Thermal efficiency

Optimising your windows and doors' thermal performance

A thermally efficient house is designed to harness the available natural resources to create a stable, comfortable environment. Everything works together for a home that's healthier and cheaper to heat and cool, year-round.

Watch our overview of key considerations for creating a home with improved thermal performance. 

Factors to consider

On average, windows and doors make up just 10%* of the building envelope but are responsible for 35-50% of a home's heat loss so they're an important element to get right.  As such, changes to the H1 Energy Efficiency clause of the New Zealand Building Code are coming into effect throughout 2023 to improve the overall thermal performance of our homes. Meeting these new minimum insulation requirements, and consideration of the below key factors will help you create a warmer, drier home with year-round comfort. Use this handy tool to better understand the insulation requirements and suitable products for your zone. 


Windows gain more heat in summer and lose more heat in winter than any other surface in the home so you'll want to consider how the sun moves over your home through the year so you can optimise the position of your windows and doors. In the winter you'll likely want to capture the sun and in the summer you'll likely want to reduce the solar gain to keep the house cooler.

Positioning windows and doors in such a way to encourage air movement means that a breeze will pass through the whole of the room to move unwanted warm and damp air. Ideally, windows would be at different heights and not directly in line with each other across a room to achieve cross-ventilation. Good ventilation removes the airborne moisture that is harder to heat and contributes to condensation.


Window systems are responsible for at least 40%^ of heat lost from your home so any improvement you make will have a big impact on the overall thermal performance. Double glazing with an inert gas fill such as argon or krypton between the glass sheets helps to slow the flow of heat in and out of the house as the gas is a poor thermal conductor. This can be further improved by choosing a thermally efficient frame.

A thermally efficient frame, such as ThermalHEART® or uPVC Klima Series helps to reduce the transfer of hot and cold temperatures through the frame. In May 2023, the minimum R-Value requirement for four zones increased meaning buildings in these regions will need Low-E glass in a thermally improved frame. This expands to all zones from November 2023.

Choosing Centrafix™ installation will further enhance the performance of the window system by over 20%[2]. This method is a BRANZ appraised alternative solution to NZBC E2/ES1 and aligns all insulation elements – wall, window frame, glazing – to prevent cold air from circulating around the window and door frames, improving the whole window system’s thermal performance.


Glass plays an important role in the thermal performance of the whole window system. Low-emissivity (Low-E) glass has a transparent coating that reduces the amount of heat that passes through the glass while still letting light through. Low-E coatings can be tailored to maximise or minimise the sun's energy coming in making them a popular choice for improving the thermal performance of your home - no matter your home's climate or orientation. Look for products that are designed for New Zealand's conditions such as Solux-E by AGP which blocks excessive heat and also provides excellent light transfer and clarity for light, bright rooms.

To further improve the thermal performance of your glazing use an inert gas fill such as argon or krypton and a warm edge spacer to add insulation between the panes of glass. 

R & U Values

You will come across R-values and U-values as you research thermally efficient products. Architects and engineers use these values to calculate the thermal efficiency of the whole building envelope. R-values refer to a material's resistance to conductive heat flow. A U-value is the measure of how much heat is transferred through the material. A lower U-value indicates better insulation properties so for better thermal efficiency you'll want to choose products that have a low U-value to bring down the overall U-value of your home. Consult your architect or manufacturer to learn more about these values in relation to specific products or your building. You can also use this zone map to understand the Regional R-Value requirements.


The Window Energy Efficiency Rating System (WEERS) is a 6-star system assessing the thermal performance of the whole constructed unit. WEERS combines the thermal performance of the frame and glazing, together with the size of the window to calculate a rating between one and six stars, six being the system that offers the highest efficiency. Currently, the highest performing system is our Klima Series uPVC system when used with low-E double glazing. Our manufacturers can assist with these calculations.


Improves the thermal performance of aluminium windows and doors by keeping warm and cold areas separated. ThermalHEART® frames deliver almost double the thermal performance of standard aluminium windows and doors1
Innovative, uPVC windows and doors that are thermally optimised for New Zealand's conditions. Klima® is rated a maximum 6 energy stars under WEERS (Window Energy Efficiency Rating System) when used with high-performance glazing.


On average, we spend around 90% of our day indoors so the quality of the air in our homes, work and schools impacts our health and comfort and the efficiency of heating systems. Our most basic day-to-day activities create moisture in our homes which appears as condensation when it meets a cold surface and is much harder to heat than fresh air.

Thorough ventilation is the best way to keep our indoor air quality high and flush out moisture. We've developed a number of passive ventilation products which can work with your lifestyle and bring less-humid fresh air in to help the efficiency of your heating systems.

Passive Ventilation options


Improves airflow with less visual intrusion.
  • A cover on the inside also acts as a manual slide for opening and closing
  • Air movement is directed upwards to prevent draughts
  • Internal insect mesh barrier ensures only fresh air is allowed in
  • Ideal for cross-ventilation and air flow.


Delivers background ventilation through the bottom of an opening window.
  • Installed into bottom-opening windows (awning), meaning it is hidden away
  • A manual slide allows the vent to be opened and closed
  • Insect mesh barrier ensures only fresh air is allowed in.


Naturally automated trickle ventilator suitable for high-end residential and commercial projects.
  • Opens & closes based upon air temperature. This ventilation option is fully automated without the use of power, sensors or human intervention
  • Installed in Metro Series or APL Architectural Series windows or doors 
  • Mesh prevents insect entry to the aluminium Ventient box, and dust and pollen filters may also be incorporated.

*The Proposed Buildings used as examples in NZS 4218:2009 have windows making up between 9.8% - 15.5% of the building envelope.
^Share of envelope heat loss from a house insulated to pre-2007 requirements. Based on a single level home, including the roof and floor. Source: BRANZ.
1. Comparison figures are based on complete thermal modelling of an average house lot of Metro Series ThermalHEART® products with The AGP System® double glazing using the WEERS modelling methodology. The benchmark figures were calculated by modelling the same average house lot of products using the equivalent non-thermally broken products glazed with standard double glazing. A “standard aluminium window or door” refers to a non-thermally broken aluminium frame with standard double glazing.

2Centrafix™ comparison figures are based on complete thermal modelling of an average house lot of products installed in 90mm timber framing. The standard installation benchmark was calculated by modelling the same house lot of comparable products using a typical E2/AS1 cavity construction installation on 90mm timber framing.

A thermally efficient 8 star Green Star house can be up to $900 cheaper to warm than a house built to code. Source: NZGBC.

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