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A direct, informal summary, using colloquial and easy to understand language, for everybody out there. For architects, builders, fabricators, the public.

Before we start

Please note that this blog was posted during the consultation session by MBIE, before the code changes were announced. Interesting to see how we thought back then! This blog is now out of date, stay tuned for our next blog. 

What do we know?

Change is definitely coming, to a greater or lesser degree. MBIE has proposed three different levels of change, and we know they will settle on one of them, possibly with some modifications.

There will be 3 new climate zones on top of the 3 current zones as per this map:

We are pretty sure (but can’t be certain) that the required R values of residential windows will change to option 2 as per the below chart:

This means, critically, that for most of the country, standard aluminium window frames will need very high-performing glazing to comply with H1/AS1, and if not getting them past council will either be difficult or impossible.

Note that this table doesn’t apply to large/commercial buildings – there is another table for that.

Is it really that simple?

No. There are holes and complications in the proposal, and how it affects aluminium joinery. The biggest is that this proposal is treating the individual building components separately, rather than as a system, which ignores the way that all the building components work together for overall performance. However, that is too big a topic for us to cover here.

Other complications:

Window Size:

Larger windows have better performance than smaller windows because the ratio of glass/frame is higher. This means that every single window in a house lot will have a different rating…

Window configuration:

A window with a mullion in it will have worse performance than the same window with no mullion in it. Every change of configuration will affect the window performance.

In the proposal are “generic” U value performance numbers for different types of window frame with different types of glass. However, these generic numbers are old and out of date… modern glass and modern frames could vary substantially from these numbers.

How the windows are installed has a massive effect on thermal performance, which is not mentioned in the proposal. Lower quality windows installed well might outperform high-quality windows installed badly… but this would be missed.

What we dont know

We don’t know for certain which of the three proposed options MBIE will choose, or whether they will modify any of the proposals which they choose. However, it does seem 90% likely that they will choose the middle option.

We also don’t know how long a “roll-out” period they will determine. However, it seems likely that it will be tied to the extremity of the option they choose – option 1 could be implemented within 12 months, option two could probably be implemented in 24 months, and option 3 in 36 months or more.

We don’t know how the system will all work in practical reality. For example, the status quo is that windows need an R-value of 0.26, however, this is not a number currently asked for or measured. If they don’t police it at the moment, what method will they use to police it moving forward?

How are the r values of joinery calculated?

The thermal performance of windows is generally calculated in R values (in NZ) or U values (overseas). There are plenty of articles on the internet explaining these in more detail, which we recommend looking up.

What we need to know here is this: U values are loosely the inverse of R values. So dividing one value by the numeral 1 gives you the other; for example, 1/R = U, and 1/U = R.

The way to calculate the R-value of a window is roughly as follows:

(Uglass x Area of glass) + (Uframe x area of frame) + (PSI glass spacer x length of glass spacer)

                                                                     Total area (glass + frame)

Once you have this number (you can get the U values of the glass, frame, and the PSI value of the spacer from suppliers), then you can get the inverse of it, and that is the R-value.

What this means is that there are six variables to play which affect the R-value:

  1. U value of glass (normally somewhere between 0.6 and 3)
  2. Area of glass (dependent on the size of the window)
  3. U value of frame (normally varies from 8 for standard aluminium, to 3.5 for thermally broken aluminium, and down from there for UPVC and timber.
  4. Area of the frame (depending on frame design and size of the window, number of mullions, sashes, etc)
  5. PSI value of the glass edge spacer.
  6. Length of glass edge spacer (dependent on window layout and size)


Currently, we aren’t required to consider the performance of the install detail, so these are the only six variables – simple right! Of the six variables, three of them are tied to the actual size of windows, not the design of the suite, so will be a consideration for architects and designers when planning the size, location, and shape of the windows.

The other three variables relate to the thermal performance of the frame, glazing, and glass spacer. The easiest two to change are the glazing and glass spacer; there are “warm edge” spacers available, and glazing can be specified with increasing levels of Low-E coating, thicker spacers, and gas infill, all of which improve performance incrementally, but also increase cost.

The last variable is the selection of the joinery suite, which goes from standard aluminium frames to thermally broken frames, with timber and UPVc being at the highest performing end of the market; once again increasing cost with performance increases.


We don’t know how this will all work yet. However the three broad options are below:

  • Per window – this will almost certainly be difficult to implement, as we can see that large windows will have better ratings than small windows. This would mean that small bathroom windows wouldn’t comply, even though in reality they may not cause a problem.
  • Per houselot – this is somewhat likely what will happen, that the windows will be considered together in a houselot. This would need to be calculated at the design stage by the joinery software.
  • Per sample size – this is also somewhat likely. MBIE have put forward a window of 1800 x 1500 with a central mullion, with an awning window on one side and a fixed light on the other. It could that each suite and glass makeup is tested by a third party, and a certificate issued. Then it can be assumed that no matter the size or makeup or the window, it would be deemed compliant.

What else do i need to worry about?

This covers most of it for windows…! However, the changes don’t just affect windows, they also affect quite several other areas of the building, from walls, roof, and underfloor details. If you are involved in these areas of building, there’s lots more learning to do!

It should be mentioned that the acceptable R values for commercial buildings will also be increasing – from the current status quo of zero. It will likely mean that double glazing is required for all exterior joinery in these situations, which shouldn’t cause much of an upset to the market – lots of this is already being done.


We here are Stärke are very excited; it’s a long overdue revision of the standards, and it brings significant change to a market which is somewhat stagnant… proof of this is that the current standards, as admitted by MBIE, are less than half the performance of internationally accepted standards!

Stärke currently has 4 broad suites, from our 35mm residential40mm architectural52mm thermally broken aluminium, and commercial suites. We are extremely confident in the performance of the suites and are eagerly awaiting confirmation from MBIE of the new standards. We also are passionate about the install detail, which currently isn’t mentioned, but we’re going to go right ahead and get a new one approved anyway.  More on that in another blog post soon… stay tuned!