Window ratio to floor area advice

Question

I have seen quotes indicating window-to-floor ratios but I am not sure what it indicates. Is there an optimum for different locations? Does the ratio change depending on your location and construction methods?

— Sandy and Neil

Answer

Windows and doors are an important aspect of any house design. They are required for physical and visual connections, but their interaction with heat gain/loss and natural ventilation make them and their design critical to a home’s good passive design.

A window-to-floor ratio provides a rough rule of thumb for determining optimum areas of window in relation to the floor area of a room or house. As with all rules of thumb it should only be used as a starting point for a design and firmed up by a skilled designer and computer modelling. This will help you account for the complexity of the thermal interactions in a building.

In any house, window type, area, orientation and shading should be jointly considered in order to effectively control the heat gain and heat loss of a building. They will be dependent on the opportunities of the site and the climate it is located in, and should be shaped further by the construction method employed. In temperate climates, higher levels of exposed thermal mass will enable greater areas of windows.

As a general guide, the total window area should be less than 25 per cent of the total floor area of the house. Most of the windows should be located to the north where good solar access is easiest to manage, with minimal amounts on the east and west facades. Windows on the south can help encourage good ventilation, but can be the source of heat loss. They should be used sparingly.

The types of glazing, framing, openings and seals used in the windows and doors will have an enormous impact on their effectiveness. It is well worth investing in what is commonly regarded as the weakest link in the building envelope. [Ed note: Read more about windows and glazing in Sanctuary 20.]

— Caroline Pidcock