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24 June 2016

More Sophisticated Thermal Performance Ratings for Windows

The WEERS (Window Energy Efficiency Rating System) was recently developed by BRANZ along with the Window Association of New Zealand, EECA and MBIE.

Both WERS and WEERS ratings are voluntary schemes to provide the construction industry and building owners with an indication of the thermal performance of the windows to be fitted into a building. Even though these are industry recognised levels of performance, all windows and doors fitted to a building must still comply with the requirements of the New Zealand Building Code, and other standards and regulations.

Whereas the WERS system takes a generalised approach to rating the thermal performance of windows based on a general description of the framing material and type of glazing, the WEERS scheme is more comprehensive in that it rates a specific window (or door) unit as it has been fabricated for delivery to the construction site. There is a Star-Rating (up to 6 Stars) allocated to each unique window assembly. Should the assembly achieve a minimum R-value of R0.32 then the glazing and frame unit can also be given a separate Energy Star Mark (as administered by EECA, the Energy Efficiency and Conservation Authority). The WEERS scheme also allows for all the windows and doors on a job to be bundled so that an average overall rating can be given for the project.

The WEERS rating is calculated for each glazing and frame unit taking into account:

  • The thermal performance of the frame material,
  • The thermal performance of the glass and Insulating Glass Units (IGUs),
  • The size and shape of the window, and
  • A factor accounting for the IGU spacer material and its interaction with the frame.

Because of this detailed analysis, the rating is unique and specific to any particular window. This then allows for the design possibility of tailoring each window for its location within a particular building elevation, and the purpose of the room behind.

The primary factors influencing the performance on a window are the frame material and the glass, hence these become the primary considerations when the rating is being calculated. Any framing material can be accommodated in the analysis, but in my opinion, if the full benefit of the WEERS rating is to be maintained for a thermally broken aluminium framed window, then there must be a full continuation of the adjacent wall's thermal envelope across the window opening construction gap to the frame's thermal break materials. If not, then a permanent cold bridge is formed with the possibility of condensation forming on the inner portion of the aluminium frame. The same need for careful construction also applies to other frame materials.

One type of glass often used in IGUs is 'E-glass'. In addition to the WEER's label, a careful check should also be made of the location of the E-glass within the IGU. The coated surface should be at either surface two or three, ie. not one of the outside faces where it can be damaged. The radiated energy passing through the glass is reflected from the back (the glass interface) of the 'E' coating. Before specifying windows a decision needs to be made regarding the purpose of the IGU — is it to retain interior heat, or to reduce excessive summer solar gains — and therefore the positioning of the coating is important. The replacement of the air within the IGU with various inert gases improves performance of the glazing.

The size and shape of the window is included in the WEERS assessment because of the proportional influence of the frame and the glazing. A small window has a greater percentage of exposed frame surfaces, and for a large window or door the performance of the glass will dominate.

All IGUs have a structural spacer around their perimeter to hold the two glass panes apart (in additional to the gas sealants). This provides a thermal bridge between the interior and exterior panes, especially for thermally-broken aluminium or timber frames. The WEERS rating can be significantly affected by the choice of material. The commonly used aluminium spacer has a much higher thermal conductivity than stainless steel, and some form of non-metallic spacer will substantially reduce this thermal-bridge, especially for smaller windows.

The introduction of the WEERS assessment and labelling of windows will lead to a much improved thermal performance of the most significant weak point in a building's thermal envelope. Even so, there are situations where single pane windows are a better overall solution.

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