Windows define the relationship between a building and its surroundings. A well-placed window creates dynamism and drama, accentuating and playing with our perception of space. While rooms create the bones of a building, providing function and defining the use, windows bring the magic.

There is, however, much more to windows than manipulating the visual experience of space. They have a massive impact on our other senses, providing warmth, sound quality and the movement of air over our skin, and while these may all sound rather abstract, they play a key role in forming our perception of a space.

Sustainability

Beyond these experiential issues, as we push towards low-energy architecture, windows increasingly play a technical role and become a key part of a building’s energy strategy. Sustainable buildings not only need to eliminate heat loss, they also need to exploit all available local energy sources — the biggest of these being the sun.

When most people think of using the sun’s energy in buildings, they think of solar panels, but the best way is through passive solar design and harvesting the sun’s energy by capturing the heat entering through windows. Designed right, it provides free energy for as long as the building stands; designed wrong, you lose heat rather than gain it and can suffer from overheating.

Balancing Solar Gain and Heat Loss

As with much in life, it is a balancing act between the energy a window creates over a year through solar gain and the amount it loses. In the UK this means that as a principle you need to:

  • reduce glazing on north elevations because north-facing windows rarely capture the sun’s energy and will steadily lose heat throughout the year
  • concentrate glazing on southern, eastern and western sides. These windows are far more likely to be net sources of heat gain and will therefore reduce your space heating needs.
North-facing elevation of a home with small windows to allow lightin but reduce heat loss

This is the north facing elevation of the home seen above. Glazing has been limited to allow light in, but limit heat loss

Overheating

Anyone who has been in a conservatory on a sunny day knows that this too has to be done carefully as overheating can become a real issue. As the planet heats up, it is predicted that by 2050 the UK’s climate will become more like the south of France; consequently overheating will become a more pressing issue.

Given that the average self builder moves every 20 years, if you are thinking of building your house at present, you will experience big changes in our climate before you move on, so plan now.

Thermal v Architectural Demands

There are two different but critical functions that windows play in our buildings — the architectural role of light, view and experience, and the thermal role of providing passive solar gain. Once you start getting down and dirty with an actual building, these two requirements are often in conflict.

Buildings frequently want to face north due to the site and their internal layout, and windows have a tendency to end up where, from a thermal perspective, they shouldn’t be. As with so much of architecture, it is the art of good compromise that defines a great building — a balance of reconciling the tension of form (great space) and function (solar gain).

There are no easy answers as each site and building has unique requirements, but thermal modelling is key. This can tell you how each window will perform thermally so you can judge whether the architectural function of the window outweighs any possible thermal losses. But, sometimes a lovely big north-facing window is worth it, because it’s good for the soul.

Information from the outset is the key and the more you know about a design before it is built, the better informed you are to make these decisions and get the location and sizing of your windows just right.

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