Radiant Godzone

New Zealand’s climate presents a unique challenge for architects. While ambient temperatures may be lower than those experienced in many parts of Australia, Europe or North America, buildings here are subjected to some of the harshest weathering conditions in the developed world.

The combination of high ultraviolet (UV) radiation, intense solar exposure, humidity, salt-laden winds and fluctuating temperatures creates an environment where colour and coating performance must be considered as carefully as any other aspect of the building envelope.

Why do buildings in New Zealand seem to weather differently from those in many overseas markets?

The three main causes of paint weathering are UV radiation, moisture and heat. Of these, UV is generally the most aggressive. New Zealand’s relatively clear atmosphere and southern hemisphere location mean coatings can be subjected to significantly higher UV levels than those experienced across much of Europe.

A summer’s day (when the sun is at its closest) in the north of the country can have a UV Index of greater than 14, while a winter day in the South would typically have an index of about one (measured on a clear day around noon). When moisture and UV are both present, like early morning whilst dew is still present, these twin aggressors can produce the most damage. Whilst heat doubles the rate of chemical change for every 10 degrees Celsius increase (rule of thumb), it also drives away water giving an overall positive effect.

Is heat or UV the bigger problem for paint performance?

Both matter, but UV is typically the primary driver of long-term coating degradation.

UV radiation attacks paint binders and pigments, causing fading, chalking and loss of gloss over time. This is particularly true of many synthetic bright reds, which can be very sensitive to UV fading. Interestingly, red iron ochre pigment is totally UV resistant.

Heat then compounds the problem by increasing surface temperatures, accelerating ageing processes and creating greater movement in substrates and paint films.

Resene has invested time – 80 years – and money into the research and development of some of the world’s most durable pigments and tinters, to produce a range of products specifically manufactured to withstand the harsh climatic conditions across Godzone.

How significant is radiant heat compared with air temperature?

Often more significant than people realise. Anyone who has stood in direct sun and then felt immediate relief when a cloud passes overhead has experienced the impact of radiant energy. The air temperature changes very little, but the reduction in solar radiation is noticeable instantly.

Research carried out by Nomoto et. al. (2024) into thermal comfort suggests that some wavelengths of infrared radiation create a greater sensation of heat than others. They consistently found that for the same energy quanta, far-infrared radiation produced a greater degree of ‘uncomfortable heat sensations’ than near- or mid-IR wavelengths, reinforcing the idea that radiant energy can have a significant effect independent of air temperature. The same principle applies to buildings and exterior surfaces.

Does New Zealand’s climate create challenges beyond UV exposure?

Absolutely. New Zealand’s long, thin shape and alignment with the prevailing nor’westers, mean many buildings experience wind-driven rain and salt deposition. Salt aerosols can travel surprisingly long distances inland, up to 100kms from shore, affecting buildings well beyond coastal locations.

Many New Zealand buildings are also constructed from materials such as timber and galvanised steel, which require coating systems capable of withstanding environmental stresses over long periods.

Why can’t overseas testing alone predict performance in New Zealand?

Because New Zealand conditions are unique. Paint manufacturers use both accelerated laboratory weathering and natural exposure testing to evaluate products.

However, artificial testing cannot perfectly replicate the combination of UV intensity, moisture, temperature and environmental conditions experienced outdoors.

Multiplying the effect of natural weathering can be achieved by exposing panels at a site closer to the equator; angling the panels to maximise UV exposure; rotating the panels to follow the sun from rise to set; adding reflecting mirrors to increase the units of UV hitting the panels and adding water spray to the assembly. Resene has such sites at Upper Hutt and Tauranga and a non-endorsed tropical site just outside Nadi, Fiji.

This is why local exposure testing remains so important. Products that perform well in Europe or North America do not automatically deliver the same results in New Zealand.

What’s the key takeaway for architects?

In New Zealand, colour specification is about more than aesthetics. High UV levels, radiant heat, moisture and coastal influences all affect how a coating performs throughout its life.

Understanding these environmental factors – and specifying products that have been designed and tested for local conditions – can make a significant difference to durability, maintenance cycles and longterm building performance.

When it comes to paint performance in New Zealand, what you don’t see in the atmosphere can be just as important as what you do.

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blackwhite magazine

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