When you are comparing insulation, the R-value is the number that matters most. It is also the number that people most often get tangled up in. Here is what it means, what the minimums are for rentals, and how new builds differ.
What an R-value is
R stands for resistance. An R-value measures how well a material resists heat flow. The higher the R-value, the better the insulation slows heat moving through it, so the more it helps keep warmth inside in winter.
A few things to know:
- It is printed on the packaging. The R-value of a product at a set thickness is displayed on the packaging, so you do not have to guess.
- Layers add up. If two products sit on top of each other and both are in good condition, their R-values are added together.
- Thickness is not the whole story. Different materials have different R-values at the same thickness, so a thicker product is not automatically better.
Minimum R-values for rentals
Rental homes have a minimum standard for ceiling and underfloor insulation. MBIE’s Healthy Homes insulation guide gives the minimum R-values for new or topped-up insulation:
| Zones 1 and 2 | Zone 3 | |
|---|---|---|
| Ceiling | R2.9 | R3.3 |
| Underfloor | R1.3 | R1.3 |
In most situations, existing ceiling insulation also needs to be at least 120mm thick. The guide has a map of the climate zones for this standard.
These are minimums for rentals, not a target for every home. You can read more in insulation requirements for rental properties under the Healthy Homes standards.
New builds are different
New buildings follow Building Code clause H1, which has six climate zones and its own rules. For a time, H1 had a simple table of minimum R-values for each part of the house. In the fifth edition of the H1/AS1 Acceptable Solution, for example, that table listed R6.6 for roofs and R2.0 for walls in every climate zone.
The sixth edition, effective 27 November 2025, removed that table. New builds are now shown to meet H1 through a whole-building calculation or modelling. That means there is no single R-value for every new build, and your designer works out what your plans need.
If you want to see where those figures come from, they are in MBIE’s fifth edition and sixth edition documents. We explain the changes in understanding the H1 insulation standards.
Why the installed R-value can differ from the label
The number on the packaging is for the product as designed. How it is installed affects what you actually get.
- Compression. Insulation that is squashed loses effectiveness. That is why ceiling batts should sit fluffy and full, not be pressed down by storage or foot traffic.
- Gaps. A gap at the edge of framing or between batts lets heat bypass the insulation.
- Dampness. Wet insulation does not perform as it should, and the source of the moisture needs fixing first.
- Clearances. Around downlights and flues, insulation has to stay back for safety. Along the edge of a roof space there is also a minimum gap between the insulation and the roof underlay, so the product at the edges is sometimes thinner.
Choosing the right R-value for your home
For most existing homes, the practical question is how much your ceiling and underfloor can take, given access and clearances. Our assessment looks at what you already have, then recommends what fits.
Our ceiling insulation uses Knauf batts, and our underfloor insulation uses Mammoth products. If your ceiling has old loose-fill insulation, we may recommend Insulfluff removal before new insulation goes in.
Guide prices are $1,600 to $2,000 for a ceiling and $2,200 to $2,500 for underfloor, for a 100m² home. Your quote depends on access and materials.
Get a clear answer for your home
Get a free, no-obligation quote and we aim to call you back within 48 hours. You can also call us on 027 482 0333.