When talking with people interested in our product, they love the insulating properties of vinyl, the integrated fly screens, and the cost. The next question often asked is about vinyl stability in the harsher New Zealand sun. There may even be an anecdote of some other vinyl product (most often cladding) that was installed 20 years ago and just didn't stand up to the test of time. I can assure you that those vendors are out of business now.
The Window Association of New Zealand (WANZ) is an organization that ensures that windows meet all the relevant performance standards, including durability. Other important performance standards are strength, weather tightness and air leakage. The page concerning PVC windows can be found here. In summary, they recognize that New Zealand has nigher UV exposure than northern hemisphere countries and requires more strict UV resistance. As such, the normal EN12608 “severe climate” test is approximately equivalent to 2 years outdoors exposure in the New Zealand climate, further modification is needed to approximate the minimum 15 years durability requirement of the New Zealand Building Code. One modification is the change the formulation of the uPVC blend so that it has a minimum level of 8 parts per hundred resin, (8phr) Titanium Dioxide acting as a sunscreen to protect it from the damaging UV light.
As quoted from WANZ, "this option is based on the knowledge and experience of the uPVC manufacturing industry in New Zealand over the past several decades for other uPVC construction products. The New Zealand sunshine strength requires a higher level of “sunscreen” in the formulation mix to protect the uPVC from the strong UV light we have in New Zealand."
ThermaScreen Windows sources it's windows and doors from Pella Corporation. Pella has been around since 1925, and has been innovating since it's inception. A quick summary of achievements can be found at http://www.pella.com/pella-difference/. The PVC that is used in Pella windows exceeds the New Zealand requirements for UV protection by as much as 25% more 'sunscreen' than is needed.
In addition, I would like to share a personal anecdote. We were recently working with a remodel which had vinyl gutters installed decades ago. The gutters were removed and still found to be in good condition. They were still flexible and structurally sound even though they had been in full New Zealand sunlight every day. The gutters were exposed to more sun than windows are since the gutters aren't protected by an overhang or eaves. The same gutters were put back up again and are still in use. This is an example where vinyl with the correct formulation does stand up to the NZ sun.