Sunday, April 24, 2011

Effect of Window Size on Energy Consumption

Passive House Design centres its philosophy on a number of key principles. These are: ensure high air tightness, provide high levels of thermal insulation and optimise internal gains by providing large high performance solar collectors (windows) in the optimal solar orientation (i.e. towards the equator).

There are often issues of summer overheating and glare for consideration in the design as a result and this is often overcome by providing shading in some for or other that does not compromise solar gain in colder periods when it is most needed.
Large windows provide extra solar heat gains but they also have a much poorer thermal performance than other building elements which is a challenge for designers looking to achieve maximum solar gains without compromising thermal comfort.
This often means that walls that are not facing the sun typically have very small windows or none at all resulting in a lack of daylight and outside perspective for the rooms on these sides.  

A study was carried out in Gothenburg, Sweden in 2001 of 20 terraced houses built to passive standards built with large south facing windows and small north facing windows to maximise solar gain and minimise thermal loss (see below). The total floor area is 120 m², and the original window area is about 16% of this, which is more than what is recommended in the Swedish building regulations for sufficient lighting conditions.
The study ran simulations on how varying the window sizes (south reduced north increased) would affect the energy consumption and maximum power load to maintain an indoor temperature of between 23 and 26 °C. They used a simulation tool DEROB-LTH (Dynamic Energy Response of Buildings LTH) which was capable of modeling both the diffuse and direct sunlight effects on these buildings for the various window sizes as well as orientation and window type.
The simulations showed that neither the size nor the orientation had a significant influence on the heating demand in the winter, but is relevant for the cooling need in summer (see results below).

Therefore it is possible to enlarge the window area facing north and get better lighting conditions, provide more aspects for the user to view and to provide extra cooling in the summer to reduce the cooling load on the HVAC system. This means that it is possible to have greater flexibility when planning window areas and glazing layouts in low energy buildings and passive house designs.  

While the results of the study are particular to more northerly situated countries (approximately 60 degrees latitude) the principal findings of the study remain applicable to passive house design where heating demand is expected to exceed cooling demand.  

In conclusion over glazing can lead to increased energy demand for cooling and poor internal lighting and aspects for the user. Reducing the window area for solar collection slightly can be beneficial to summer cooling energy costs, lighting and views without compromising on winter head demand.


  1. HI Sean,

    Great blog! I think the results from that study highlight the problem of large windows facilitating possible overheating in summer months. This can obviously be detrimental to the Passive certification of the house, as the Passive House Institute clearly outlines that in warmer climates and/or during summer months, excessive temperatures may not occur more than 10 % of the time. The windows are clearly doing what they were intended to do, but for the sake of comfort, a balance must be achieved. However, do you think the geographical location of the study had would limit its relevance to others countries, for example Ireland?

  2. Hi Derek, I think the principle findings of the study were that a passive house may not require as much sun orientated glazing to generate heat gains in cold climates as previously thought. They found no significant increase in heating demand by orientating the house 180 degrees and having the majority of windows facing north. This seems to be saying that with good thermal insulation and airtightness along with optimal internal layout the heating demand can be met with less solar gains. THerefore the risk of overheating in summer can be minimised by reducing window size slightly and avoiding extra costs of expensive shading and airconditioning. In terms of geographical location the study is applicable to countries above 60 degrees latitude (which is roughly above Scotland). However, generally speaking, the further South you are the higher the solar irradiance on the building and hence the greater the risk of overheating so you could say this study is more applicable. Although bear in mind that passive design in countries like spain for example look to providing north facing windows with minimal south facing windows as normal practice as their problem is cooling demand rather than heating demand. Therefore you could say that countries in mid Europe need to strike a balance between north and south orientation. Also PHPP accounts for geographical location, window size and orientation when calculating solar gain. However this study may be asking questions of the accuracy of the calculations for all locations.