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) — loadings on building enclosures. An enclosure carries several physically independent load types, and a durable design must satisfy each one, not just the structural one. GRAVITY (dead + live) loads are the self-weight of cladding, glazing and structural elements plus superimposed snow, transferred to the structure through anchors and fasteners sized for both short- and long-term (creep) behaviour. WIND load acts as both a mean and a fluctuating (gust) pressure on the enclosure's exterior face, positive on the windward side and negative (suction) on leeward/side faces and at corners/edges where local pressure coefficients spike; cladding, glazing and their attachments must resist this without excessive deflection or fatigue. SEISMIC load requires the enclosure to accommodate the building's interstory drift without cladding panels cracking, glazing lights popping out of their frames, or brittle finishes shattering. THERMAL load is the day-night and seasonal temperature swing across the assembly, which drives differential expansion/contraction between dissimilar materials (requiring movement joints) and the steady-state heat FLOW load addressed by insulation (Q2/Q4 below). MOISTURE load has several distinct sub-mechanisms: bulk rainwater deposited on the exterior face (Q6), capillary suction of groundwater at grade, water VAPOUR diffusing through the assembly from the warm, humid side (Q4), and built-in construction moisture drying out over the first few years. AIR-PRESSURE load is the differential air pressure across the enclosure from stack effect, wind and mechanical system operation (Part B below), which drives both bulk air leakage and, critically, moisture-laden air INTO the assembly at a rate that dwarfs pure vapour diffusion if the air barrier is not continuous. Finally, in-service SUSTAINED and IMPACT loads (occupant contact, maintenance access, hail, wind-borne debris) and UV/solar radiation load (surface heating, cladding degradation) round out the set. Good enclosure design treats these as a load PATH problem exactly like the structural system: each load must have a continuous, engineered path from where it acts to where it is safely resisted or shed.

Part (B)(i) — why controlling air flow matters. Uncontrolled air leakage carries far more heat AND moisture through an enclosure than diffusion alone: air can carry roughly 100 times more water vapour per unit area than vapour diffusion through even a poor vapour retarder, so exfiltrating warm humid air is the dominant cause of concealed condensation and mould in cold-climate walls, not vapour diffusion. Air leakage also drives direct energy loss (both the sensible heat carried by the escaping air and the latent heat of any moisture it carries), degrades occupant thermal comfort (cold draughts, localized radiant asymmetry near leakage paths), and can compromise acoustic performance and indoor air quality (uncontrolled paths admit outdoor pollutants, and pressure imbalances can draw combustion gases back into occupied space). Controlling air flow with a continuous, structurally-adequate, sealed air barrier system is therefore treated as at least as important as insulating and vapour-controlling the same assembly.

Part (B)(ii) — necessary conditions for air flow. Airflow through an enclosure needs exactly two things simultaneously: (1) a PATH — an opening or interconnected network of openings (joints, penetrations, gaps around service penetrations, porous materials) through the assembly from the warm side to the cold side, and (2) a PRESSURE DIFFERENCE across that path to drive flow from the higher- to the lower-pressure side. Removing either condition stops the flow: sealing every path (the air barrier's job) works even in the presence of a large pressure difference, and eliminating the pressure difference (impossible in practice, since stack effect and wind are always present to some degree) would work even with paths present. Because a fully sealed enclosure is unrealistic, practice targets the path (continuous air barrier) as the controllable variable.

Part (B)(iii) — mechanisms generating the pressure difference. Three independent mechanisms superimpose to set the net pressure difference across any point of the enclosure. STACK EFFECT arises from the density difference between warm interior air and cold exterior air in a tall building: it pressurizes the upper portion of the building (driving exfiltration there) and depressurizes the lower portion (driving infiltration there) relative to outside, with a neutral pressure plane somewhere in between; the effect scales with building height and the indoor–outdoor temperature difference, and reverses direction in summer. WIND generates a positive (windward) or negative (leeward/side, roof) pressure that varies with wind speed squared, building shape, and local pressure coefficients (highest suction at corners, edges and the roof). MECHANICAL SYSTEM (HVAC) operation imposes whatever supply/exhaust imbalance the design intentionally or unintentionally creates — a building can be deliberately held slightly positive (contaminant/dust exclusion) or negative (odour/moisture containment) by fan balancing, and this often becomes the largest and most controllable of the three drivers. The pressure difference at any given point and time is the vector sum of all three.