Getting this right is not only crucial to the energy efficiency of your villa but also to the comfort and appearance of your home.
Heating and cooling consume by far the greatest proportion of household energy use. Data from the European Aluminium Association estimates that 40% of the heating energy lost from homes escapes from windows or the gaps around windows.
Window technology has come a long way in the last 30 years. Windows need no longer be the weak point in a buildings thermal envelope. However, if you are to keep costs down then you need to be thoughtful over the design.
From a technical perspective the variables are the specification of the glass, the frame, the airtightness of the frame and the window placement i.e. its size and orientation.
Thermal transmittance, commonly expressed as a U-value, is a measure of the rate of heat loss through or across a building component. It is expressed as watts per square metre, per degree Kelvin, W/m2K. The U-value is calculated using the combined thermal resistance of the materials in the element, air spaces and surfaces. The lower the U-value, the higher a material’s heat resistance and hence the better the insulation.
For windows, the value of the frame is referred to as Uf, the glass as Ug and the combined value as Uw. Factors that affect the Uf-value include the profile design, profile material, number of chambers, use of a thermal break, glazing gasket and weather gasket.
It’s important to note that the Uw value does not take into account any thermal bridge effects at the joint between the window or door frame and the rest of the building envelope. The calculation also does not include effects of solar radiation or heat transfer caused by air leakage.
Double glazed windows are seen as a minimum basic standard these days. They can be filled with an inert gas such as krypton or argon which will significantly lower the U value. Triple glazed windows provide even lower U values. The disadvantage is greater expense and greater weight and that can mean more expensive surrounding structures and support depending on the placement.
Low-emissivity (low-e) glass has a coating that reflects heat back into the house, reducing heat loss through windows and doors. This technology was introduced to the mass market in the early 1990s as K glass. Low-e glass can be manufactured with a hard coat or, more recently, a soft coat. The soft coats generally give a better thermal performance, and the hard coats tend to have a slightly greyish tint to them. Specifiers should ensure that there is consistency on each facade.
Frames can be wood, plastic, aluminium, steel, composite, or pvc. All frames should have a thermal break, in other words, a layer of insulation that meets the thermal point of the glass and avoids a thermal bridge. A thermal bridge is any point in the buildings envelope that conducts heat. A lack of thermal bridges is a key feature of an energy efficient house. A single thermal break can render useless all the design effort and expensive materials used in the home.
South facing windows maximise solar heat gain to the house. As a general rule in Andalucia they will be shaded to avoid overheating in the summer. Exceptions might be triple glazed argon filled e-coated windows. Normally this is not necessary because the climate is suitable for shaded outside living so we are looking for large shaded south facing windows that integrate the outside and the inside space. When they are closed in the winter the inside space still feels connected to the outside space.
In Northern climates where you want to maximise solar heat gain you may want to expose the south facing windows.North facing windows, in the northern hemisphere, are exposed to significantly less solar radiation.
Thoughtful window placement and sizing will take into consideration the solar orientation of the house, the views and how the home will be used, breakfast areas, evening areas, shaded areas swimming pool kitchen and garden, window specification and budget and the climate.
Author: John Wolfendale
Bio: John is a founder of Eco Vida Homes and is passionate about bringing modern design and construction practices to Spain. He believes a home which is warm in winter and cool in summer is largely a matter of design and selective use of materials. He is British and a Chartered Surveyor with over 24 years experience living and working in Spain.