Sure window frame material matters, but so does geometry (NZBS6)

The models above show a really simple concept: window frames will conduct heat differently depending on both their material (obvious) and geometry (maybe not so obvious). All three of those models are aluminium frame windows with the same glass, same interior and exterior conditions, and same material. Drastically different temperature gradients and drastically different R-values due to just the differences in geometry.

In New Zealand, the thermal performance of windows in determined through NZS 4218 (kind of). This standard gives you a table to pick your window R-values. This method allows you to assume every aluminium window frame conducts heat exactly the same. R0.26 is the default value for a double glazed aluminium window in New Zealand. That is where the building code is set. Frame geometry need not be considered in this country, just the material and the glass.

Strangely, the same generic non-thermally broken aluminium windows with double pane glass are widely considered R0.17 in Canada. That’s because thermal modelling software, per NFRC, calculated that value (New Zealand theoretically also uses NFRC-100 for modelling, more on that below). Sail those same windows across the Pacific and apply far less sophisticated (and less accurate) methods and they become R0.26. Canadian building code requirements for windows left this technology behind 15 years ago…

Of course you could prove the actual R-value of your product in New Zealand using free software (THERM) and an experienced thermal modeller. NZS 4218 let’s you do this (if you’re that interested) and directs you to BRANZ “Standard Window R-value Calculation Procedure (SCP)” which then directs you to find a list of approved modellers from WANZ. Here’s the fun bit, there is no list! So you can’t follow the thermal modelling pathway to determine the effect your frame geometry has on the actual R-value of your product which leads the whole industry back towards mediocrity; gleefully thinking all aluminium windows are created equal.

You can’t generically combine an IGU with a frame and get a standard R-value, it’s just not the simple. The spacer bar, the position of the window in the opening (align with the insulation) and geometry can vary the actual performance by orders of magnitude.

WEERS is a new voluntary performance standard aimed at moving these generic R-values towards more realistic values. This is a shift towards more accurate performance numbers and is the first step towards getting better windows and homes in New Zealand. Unfortunately, I suspect modelling will show their windows are worse than than the building code so it’s hard to see voluntary adoption taking off any time soon.

Generic schedule tables are useful if they are granular enough to give you accurate results. Unfortunately, windows don’t fall into this category.

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