Sun-smart materials

This Issue This is a part of the Uniquely NZ feature

By - , Build 142

Polymeric building materials can perform poorly under our very high solar radiation and uv levels. That makes it very important to confirm the building materials you are using have the appropriate uv stabilisers.

Figure 2: Solar irradiance figures for New Zealand. Map provided by NIWA.
Figure 1: Our most populated centres correlate to latitudes equivalent to the Mediterranean.
Figure 3: Maximum annual UV Index values across New Zealand. UV Index map provided by NIWA.

THERE HAVE BEEN NUMEROUS ENQUIRIES at BRANZ recently about weathering of polymer-based materials for coatings, building underlays, flashing tapes, spouting, wall coverings and glass-reinforced plastic.

Some people wanted to know how to specify and commission testing that demonstrates fitness for purpose, and others how to confirm and demonstrate that a supplier’s testing covers sunlight exposure in New Zealand.

Our sunlight and UV

The intensity of sunlight varies with latitude, and the areas with the highest solar radiation are generally in the band of latitude 15–35°. New Zealand lies between 34–47°.

The equatorial belt – 0–15° – has less solar radiation due to high atmospheric humidity and more cloud cover. Higher total solar radiation usually results in more UV radiation.

This means that, while parts of Australia receive much more solar radiation than New Zealand, UV levels here are still higher than Europe and parts of North America. Our most populated centres correspond to latitudes equivalent to the Mediterranean or California (see Figure 1).

NIWA estimates that, on average, only about half of the radiation at the top of our atmosphere reaches the ground. The rest is absorbed or reflected by gases, clouds and dust in the air.

Figure 1: Our most populated centres correlate to latitudes equivalent to the Mediterranean.

Cloud is the most variable of these factors, changing with both time and location. On average, cloud reflects about 20% and absorbs about 4% of all incoming radiation. Available solar energy under clouds can be less than a tenth of clear-sky values.

Getting specific with solar irradiance

Overall, New Zealand is sunnier than most of Europe and North America, and its peak UV is higher due to the lack of pollution and closer approach to the sun during summer than in the northern hemisphere.

The annual solar irradiance figures for New Zealand (see Figure 2) show that the irradiance in the north, including Kaitaia, Auckland, Bay of Plenty, Hawke’s Bay and Taranaki, are marginally higher than the Wellington region, with a typical annual solar irradiance of over 5.3 GJ/m² per year. Miami, in the US, typically has 5.6 GJ/m².

Nelson, meanwhile, has similar irradiance to more northern areas. The higher proportion of clear-sky days around Nelson and Blenheim and over the Manawatu, Bay of Plenty and Hawke’s Bay, explains the higher solar irradiance there. Northland benefits from its higher sun angles.

The trend of decreasing solar radiation intensity continues for locations further south, with Westport and Christchurch having typical annual solar irradiance of 4.67 and 4.83 GJ/m² per year respectively.

Due to altitude and clean air, Central Otago, and the central plateau of the North Island have higher annual solar irradiances. Queenstown has an annual solar irradiance of about 5.2 GJ/m², as does Ohakune.

Finally, in the extreme south, Invercargill receives an average annual solar irradiance amounting to roughly 4.4 GJ/m² per year. This is still considerably more than the 4.0 GJ/m² received by the southern UK.

UV levels high in NZ

Total solar irradiance at a given location doesn’t tell the whole story when thinking about the photodegradation of building materials or burning of the skin.

Most polymeric materials are mainly degraded by the UV component of sunlight, as is skin. In other countries, the UV is attenuated by pollution and dust in the atmosphere. This doesn’t occur to the same extent in New Zealand, and as a result, our UV levels are high.

UV intensity can also be simply represented in terms of the UV Index, a scale derived from the intensity of the incident sunlight, weighted to the sensitivity of human skin. As an approximate guide, a UV Index of 12 corresponds to a burn time of about 12 minutes, a UV Index of 6 corresponds to a burn time of 24 minutes and so on.

From the summer of 2010–11, public UV information has been provided with the sun protection alert period, which corresponds to the times when the forecast clear-sky UV Index exceeds 3. Figure 3 shows the maximum annual UV Index values across New Zealand.

Daily UV Index (UVI) forecasts for specific locations are available from the NIWA website. UVI typically exceeds 13 in Northland during summer. For comparison, the peak UVI in Miami is 11.

Figure 2: Solar irradiance figures for New Zealand. Map provided by NIWA.
Figure 3: Maximum annual UV Index values across New Zealand. UV Index map provided by NIWA.

Impacts on building materials

Past BRANZ research into the natural weathering of building plastics at nine exposure sites around the country showed significant differences in the rates of photodegradation of polymeric materials across New Zealand.

The results were broadly as expected given the variations in solar, and particularly UV, intensity. Detailed results are available in BRANZ Study Report 182 Weathering of polymeric materials in New Zealand, available from the BRANZ website www.branz.co.nz.

Quick checklist

Consequently, when assessing test results from a supplier:

  • check the tests are to established standards or procedures, for example, ASTM, or are documented and available
  • check the irradiance meters and other instruments used have been calibrated so they are traceable to the national measurements standards of the laboratory or another country.

Most of New Zealand in severe zone

Measurement standards for polymer building materials, for example, BS EN 12608, typically set different requirements for moderate and severe climate zones.

These zones are defined by the annual solar energy on a horizontal surface and the average daily maximum temperature of the warmest month. A significant fraction of New Zealand rates as a severe zone, and products sold for use here should meet the more stringent overseas requirements.

In reality, most commercially available products will contain additive packages, such as UV stabilisers and antioxidants, to enhance durability.

However, manufacturers and specifiers should be aware that the correct choice and accurate incorporation of these additives is crucial to obtaining appropriate service lives from exposed materials.

For more

Watch this research in action on YouTube at www.youtube.com/watch?v=vkTv79KrfzI.

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Figure 2: Solar irradiance figures for New Zealand. Map provided by NIWA.
Figure 1: Our most populated centres correlate to latitudes equivalent to the Mediterranean.
Figure 3: Maximum annual UV Index values across New Zealand. UV Index map provided by NIWA.

Advertisement

Advertisement