Insulation & Thermal Envelope
The Challenges:
NZ homes are subject to extremes of temperature across the seasonal cycles and across the geographical differences between subtropical Northland Te Taitokerau and chilly Southland Murihiku. The challenge of maintaining steady consistent temperatures – for the health, wellbeing and comfort of the people living in them and to support the efficient operation of ventilation and other systems in the home – requires effective insulation.
The principle is simple – a basic protective layer to snugly secure the thermal envelope of the building, minimising heat loss or overheating, and reducing the load on the heating or cooling systems, so reducing ongoing energy costs.
Insulation works by slowing the transfer of heat, whether inside warmth leaking out in winter, or outside heat raising interior temperatures in summer. Good insulation will help maintain liveability, health and energy efficiency throughout the year.
“While insulation is often thought of as something mostly important for winter, it’s still effective in the summer months, keeping homes cooler and minimising the transfer of outside heat to the inside.”
Archipro:
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Different types of insulation:
A range of different products and methods are available to deliver optimal insulation for each home and budget:
Fibreglass (the famous pink Batts) commonly used in walls, ceilings and roof spaces – can be made from recycled glass and include fire retardant and anti-microbial elements
Rigid foam board commonly used in exterior walls, foundations, roofs and under concrete slab floors – manufactured from expanded polystyrene (EPS), extruded polystyrene (XPS), or polyisocyanurate (PIR)
Polystyrene, commonly used for underfloor insulation and in walls and ceilings
Structural insulated panels (SIPs) which can utilise a range of materials
Wool insulation, used in walls, ceilings and roof spaces – a natural, sustainable, renewable product which can be made from recycled wool and recycled itself
Straw bale insulation, used in walls and ceilings – also a natural, sustainable, renewable product
Mineral wool or rock wool, used in walls and ceilings – made from stone and industrial waste, usually with a high recycled content
Polyester, used in walls, ceilings, roofs and underfloor – some products include recycled plastic
Spray foam insulation – often used in renovations of existing homes
Sisalation or metal foil insulation still found in some older homes, although it is now banned to install or repair sisalation due to safety risks with potential exposure to electrical wiring
What is the R-Value?
The standard measurement of the effectiveness of insulation systems and materials is the R-value. This records the resistance to heat transference – the higher the R-value, the better the performance of the insulation. Other factors are also important:
“The R-value of any part of the building depends not only on the insulation but also on the thermal performance of other elements such as the framing and cladding… High density materials such as concrete, brick or stone… have low R-values and so are poor insulators. Thin metals such as profiled steel claddings and fibre-cement sheets also have low R-values and are therefore also poor insulators.”
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What is Thermal Bridging?
Thermal bridging is when a structural element cuts through the insulation, lining or cladding, making a connection, or bridge, between the interior and exterior of the building envelope. This creates pathways for heat to escape, leading to inefficiencies, heat loss and inconsistencies in internal temperature. Even small penetrations can have a significant effect.
Many things can create thermal bridging:
Steel framing
Timber framing in walls – timber conducts heat more than insulation, and buildings with a high proportion of timber framing in external walls may compromise thermal performance
Aluminium window frames that are not thermally broken
Nails, screws and fittings penetrating through the linings or insulation
Downlight fittings and other lighting
Piping and wiring conduits
BRANZ report on timber framing and thermal bridging:
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It is important to work with a designer or architect who understands these risks and proactively designs the structure of your build or renovation to avoid thermal bridging, and to work with a builder and other trades who take appropriate care.
Outsulation:
Another option for effective thermal efficiency is “outsulation” or external insulation, where a continuous layer of unbroken thermal protection – along with a water resistant and vapour control layer to prevent moisture problems, and a weatherproof external render coating – are installed around the outside of the building.
This can be useful for renovations of older homes, where it can be difficult or hugely disruptive to access wall cavities to retrofit insulation from the interior.
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Insulation under concrete slab floors:
Many people are familiar with insulation in the roof space and in the walls, but perhaps not with the importance of insulation underneath and particularly around the edges of concrete slab floors:
“Modelling shows that approximately 80% of the heat lost from a concrete floor slab is at the edge… This suggests that insulating slab edges is far more effective for preventing heat loss…”
BRANZ:
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This insulation provides a thermal break and a moisture barrier, and can protect the concrete from cracking. It is regulated under the Building Code clause H1 Energy efficiency. Concrete slab insulation is typically rigid foam insulation (expanded polystyrene (EPS) or extruded polystyrene (XPS)):
“Because a significant amount of heat loss can occur through the exposed vertical edge of the slab, slab edge insulation is a very effective way of improving slab thermal performance… To be effective, the insulation must be installed on all exposed faces of the slab edge from the top of the slab to the bottom of the footing.”
BRANZ advice:
Insulated unheated slab-on-ground concrete floors
Edge insulation of concrete slabs
BRANZ Bulletin:
Download PDF
The regulations for insulation:
Until the 1970s insulation was rare in NZ homes and many older homes still lack effective insulation. In 1977 legislation was introduced mandating insulation in new homes from 1978, and since then a series of upgrades to the regulations have progressively improved the performance and efficiency of our housing:
BRANZ:
Build: Thermal Insulation
The Building Code clause H1 Energy efficiency provides for the efficient use of energy in the home and sets physical conditions for energy performance including adequate thermal resistance:
New increased H1 insulation requirements for new housing came into effect in 2021-23, intended to reduce the energy needed for heating by approximately 40% over previous requirements, and to generate carbon savings. The new H1 requirements established six climate zones across NZ, rather than the previous three zones, to be more specific for local conditions:
H1 Energy Efficiency updates reach implementation milestone
In November 2025 more changes have been introduced to the Acceptable Solutions and verification methods for achieving compliance with clause H1 of the Building Code, including new requirements to account for thermal bridging from wall framing:
The Healthy Homes Standards for privately-owned rental properties became mandatory from 1 July 2025. All private rental properties must have thermal insulation installed in ceilings and under suspended floors where reasonably practicable:
Tenancy | Insulation standard
The Energy Efficiency and Conservation Authority (EECA) runs the Warmer Kiwi Homes programme which provides grants to eligible homeowners to improve ceiling and underfloor insulation in uninsulated homes built before 2008. The programme offers different levels of support to meet the greatest needs:
“Start off with the foundation. It's really important to isolate the foundation from the outside environment, from the sides and underneath – applied 50 mm polystyrene with a membrane protection – the foundation is really key, start from the bottom.”
Guy Gillespie: Character Homes
“Polystyrene is the best way to insulate some of those slabs – back to putting what we call hot edge on the slab now, but we can still separate the majority of that slab at end of life – using hot edge and recycled plastic pods.”
Bernice Cumming: Firth Industries Ltd
“Looking at how a Firth hot edge insulated concrete slab was going to capture that seasonal winter sunlight and passively heat the house, combined with centralised thermal mass walls – we had performance panels on the exterior walls, and then to save some money we went for a purlin rafter roof and insulated that.”
Duncan Firth: Solarei Architecture
“Most clients don’t tell you what kind of concrete slab they want, they just want a house with a floor, so it’s up to me to specify the kind of concrete slab, not a very big step for us to specify a better kind of concrete slab with more insulation – I just do it because it’s the right thing to do, it’s good practice.”
Tony Biesiek: Imagine Architecture
“Non-negotiables – double layer of framing, 90 mm framing which is standard Buidling Code but inside that another line of 45 mm framing that runs counter to the studs, that is also insulated, so that secondary insulation layer bridges all the studs and lifts the thermal performance alot more than just increasing the wall thickness.”
Rob d'Auvergne: Character Homes
“Framing – depth of 140 mm can achieve resistance value of 3.6 insulation – you could go another step and put an interior membrane and then another layer of insulation on the inside. As you increase that thermal resistance you increase the performance of your home.”
Guy Gillespie: Character Homes
“Also included – 6x2 walls rather than 4x2 walls to increase the insulation around the perimeter envelope of the building. Plus insulation around the foundation slab.”
Clayton Mitchell: Statement Homes
“Another product K17 – a Cooltherm insulated plasterboard, it’s got 10mm of GIB adhered to it. It offers a solution to continuously insulate on the inside of your framing – it can be just mechanically fixed to timber frame walls or up in the ceiling fixed to rafters.”
Candice Smith: Kingspan Thermakraft
“For us what’s really important is insulation, comfort, efficiency.”
Simon Fenwick: Nest Residential
“Warm Roof systems – a higher R-value per thickness using poly iso insulation or PIR insulation and our membrane systems – the Warm Roof comes with lots of benefits as the insulation’s kept on the outside of the building, it’s a higher insulation rating per thickness so you can reduce depth and height where you can.”
Jeff Fitness: SealCo
“Using product called K7 which is a pitched roof board, high performing phenolic foam insulation – often we can increase the thermal performance of the roof – a fantastic way to quickly and effectively increase thermal performance.”
Candice Smith: Kingspan Thermakraft
“Both savings on the up front costs can be in the design, reducing the cost of insulation by putting it outside – and then of course with the added bonus of the long term benefits is the insulation creates a better envelope, a better insulation envelope for your build which also reduces the heat loss or heat build up.”
Jeff Fitness: SealCo
“Out-sulation – where you insulate on the outside of your external frame of your building. A choice of finishes, texture, coating systems – also systems to go over masonry block or brick, stucco system over stainless steel lath for curved walls.”
Harry Puttock: Sto NZ
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