The Building Envelope: Separation of Elements
The "envelope" is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer.
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.