Building Envelope Design: Water, Air & Thermal Control
The building envelope is the physical separator between the conditioned and unconditioned environment. Effective design must manage four critical control layers to ensure durability and comfort.
1. The Four Control Layers
To build a durable envelope, you must manage four key flows, in this order of importance:
- Water Control: Rain/groundwater. If this fails, the building rots. (e.g., Roof, cladding, sarking).
- Air Control: Stopping drafts. Critical for energy efficiency and moisture management. (e.g., Building wraps, tapes).
- Vapour Control: Managing condensation. In humid climates, keep vapour out. In cold climates, let vapour escape.
- Thermal Control: Insulation. Slowing heat transfer.
2. Facade Systems
Facades have evolved from simple structural brickwork to complex rainscreen systems.
Brick Veneer
The Australian standard. A timber/steel structural frame with a non-load-bearing skin of brickwork.
Key Detail: The 40-50mm cavity. It acts as a capillary break. Water that penetrates the brick must be able to drain down the back of the wall and exit via weep holes. Blocked cavities = damp houses.
Lightweight Cladding
Fibre cement (FC) weatherboards, matrix panels, or metal sheet.
Pros: Fast construction, contemporary look.
Cons: Relies 100% on the "sarking/wrap" layer for waterproofing. The cladding is often just a "rainscreen" that deflects 90% of the water.
Rendered Systems (Hebel/Polystyrene)
Hebel (AAC): Aerated concrete panels. Good thermal mass, fire resistant.
EIFS (Polystyrene): Excellent insulation but requires strict installation adherence to prevent moisture entrapment and fire risk. Non-compliant poly on high-rise buildings is a major industry issue.
3. Roofing & Flashings
The roof is the primary water shield. Most leaks occur not through the sheet, but at the junctions.
Metal Roofing (Colorbond/Zincalume)
Corrugated (Custom Orb): Requires a minimum pitch of 5 degrees.
Concealed Fix (Klip-Lok): Can go down to 1 degree. Excellent for flat-roof aesthetics but harder to flash.
Flashings: The #1 Failure Point
A flashing is a folded metal strip used to bridge the gap between two materials (e.g., roof sheet and wall).
The Rule: Water must always flow downwards and outwards. Over-flashing (relying on silicone instead of gravity) is a sign of poor workmanship.