SIZE MATTERS: reducing building size is a great way to decarbonise.

When it comes to decarbonising buildings, size really matters. In fact, it matters almost more than anything else, and reducing building size offers the wonderful bonus of an instant reduction in cost as well as in carbon. In the wicked problem of a housing affordability crisis in the midst of a climate crisis, that double whammy benefit is a chance we should jump at.

Here we set the context for reducing carbon by reducing building size. In the “Sweating the Detail” series we’ll explore tips and tricks to help achieve that.

In 2020 BRANZ published ground-breaking research into the carbon footprints of our houses, and compared them with our Paris Agreement commitment. It was summarised in “A carbon budget for New Zealand houses” (free at branz/pubs/research), and the picture is pretty stark: the ten stand-alone houses they studied each cleaned out their entire lifetime carbon budget before anyone had even moved in (red dotted “budget” line against green “materials” columns), sometimes several times over.

And it gets worse! That’s for a 90 year life, not the 50 years required by the Building Act, and BRANZ reports our carbon-intensive post-Covid building boom cleaned out the carbon budget that, in 2020, they had allowed to last until 2050.

But the news isn’t all bad. When it comes to size, we’ve a lot to work with, because it’s easy to forget just how big our houses have become, and especially stand-alone ones.

StatsNZ offers some striking insights, such as in the graph below, showing Auckland’s average stand-alone house floor area expanding a whopping 73% in the 22 years to 2013, then contracting 15% since (stats.govt.nz/new-homes-smaller).

Before 1991, houses were smaller again, in 1974 averaging about 110 m2 and, before that, the “3 bedroom 1000 square foot” house (93 m2) was considered an acceptable entry-level size.

The current average of multi-unit houses is about 100 m2, and it’s largely the prevalence of apartments and medium density housing shown in the chart below that has pulled down the “Total homes” line in the graph above.

Carbon footprints (or dollar footprints for that matter) don’t come down quite in proportion to floor areas, but do come down significantly. And one of our most carbon-intensive elements, the concrete floor, does come down in proportion to building floorplate.

But it’s not just about floor area. The trend to build a 2.7m stud instead of 2.4 m typically adds over 10% to cladding areas and wall linings, incurs extra wall framing (especially when extra height demands closer or larger studs), and increases bracing demand. The typical net effect is 2% more carbon.

The chart below (part of the Clearcut dashboard) shows the effect of reducing from 225 m2 to 175 m2 (dark blue and light blue), and of raising the stud from 2.4 m to 2.7 m. (light blue and red).

It’s also worth reflecting we each unwittingly add to the “shifting baseline”, the  progressive nudging up what we see as “standard”, so that yesterday’s luxury becomes today’s norm.

A few years ago, Resilienz investigated this (for a talk with the challenging title “Architecture and Extinction”!). Here are our notes just regarding children’s bedrooms:

So, the scene is set, we’re blowing the carbon budget of our houses many times over, and size is a big part of that problem. Check out our article “Smaller is Beautifull” for tricks how to tackle that.