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Published on 11 December 2018 10:34

Window Performance and Specification: Cause and Effect

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Key Learning outcomes
  • The practical impact of a low thermal performance target (U-Value).
  • Understanding the measure of solar gain and how it can impact on design.
  • Understanding acoustic terminology and how it can impact on design.
  • How each of the above can conflict with the architectural intent.
So, a window. It’s easy? A hole in the wall in which the occupants can see out of, whilst allowing light into the building and providing protection from the elements. Unfortunately, not.

Window specification can be complex, there is more to it than just keeping the cold out and the warmth in. We need to consider the levels of light required in a room, the need to create a safe and secure environment, all whilst protecting against the Earth’s natural elements and other man-made factors such as noise pollution.

There are many aspects of window specification that we are asked about as a window manufacturer, however three of the most common criteria that we are asked to comment on are: Thermal Performance, Solar Control and Acoustic Performance, as such we will focus on these throughout this article.

This article will identify how we finalise the specification of a window system, considering building regulations but also the additional requirements of third parties such as M&E consultants, acousticians or employers requirements, all of which can influence the overall design and cost of your building.

1.0 Thermal Performance – U-Value

Thermal performance is the measure of the flow of heat through a material. It is referred to as a U-Value which is measured in a unit shown as W/m²K (Watts/metre squared Kelvin). In the context of a window it allows you to understand how well a product can keep the cold out of a building whilst retaining the heat inside.

When you look at the U-value of a material it tells you the amount of energy lost every second, for each square meter of material, for every degree of temperature difference between the inside and outside of a building. The lower the U-value, the less energy is being lost, and so the better that material or product is as an insulator.

UK Building Regulations define the maximum U-Value that certain building components need to achieve. For windows and patio doors Part L of the Building Regulations set out the target requirements for new build and refurbishment projects which typically have U-Value targets in the range of 1.4 W/m2K to 1.6W/m2K.

Whilst U-values are still used in the Building Regulations to set limiting standards for the elements of a building's fabric, the overall thermal performance of buildings are now assessed using more complex modelling procedures.

For non-domestic buildings, the Simplified Building Energy Model (SBEM) developed by the BRE determines the energy performance of a proposed building by comparing its annual energy use with that of a comparable notional building. For dwellings, energy performance is assessed using the Government's Standard Assessment Procedure (SAP).

Discussions should be had with manufacturers with regards to the energy performances achievable within their product range. Choosing windows with an enhanced thermal performance (lower U-Value) may allow you to make savings in the cost and performance of other materials used within the construction of that particular building.
Thermal image of a composite window to show the flow of heat through the material
Thermal image of a composite window to show the flow of heat through the material
 
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