The BRANZ House insulation guide (HIG) has been an integral part of the process of insulating New Zealand homes for 45 years – with the first five editions available in print form and the latest 6th edition comprising a set of spreadsheets with more functionality than earlier versions.
When nationwide mandatory requirements to include thermal insulation into the exterior shell of houses were first introduced in 1977, a 37-page BRANZ publication Paper C1: A Construction Guide to Home Insulation provided examples of typical house construction and the impact of various levels of insulation added to floors, walls and ceilings.
Establishing minimum R-values
The minimum component R-values were in turn set by NZS 4218P Minimum thermal insulation requirements for residential buildings. The P represented provisional because it was envisioned that those minimum requirements would be increased in a few years once the building industry was familiar with the process of including insulation.
That was at least the plan. In practice, NZS 4218P didn’t become NZS 4218 until 1996, and it wasn’t until 2000 that it was an Acceptable Solution in the New Zealand Building Code. This was some 23 years after the start of the process and then only with an increase in minimum requirements for suspended floors to account for the typical draped foil insulation. The requirements for walls and ceilings were unchanged from what they were back in 1977 and much less than what was envisioned.
Meanwhile, Paper C1 evolved into the 1st edition of the HIG in 1995 ahead of the much-anticipated NZS 4218:1996. The 2nd edition was published in 2005, the 3rd in 2007, 4th in 2010 and the 5th and last edition in print format in 2014.
Evolving with schedule changes
I’ve had the pleasure of being involved with the development of the HIG since the 1st edition, but it wasn’t until the 3rd edition that I played a significant part in its production. The 3rd edition in 2007 coincided with the first significant increases in the minimum H1/AS1 schedule method values when ceilings went from R1.9 to R2.9 and walls went from R1.5 to R1.9. Without a doubt, the most challenging has been the latest edition – both because of the change from print to spreadsheet format and because of the change in purpose.
The timing of the first five editions was tied to changes in the Building Code requirements, the way houses are constructed and the materials that are used. The range of options available to the industry has continued to expand and the HIG has had to correspondingly expand until it became clear after the 5th edition that the print format was becoming impractical.
For the first five editions, the HIG updates simply reflected the established construction practices at the time, and the changes to those practices were generally being introduced ahead of the mandatory
requirements for higher component R-values so the additional material needed for the HIG revision was already evident. For example, when the 2007 requirements were introduced, the R-values of walls, other than masonry, were typically already meeting the new R1.9 requirement.
The impact of H1
The changes introduced by the 5th edition of H1 are arguably the most dramatic of all the previous updates to the Building Code energy efficiency provisions for housing. The 6th edition of the HIG didn’t have the luxury of simply reflecting what the industry was largely already doing as with the earlier editions.
The building industry was in effect waiting for the HIG to help them design new insulation products and construction solutions. Without knowing what would become the preferred solution, the BHIG needed to provide a much larger range of options – many not currently in use.
Traditionally, the HIG was seen as only being of relevance to the schedule method of H1/AS1 but its purpose has expanded to providing reliable and traceable inputs for both the calculation and modelling methods, including how to insulate to levels well above the minimum values specified in the Building Code. A good example is the greatly expanded range of options for walls despite the minimum requirements for walls remaining at the same level as previously.
Advantages of the spreadsheet
A major advantage of the online spreadsheet format is the ability to regularly update and expand the HIG as new solutions are developed and solutions found for how to install thicker insulation and how to address moisture management concerns with higher levels of insulation or new materials.
Another advantage is the ability to include tabular data with the higher resolution now needed by industry. Most of those updates have been developed in conjunction with the building industry, including:
- difficulty including floor slab perimeter insulation between house and garage and under the garage door
- the need to determine the component R-value of an opaque door leaf and frame
- compressing insulation to achieve the required minimum R-value at the edge of a roof
- better accounting for excess framing in walls and some framing fabricators now able to provide detailed information on the actual framing fractions/ratios
- future needs for deeper wall framing to allow for higher component R-values and/or lower embodied carbon insulation materials such as wood fibre
- a greater range in the thickness of the metal used for steel framing
- external or internal secondary insulation layers
- the inclusion of insulation under the footings of slab-on-grade floor systems
- filtering on input data to ensure more reliable outputs
- warning messages on some inputs to encourage better design
- more specific details of two-layer insulation solutions for roof spaces
- greater resolution for input data such as product R-values and material thermal conductivities.
Feedback from users has also been an essential part of the development process. We are very grateful for that input and look forward to more in future.
Still to come
Plans for future updates to the HIG include:
- linking the HIG to the BRANZ H1 calculator
- inclusion of product data from BRANZ Appraisals, CodeMarks and testing
- adapting the output format to better target council needs.