Insulation not only keeps heat in during the winter months, it also keeps heat out during the summer months. It is a crucial aspect to get right when designing a new home or renovation – and a significant part of our design process.

Insulation essentially limits temperature transfer between outside and inside. A large proportion of NZ’s housing stock (typically villas and bungalows) have little or no insulation, and the industry was only starting to become more aware of its benefits from the 1960’s onwards.

This means that many homes of today would fail the current New Zealand Building Code, Clause H1: Energy Efficiency. Each design, whether new home or a renovation, needs to show compliance with this clause. Compliance must take into account the climate zone, method of construction and thermal mass of all major components (eg: floor, roof, claddings etc).

Some Basics | R-values and Criteria

When it comes to any discussion around thermal performance you will come across the term ‘R-value’, which stands for ‘absolute thermal resistance’. R-value is calculated using a formula, the result of which gives a measure of resistance to heat flow through a given thickness of material. In the most basic of terms, an R-value is a measurement of insulation’s effectiveness, and the higher the R-value the better. A high R-value means an insulator is more effective at preventing heat loss (or gain), than an insulator with a low R-value.

When it comes to choosing your insulation you need to ensure it’s;

  • intended for installation in the area you are putting it (walls, underfloor or roof).
  • correct thickness.
  • highest R-value possible for the thickness of your walls.
  • right width for the stud spacing in your walls.
  • compliant with the New Zealand Standard AS/NZS 4859.1 (check the compliance statement on the insulations packaging).*

Building Code v Best Practice

The required amount of insulation for the various areas of home (in R-value) will depend on the climate zone the home is being built within. As seen in the map below, the Wellington region is within climate zone 2.

The table above outlines the minimum R-Value’s required for the different areas of the home, depending on climate zone. We’ve previously discussed in our Homestar post, that the building code sets out legal requirements and not what would be considered best (or even sufficient) practice. In order to create a warm, dry and healthy home that is cost effective to heat and cool – we always design and specify to a standard superior to the building code, when it comes to insulation.

Types of Insulation

Insulation is available in many different variations, from a sheep wool blend to polyester. Motivations behind selecting insulation go beyond just what will get the job done. Some people are keen to opt for a more environmentally friendly choice, and some options are better suited to certain projects than others – think renovation v new build. For example, in some existing homes where pulling off wall linings and cladding to install insulation is not feasible, a foam based insulation can be injected or ‘blown in’ to small holes drilled into the cladding or wall lining. However for the projects we design, the two most commonly specified options are;

  • Soft glass wool – The typical go-to which includes brands like Pink Batts. This has a wide range of R-values and is available in various thicknesses and sizes to suit typical NZ framing spacings and a range of applications (roof, wall, floor).
  • Rigid polystyrene – Usually EPS, expanded polystyrene. We specify this for under concrete slabs, as cutting the sheets exactly right can be difficult between floor joists. It can be used around the perimeter of a slab too, to provide edge insulation. It is also used for insulating retaining walls, and available in different structural strengths and R-values.

Next comes the design of the walls themselves. The easiest way to achieve a higher level of insulation in walls, is to allow for more space to pack the insulation in – although, while we talk about ‘packing in’ insulation, you actually do not want to squeeze or compress a thick insulation into a small space. Insulation is designed to perform most effectively when it is in its natural, uncompressed state. Ideally, we would design 140mm thick walls, instead of the standard 90mm, in order to fit more insulation. This can be achieved easily for a new build or extensive renovation. However if you’re insulating existing walls, you’ll be limited to how much insulation you can fit within the existing wall cavities.

Tip: Up to one third of your heat can escape through the roof, so if you are thinking about upgrading your insulation, tend to the roof first. Replace or top up your level of insulation to at least a R value of R3.2. Next, look at the floor, if you are on timber piles and bearers then replace/top up with at least a R2.6. Both of these values are our recommendations, but if you can afford or feel the need for a higher spec, go for it. We’d advise remedying these two areas first, before even thinking about looking at your walls – but if you do want to do this, get a specialist involved.

Questions? We’d love to hear from you. At Mackit Architecture we design bespoke residential architecture for discerning clients. Based in Greytown in the Wairarapa, our team design beautiful, practical and enduring new homes or renovations both locally and New Zealand wide.

*guidelines adapted from Energywise’s guide to wall insulation – available here.