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Windows... Size Matters - Omega Windows & Doors

Windows... Size Matters

Why does size matter?

It matters a great deal when considering the proposed options for Window R values in H1.

H1 and NZS:4218 use an arbitrary unit size of 1.8 x 1.5m to generate generic Rvalue capability within one unit type, a fixed window with one sash. This generic offering of an Rvalue does not provide adequate information with which to make informed joinery and therefore Rvalue selections.

The MBIE proposed building code change document does not propose to use the industry developed window energy efficiency rating system or WEERS. The document doesn't even guide as to how to generate the R values.

What is WEERS?

The Window Energy Efficiency Rating System (WEERS) is a 6-star rating system for assessing the thermal performance of new residential windows. WEERS combines the thermal performance of the frame and glazing, together with the size of the window, to calculate an individual thermal performance rating for each window.

How was WEERS created?

WEERS was originally developed in a joint committee with EECA (Energy Efficiency and Conservation Authority), WANZ (Window and Glass Association), GANZ (Glass Association of NZ, note WANZ and GANZ are not combined to be WGANZ) and BRANZ (Building Research Association of NZ). EECA had originally put a considerable marketing budget on the table to allow our organisations to push the outcomes of WEERS. With the understanding that EECA would use the technical procedures outlined in WEERS to endorse an energy star label for windows. BRANZ created BAU579 as a working document, which was always meant to keep up with the pace of change in energy efficiency requirements.

At the beginning of the working group, WANZ & GANZ had expressed concern that there was ZERO ENERGY SAVINGS when isolating just the energy effects of increasing the Rvalue of windows. Then, unfortunately, EECA pulled out of the venture when their calculations proved that there were indeed ZERO ENERGY SAVINGS by isolating the Rvalue of windows.

However, it's a systemic failure of all parties when the working group (including EECA and their energy-saving calculations) failed to recognise and monetise the overall eco-system of a dwelling and the effects of health over the lifecycle of a home. Windows are but one part of a whole and that one part is often ridiculed by newcomers to the New Zealand market who seem horrified by the predominance of cold aluminium frames. But when even EECA couldn't recognise a reason to support increased R values, WEERS was created with little to no fanfare and just a small deliverance to industry.


Because just like the star rating of appliances, WEERS provided a platform for consumers to make informed decisions, based on comparable data. When window products are imported, and comparisons are made unit type to unit type, how do you compare NFRC against CEN modelling methods? What size was the unit when it was modelled? Hence, WEERS provided a methodology for comparison using CEN modelling methods.

Studies have found that the CEN and NFRC methods produce different U-values for the same window.

Additionally, prior to WEERS, the window and door industry was using a software package called THERM, which is the worst piece of software ever written (just my opinion). If anyone has ever undertaken a calculation using THERM, you will know the pain of errors and a 2-hour wait for the software to have a think about a calculation.

Moving to WEERS and using CEN modelling methods introduce the industry to the beauty of Flixo - Flixo's software is elegant, easy to use, and, reduced a half-day calculation to less than 20 minutes.

Hence, WEERS is a great tool that has not been supported by MBIE, even though the window and door industry in New Zealand has lobbied for the WEERS procedure to be referenced to ensure comparable methods of calculation can occur.

So why does size matter?

As stated above, H1 and NZS:4218 use an arbitrary unit size of 1.8m x 1.5m with one fixed light and one opening sash. But even with superwindows, the best performing part of any window unit is the glass. The bigger the glass, the better the performance.

Air matters, argon matters, edge spacer material matters. It all ends up mattering, and size is a huge factor. Looking at the Ucog (Uvalue of the centre of glass) you will see the variations of U value as the width of the spacer increases, with 16mm being the optimum, and beyond 16mm, the performance starts to decrease.


Let's next look at size compared with cold aluminium and thermally broken frames. I've shown the data 2 ways. Size compared to thermal frame and glass type. And thermal frame against glass type. It depends how your brain uses the data to show the comparable difference as the size increases.


These results tell me two things:

a) Never use thermally broken joinery with clear on clear glass. The cost increase compared to the results is not worth the effort. (please and thank you)

b) Rvalues are arbitrary and, these graphs explain why the construction industry continues to push cold aluminium with low E coatings - because cold frames achieve some pretty good Rvalues when manufactured with high-performance low E glass.

Actually, there are more than two things, but I'm running out of wind, and I'm sure you are running out of time to read this article.

To conclude:

My argument is a) size matters and b) Rvalues are not the right method with which to determine the best outcome for NZ homes and our population's long term health.

Working from a place of WHO's recommended internal temperature of 18degrees, the use of cold aluminium joinery is not the only cause of internal temperatures reaching below 18 degrees. My 1974 timber windows with 3mm glass reached a balmy 11 degrees in winter with zero insulation in the walls and floor. Windows are a part of an eco-system within a home, they are not an isolated component that causes devastation. Windows were the last commodity I upgraded (with floor and ceiling insulation being the first). I look forward to this year being my first winter with thermally broken windows.

But windows and doors are only a piece of the puzzle, not the entire puzzle itself.


Therefore, I know we are stuck with the "pick an Rvalue option and run with it" mentality. But I would like to argue that size does matter, and then internal temperature matters more.

Therefore, option "status quo" or option "halfway" does not cut it.


I will be supporting option 2, with the understanding that Rvalues should be a moving target with future endeavours to move industry to ongoing better performance, not a static diatribe consisting of a race to the bottom.

Windows & Glass Association