Second, it must be structurally adequate, able to carry the full range of wind, stack and mechanical pressurization loads expected over the building's service life without rupturing, delaminating or deflecting enough to open a leakage path at its fasteners or laps — which is why flexible sheet air barriers carry a rated design wind pressure rather than being assumed adequate on the strength of low material permeance alone. Third, it must be continuous, or made continuous in the field: a material performs as a system only once every seam, penetration and transition to roof, foundation, window or door is sealed with a compatible tape, sealant or membrane, since a perfect sheet with unsealed laps is not an air barrier. It must also be durable (resisting UV exposure during construction, embrittlement, and loss of adhesion over decades) and compatible with the layers on either side of it, and its vapour permeance must be chosen deliberately for the climate and assembly — an air barrier is not automatically a vapour barrier as well; combining or separating the two functions is a design decision.

Part (B).

Given. 8 mm plywood sheathing, total surface 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 the stated pressure difference, and whether plywood is adequate as an air-barrier material.

Approach. Treat the plywood 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.

1. Adopt the reference air permeance of plywood sheathing. Uncut, unpunctured plywood is one of the most airtight common sheathing materials, with a measured permeance at or below the NBCC/ASHRAE air-barrier MATERIAL limit: take C₁ = 0.02 L/(s·m²) at ΔP₁ = 75 Pa.
2. 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, $$q = 0.02\times\frac{24\,000}{75} = 0.02\times 320 = 6.40\ \text{L/(s.m}^2\text{)}$$
3. Scale to the full sheathing area. Multiplying by the 30 m² surface area, $$\boxed{Q = 6.40\times 30 = 192\ \text{L/s} = 0.192\ \text{m}^3\text{/s}}$$
4. 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.

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