A study in the price of wall framing construction

Fig 1: A standard timber frame for a townhouse development in Central Christchurch.

Background
The work behind walls

‍Healthy homes need solid walls to hold the roof up.
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Walls are a multipurpose item in the home, they provide height, bracing from wind and earthquakes, something to fix windows too, wind and weathertightness and of course insulation from the elements.

They are, or should be, the puffer jacket to the beenie. But we all know there are different standards of puffer jacket. Some work and some don’t.

Back in 2021 I wrote an article called “The Death of 90mm Framing”, https://www.linkedin.com/pulse/death-90mm-framing-damien-mcgill/, looking at what might happen when the new H1 regulations came into force and 90mm framing wasn’t sufficient anymore.  Unfortunately, MBIE backed down to builders and R2.0 became the minimum insulation value across the motu, purely based on the fact that this is about all the insulation you can get using candyfloss and lollipop sticks 90mm deep.  

However, research by BRANZ and Beacon pathway showed that the amount of timber in walls was being seriously under-estimated. https://www.branz.co.nz/pubs/research-reports/er53/Unfortunately, nothing has been done about this either.
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Fig 2. Copious amounts of timber reduce the ability of the to be insulated. A compromise with consideration of structural requirements is required. This is not it!

I regularly see 90mm pre-nailed frames being erected on building sites with copious amounts of timber, (almost) like the frame and truss manufacturers sell timber by the lineal metre and the more they put in, the more money they make.  
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I also regularly ask designers and builders about whether they are getting value for their framing. The response is often a shrug of their shoulders, so I thought I should do something about it, myself.  

Efficient Framing

‍An efficient timber frame is one that provides just enough timber to fix additional components too; sheet material, like ply or plasterboard do the heavy lifting in terms of bracing for stability. Cladding needs battens to fix too, and these in turn fix to the framing. So framing fixings can be dependent on the final cladding finish. Internally, there may be a requirement to have timber to fix the kitchen bench too for example. Every additional stick of timber, adds unnecessary cost to the build and reduces the potential warmth of that wall.  

‍This is because pinus radiata lumber is almost four times more conductive than standard insulation.

The easiest way to make framing more efficient is to remove the dwangs /nogs that are still being installed in our walls.

‍Did you know that each row of dwangs/nogs costs more than $11/m² using 90mm framing and $25/m² using 140mm framing?

Legislation

‍NZS3604:2011 is, or should be, the house builder’s bible. Often when I suggest removal of dwangs I get told the “Building Code” requires them.  

So, what are the requirements?

NZS3604 8.5.4 Lateral support of studs

‍All studs shall be laterally supported by either:
(a)   Exterior wall claddings complying with E2/AS1 or interior linings complying with section 12. Such material shall be fixed to the studs by direct nailing of cladding or lining material, providing that building paper or similar material not exceeding 3mm thick may separate the lining or cladding material from the stud; or
(b)   Dwangs, walings, or metal angle walings in accordance with 8.8

8.8 Dwangs and Walings
8.8.1     Dwangs, walings, and metal angle walings, where required by 8.5.4m shall be spaced at not more than 1350 mm centre-to-centre and shall be of not less than the following dimensions:
(a)    Dwangs: 45 mm x 45 mm;
(b)   Walings: 70mm x 19 mm;
(c)   Metal angle walings; 22 mm x 22 mm x 1.2 mm angle.  

‍Dwangs – moving with the times https://www.buildmagazine.org.nz/articles/show/dwangs-moving-with-the-times from @BRANZ build 176 in February 2020 starts off by saying  “The need for dwangs, those horizontal blocking pieces used between wall studs, has lessened over the years. Using fewer dwangs and making them narrower than the studs could improve a building’s thermal performance.”

‍H1 Energy Efficiency is a section within the Building Code that provides for the efficient use of energy and sets physical conditions for energy performance.  I wrote about the changes in the Abode Magazine. https://www.abodemagazine.co.nz/home/2023/4/18/building-changes-for-a-healthier-new-zealand

H1 requires all walls to have an as constructed insulation rating of R2.0 across New Zealand, using the schedule method.  The combination of high timber content and standard internally fixed insulation, means that achieving this legal requirement isn’t actually possible in many cases.  

I’ve always wondered what would happen if someone took a case to court or to the Commerce Commission?

The Promoted Solution

‍Since 90mm framing provides insufficient insulative space, the proposed solution is the change to the deeper 140mm framing, but is it? Even BRANZ is promoting 140mm framing as the solution https://www.branz.co.nz/pubs/bulletins/bu686-building-with-140-mm-timber-exterior-wall-framing/

However, I don’t think it is. In fact, I am sure it is not!
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The Study

‍I hired a Quantity Surveyor in September 2023 to price eight separate framing options; some with dwangs, some without in both 90mm and 140mm framing, to show the difference dwangs/nogs make, both to the price of framing and to the insulation rating of the wall as a whole.

The results can be found here