Principles of weathertight design

Following weathertight design principles will help ensure you have a dry and durable home.

Why weathertight design is important
The main purpose of your home is to provide shelter to protect you and your family from the weather. The first line of defence against the weather is the outer skin, or claddings. How claddings are designed and applied to your home is crucial, because over your home’s lifetime some water will inevitably get in. With good design this will be managed so that small amounts of moisture do not cause serious damage or risk ill health for you and your family.

Under the Building Code, the life of a building is normally considered to be at least 50 years, but we often expect our homes to last much longer. This means short cuts shouldn’t be made in the quality of what is built.

When to think about weathertight design
Weathertightness should be considered right at the beginning of the design process. At this stage the effect of the many forces that will impact on your home can be considered, and decisions made about the overall shape of the building and the materials used.

You and your designer will need to consider how the sun, wind and rain will impact on the building and how the overall design will cope with these forces.

At the detailed design stage you and your designer will make decisions about materials and how they will be combined in your home. At this stage you should also consider:

• Weather effects.
• Joints and junctions.
• How different materials, windows, doors, chimneys, etc are combined and detailed.

The builder constructs the building as it is designed and should follow the detailing and construct the building with quality materials, carefully constructed to ensure your and your designer’s intentions are carried out.

Thinking about weathertightness does not stop once your home has been built. It is up to you to maintain your home to ensure it continues to perform, keeping you warm and dry, as it was designed to do.

New Zealand Building Code and weathertightness design

The New Zealand Building Code (building code) is a performance based code that sets out the performance buildings and their components have to meet. It does not dictate how a building must be constructed, it says what a building must achieve.

Acceptable Solutions meet the performance specified in the building code. The Acceptable Solution for weathertightness is E2/AS1. This part of the code also contains a Verification Method (E2/VM1) for testing claddings to ensure they meet these performance requirements. The Acceptable Solution and Verification Method are not the only way to meet the building code requirements. A designer may design an Alternative Solution, but must demonstrate to the Building Consent Authority (usually your local council) that the alternative design will perform equally as well as the Acceptable Solution.

Under the building code, claddings have to meet performance requirements for:

• Structural stability (Clause B1).
• Durability (Clause B2).
• Spread of fire (Clause C3).
• Ground or surface water (Clause E1).
• Weathertightness or external moisture (Clause E2).
• Hazardous building materials (Clause F3).
• Ventilation (Clause G4).
• Interior environment (Clause G5).
• Airborne and impact sound (Clause G6).
• Energy efficiency (Clause H1).

Forces acting on claddings

New Zealand’s geography and weather patterns exert unique forces on our buildings. These forces must be considered during the manufacture, use and maintenance of building materials and how these materials are brought together in buildings. Good design will consider how to resist these forces and how building materials and buildings will be protected.

Wind
New Zealand lies in the ‘roaring forties’ latitudes and experiences strong winds. This, combined with high rainfall, places a lot of stress on our buildings in terms of keeping them dry. Wind can cause movement in building structures and pressure differences can cause moisture to be forced through quite small gaps.

Temperature
Although New Zealand has a temperate climate, temperature changes can still occur quite quickly. These temperature changes cause building materials to expand and contract at different rates. This places stress where materials with differing rates of expansion and contraction meet, e.g. aluminium window frames with timber trims.

Moisture
When absorbent building materials become wet, or their moisture content changes due to humidity (e.g. timber), they will expand. Different materials expand and contract at different rates. As with temperature, this causes stress where different materials meet.

Earthquake
New Zealand experiences frequent earthquakes so our buildings are designed to withstand them. But because our buildings are regularly given a shake, joints can open up and cracking can occur if this movement is not allowed for.

Salt
Most New Zealanders live relatively close to the coast. Buildings within 500m of the coast are subject to heavy concentrations of salt laden sea spray. Even buildings further inland (up to several kilometres) can be subject to moderate amounts of salt laden air. This means building materials need to be chosen carefully to prevent corrosion. Even quite durable materials will need regular cleaning to prevent salt build-up causing a breakdown of paint and other protective coatings. In geothermal areas the corrosive effects of sulphur must also be considered during the design process.

Ultraviolet light
We are aware of the effects of exposing ourselves to too much sunlight. Ultraviolet light from the sun can also cause the deterioration of many building materials. Plastics and paints are particularly vulnerable to deterioration through ultraviolet light.

How moisture behaves

To be able to construct your home to keep water out you need to understand how water behaves.

Gravity
Water flows downwards under gravity. Even when water gets in by other means gravity will make it move downwards through the structure. However, gravity is not always the only influence and water may track upwards or across for short distances, until gravity again becomes the major influence. When looking at how water will behave at a joint, always imagine it flowing downwards.

Momentum
Momentum builds up in a water droplet as it falls or is blown by the wind. The faster it is travelling the more momentum it has and the greater the impact it will have when it hits a surface. A water droplet travelling at speed can splatter or splash on a surface and enter a building because of momentum.

Pressure
If there is no wind the pressure inside and outside a house will be the same. When wind blows on a wall’s surface it creates an area of higher pressure. The pressure inside doesn’t change so is at a lower pressure than outside. The air will always try to equalize the pressure difference. To do this it will flow through any small gaps or passageways that connect to the inside of the house. As it flows through the house it can carry water with it. This becomes particularly important when holes are constructed in the cladding for windows and doors.

Surface tension
Surface tension can be observed when rain drives water droplets onto the car windscreen. Droplets will form on the glass and can be blown across the screen because the glass doesn’t absorb the water. These droplets form because the surface tension of the water holds the droplet together. Droplets will cling even to the underside of a non-porous surface (such as a painted surface or metal) and once formed can be transported by wind flowing over them. Droplets can be dislodged by a sharp edge or break in the surface.

Capillary attraction
Surface tension causes capillary attraction. Water will rise up between two surfaces that are in close contact. This can cause water to enter through very small gaps e.g. between flashings and a cladding. Most people can remember doing an experiment at school to show capillary attraction. You would have placed two sheets of glass close together with a liquid between them, and watched as the liquid rose between the glass sheets. This is capillary attraction. It will also cause water to ‘wick’ or soak up through porous materials by moving through very small gaps in the materials structure e.g. through stucco or unsealed fibre-cement.

Movement of water vapour
Water vapour is water held in the air. It is measured by relative humidity and can transport moisture both into and out of a house. It does this by:

• Vapour diffusion – There will almost always be a temperature and humidity difference between the inside and outside of a building. Water vapour travels to areas of lower vapour pressure. The amount of moisture transported in this way is small and may assist or slow drying within the house.
• Condensation – Condensation occurs when warm, moist air strikes a cool surface. As the surface cools the air it causes water vapour to condense on the surface. The most common sign of this is water running down the inside of a window in winter. 
• Solar driven moisture – A wet surface that is heated by the sun will develop a high vapour pressure. This can force moisture to be driven into absorbent materials.

The combination of factors that cause leaky buildings led to the development of the '4Ds' approach to weathertightness and these are included in E2/AS1. The four Ds are:

Deflection
Keep water away from entry points. If water cannot impact upon or reach a joint or junction, then the likelihood of that joint developing a leak is greatly reduced. The first means of deflection is in the design of roofs, verandahs and overhangs that protect wall surfaces. By using facings, flashings, overlaps, water is deflected away from a joint to the outside of the building. Cladding can also be used to deflect water, but cladding cannot be relied upon on its own to keep water out.

Drainage
Allow for water that does get in to drain away. Claddings fixed directly to the framing provide limited drainage if water gets behind the cladding. The cladding underlay provides some drainage for more profiled claddings such as weatherboard. But many claddings hold water against the underlay and once water has got past the underlay drainage to the outside is unlikely.

By constructing a drainage cavity behind the cladding, water on the back face of the cladding can drain away. Many traditional methods of constructing joints provide for drainage back to the outside, and this greatly increases the building’s ability to deal with unwanted moisture.

Drying
Allow for water that does get in but doesn’t drain away, to dry through diffusion and ventilation. The drainage cavity allows diffusion and ventilation behind the cladding. This means water that has not drained away will be able to dry out. This is particularly important for claddings that cannot breathe because of external waterproof coatings, such as texture coated fibre-cement, stucco and EIFS.

Durability
Use durable materials. Many leaky buildings were caused by minor leaks causing a major failure of the structure of the building. Water which wasn’t allowed to dry or drain away rotted the timber which was not treated and therefore was not durable. Fungi established very readily and caused rot that spread beyond the immediate area affected. Timber treatment will prevent, or markedly slow, the establishment of rot and isolate the damage to the immediately affected area, greatly reducing repair costs.

It is not only the timber that must be durable, but also the fixings, finishes and all materials.

The combination of deflection, drainage, drying and increased durability ensure future buildings will be much more robust than those that have weathertight problems.