The Challenges:
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NZ has many temperature, humidity and weather variabilities, from wild lashing winter rain and hailstorms, to blustering autumn gales and withering heat in summer, and our homes need to provide reliable shelter against all these conditions. Our homes must also be durable – sturdy, hard-wearing and resilient to last, and to continue to perform efficiently, over the decades.
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Many people remember the difficulties with leaky homes through the 1990s and early 2000s – many NZ houses were constructed with designs and materials that resulted in significant problems when water got in and damaged the structures:
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“A leaky building is one where moisture gets between the exterior cladding of the house and the inside walls. The walls can potentially rot, and dangerous mould can grow and cause structural problems for the building and health problems for those who live there.”

Many homes built in earlier eras also suffer vulnerability to water damage, due to inherent design and structural issues (such as 1930s Art Deco houses) or to maintenance issues over the years. And some more recent builds may also have weaknesses to weather, perhaps due to choices of materials or design.
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“Weathertightness is a building’s ability to prevent water from penetrating the building envelope. It depends on the building’s location, design, materials, construction and maintenance. Preventing damage caused by moisture makes it safer and healthier for occupants and improves the durability of building materials and elements.”

The Building Code provides for weathertightness under clause E2 External Moisture, which requires that:

• External moisture does not cause undue dampness or damage to building elements

• Water cannot enter the building through the roof, wall claddings, and external openings

• Any moisture that does get through is able to drain or evaporate without causing harm.

Clause E2 of the Building Code incorporates seven performance criteria including requirements for:

• Roofs

• Walls

• Structural elements / foundations in contact with or close proximity to the ground

• The space below suspended floors

• Concealed spaces and cavities

Practical options:
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There are many elements in the design and construction of a new home or renovation that will help ensure effective weathertightness:

• Building wraps: Alternatives to traditional building paper, wraps (or flexible underlays) are fixed directly to the framing, to protect interior structures from water and wind impacts. Wraps can add a further layer of insulation and minimise thermal bridging between the interior and framing and the exterior cladding.

• Cavities: to help drainage within the walls when water gets through to the back of the exterior cladding or onto the building wrap.

• Rigid air barriers: to manage the air pressure that acts on the exterior of the building, driving external moisture through the cladding and into the wall assembly. A rigid air barrier helps drain moisture safely downwards and away. They comprise a water and mould resistant sheet fixed to the framing with the joints and fixings covered with weathertight flashing tape.

• Choice of framing timbers: such as boron treated timbers to prevent rot or decay:

The Four D’s:
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The 4Ds principle was developed in the 1990s at the National Research Council of Canada (NRC), in response to increasing awareness of moisture-related failures in buildings, particularly in coastal and wet climates.  The 4D’s framework was formalised to guide the design of rain control strategies for walls, roofs, and other building envelopes.