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 they have a significant impact on a home's thermal performance so they're an important element to get right. There are a few factors to consider when planning your windows and doors for thermal efficiency:

Placement

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.

Insulation

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, such as ThermalHEART or uPVC Klima Series.

Glass

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.


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.

WEERS

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.

THERMALLY EFFICIENT PRODUCTS

 
ThermalHEART

Improves the thermal performance of aluminium windows and doors by keeping warm and cold areas separated. Windows using this system perform up to 35% better than standard double glazed aluminium windows.

 
Klima Series

Innovative, uPVC windows and doors that are thermally optimised for New Zealand's conditions. Klima Series is rated a maximum 6 energy stars under WEERS (Window Energy Efficiency Rating System) when used with high-performance glazing.

CONDENSATION AND HEATING EFFICIENCY

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

Aerovent

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.

Sashvent

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.

Ventient

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.

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