Double glazed windows stop unwanted heat loss.
True or False?
Note: If you haven’t already, it may be worthwhile reading Sustainable Architecture 101: Introduction before reading this article.
Sustainable Architecture Pillar 4: Windows
Contrary to popular belief, Double Glazed windows do not stop unwanted heat loss.
They may reduce conductive heat loss but they still allow radiated heat transfer.
Sustainable Architecture, however, is not about stopping heat transfer completely, but using it effectively.
The 5 Integrated Window Design Factors
Solar radiation from the sun travels through windows to the inside of a house. This radiant heat is absorbed by thermal mass, building elements and furniture, which when warmed up, re-radiates heat to the room air. This re-radiated heat is trapped inside, resulting in convective heat build-up within the room.
Tight integration of the following 5 Window Design factors pave the way for effective use of this freely available solar radiation.
1. Strategic Sizing and Orientation
The total solar radiation each window receives varies according to the time of the year and orientation:
- In Summer all windows receive heat gains.
- In Winter only windows facing north, north-west and north-east have a net heat gain.
- Windows should be orientated to the north where possible and ideally be large (provided solar access is good).
- South and east-facing windows should be kept small with south facing windows being openable – enable natural cross ventilation in summer.
- West-facing windows should be avoided or at least kept relatively small and well shaded.
Where there are obstructions to the north, clerestory windows, skylights and roof lights are a good option to allow solar energy into the building.
2. Appropriate Window Selection and Installation
The Window Energy Rating Scheme (WERS) is a program implemented by the Australian Window Council (AWC) with the support of the Australian Greenhouse Office. The windows are evaluated with stars – the more stars, the better the performance (the lower the U-Value):
- Single glazed window with a typical aluminium frame have U-values ranging from 7.9 to 5.5 W/mK.
- Double glazed windows with timber frames have U-values ranging from 3.8 to 2.5 W/mK.
An important factor when selecting windows is the frame itself – as it can negatively affect the overall performance of the window. Some frame materials, such as metal, glass or aluminium, allow heat to pass through easily and therefore shouldn’t be used. Up to 20% of heat can be lost through the frame alone – even on a Double Glazed window.
Finally, a good U-value is no guarantee for a well performing window. The installation of windows (and doors) needs to be done according to manufacturers guidelines – all gaps must be sealed and weather-stripped carefully in order to perform to the specified U-value.
3. Strategic External Shading
Double glazing won’t prevent radiated heat from coming into the building, which means that in summer windows need to be protected by means of external shading.
- North Facing external shades: Fixed eaves, verandas or pergolas over north facing windows can actually be designed to strategically block out the summer sun yet allow in winter sun. This is because the summer sun sits higher in the sky, while the winter sun sits lower on the horizon.
- East and West-facing external shades: East and west facing windows need a flexible shading device that can be completely retracted to let winter sun in but can be extended to keep harsh summer sun out. Adjustable shading includes canvas blinds, shutters, metal slats, louvres or shadecloth over pergolas.
Shading devices should always enable ventilation outside the window, as shading fitted too closely to a window can trap warm air which can be conducted into the house.
4. Quality Internal Coverings
In winter, lower room temperatures and draughts occur when window glass surfaces are noticeably colder than the warm air in the room. The Relative Air Velocity ends up high enough for occupants to feel winter discomfort. For this reason, all windows require protection from heat loss in winter in order to maintain Thermal Comfort. To minimise winter heat loss, it is important to trap a layer of insulative still air between the window and the room. This can be achieved by using internal coverings such as drapes, blinds and/or lace curtains combined with pelmets.
5. Thermal Mass
Appropriate use of Thermal Mass in conjunction with well integrated window design is its own subject, and conveniently will be covered in next week’s article.
Sustainable Architecture Windows: Conclusion
Double glazed windows alone do not stop unwanted heat transfer – nor are they designed to.
Passive Solar Design is not about stopping heat transfer completely, but using it efficiently.
The following 5 factors of integrated window design must be present in order to achieve the goal of Passive Solar Design:
- Strategic Sizing and Orientation
- Appropriate Window Selection and Installation
- Strategic External Shading
- Quality Internal Coverings
- Thermal Mass
The next article in this series will focus on Pillar 5 of Sustainable Architecture: Thermal Mass. To have this article automatically delivered to your email inbox, why not subscribe to our e-newsletter? Its free!