How to choose the best windows for your building
David Thorpe | 10 October 2017
They say windows are the eyes of a building – and your eyes are a window to your soul. To help your buildings’ windows best serve the needs of its occupants, the UK’s NHBC Foundation has released a special guide. Simply substitute northern for southern orientation if applying to Australia and other places south of the equator.
The wellbeing, health and overall experience of people within a building are enormously affected by the way windows perform. Quite apart from their aesthetic appearance, three key factors – daylighting, energy efficiency and risk of overheating – need to be taken into account.
The NHBC’s guide starts from the fact that these factors do not interact in a complementary way. Hence the need for this study, which will help architects and designers navigate the trade-offs and compromises to be made.
The advice in this guide highlights the effect of the resulting choices on occupants, whether the building is a mid-floor apartment, a mid-terrace house, a semi-detached house or a detached house.
The findings are derived from extensively exploring the parameters in a passive house modelling software program (PHPP), and cover frame materials, heat loss, solar gain, preventing overheating, daylighting, ventilation, condensation and health.
They reveal the effects of making changes to the glazing itself, to the frame design, or to the window area and orientation.
The guide also provides design aids to optimise choices depending on whether you want to prioritise designing for energy efficiency, for maximising daylighting or for minimising the risk of overheating in your building.
How windows behave
Windows admit both light and heat. They also may lose heat through air leakage, conduction and radiation. In passive buildings windows will have two or three layers of glazing and multiple seals, plus internal insulation to prevent thermal bridging (the unwanted passage of heat through the frame).
Passivhaus standard windows are typically triple glazed in higher latitudes. Double glazing is used nearer the equator.
The panes of glass may be treated with special coatings that admit and retain as much infrared heat and light as is required. A huge range of coatings is available.
It’s possible to specify panes with extra clear outer layers, letting up to 80 per cent of light and 71 per cent of the sun’s heat in, for high latitudes, or tinted for low latitudes.
The thermal resistance of air each side of the glass of a window also contributes significantly to its thermal resistance. Still air is a good insulator.
Framing the problem
Windows obviously need frames, but the proportion of the window that is frame has a surprisingly large effect on performance.
It is described by a number called the frame factor, defined as the proportion of a window’s area that is occupied by glass. Say that 53 per cent of a window area (the “hole in the wall”) is glazed, then the frame factor is said to be 0.53. Typically this figure is 50 per cent to 70 per cent.
The NHBC’s modelling shows that if the width of the frame is increased by, say, just 25mm this can reduce the frame factor by over 10 per cent, changing significantly the performance of the window.
It concludes that the “frame factor can have a more significant effect on space heating consumption than is often realised”.
Another figure on a window performance label is the g-value. This describes the proportion of the solar energy falling on the glass which will end up inside.
The range of g-values of modern glazing is typically 0.4 to 0.8, signifying that 40 per cent and 80 per cent of the solar energy gets through the window. Specifying the right g-value is key to controlling overheating throughout the seasons.
Daylighting can minimise the need for electric lighting.
To capture the requisite amount of daylight for most tasks, nearer the equator, windows will tend to become larger and equator-facing. Conversely, moving towards the poles, they will tend to become smaller, especially on the east and west sides, where the sun is lower in the morning and evening.
The correct management of daylighting will help to separate it from that of heat gain, which may not always be required. This can be achieved by choosing the light-to-solar gain of glazing.
This is the ratio between the g-value and “Visible Transmittance”. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains.
The higher the number, the more light is transmitted without adding excessive amounts of heat. In some territories you can find this number on window energy performance labels.
Applying these principles
The NHBC guide applies these principles to various types of buildings, playing with the variables to see the effect.
Just one of many examples is below.
The conclusions the NHBC draws from this are cautious, because, as it says, “the effects of making changes to windows are dependent upon the type of home, its basic specification, its surroundings and its geographical location”, adding that “designers must take care not to over-generalise”.
The purpose of this guide is to highlight what can be achieved with careful design and to identify broadly where the good solutions can be found (and conversely to help avoid solutions that are likely to be suboptimal or poor, compared to the best).
It also warns: “Window products that are outwardly similar may have very different U-values, g-values, frame factors, etc., all of which can potentially reduce performance.”
For example it is particularly easy to confuse centre-pane U-values and overall window U-values, or for the impact of slight changes in frame factor to be underestimated.