EXTERNAL ENVELOPE 59 Hit the Passivhaus balance
Vasilis Giannopoulos from Internorm explains how to meet the stringent demands of Passivhaus and EnerPHit while balancing daylight, insulation, and durability
T
he windows in any building, including Passivhaus designs, have a multi-faceted role. They need to provide abundant daylight, make the most of the views, ensure high levels of security, and unify the interior with the exterior space. Passivhaus windows must also reduce heat loss to a minimum, maximise useful solar gains in the winter – therefore optimising the energy balance – provide exceptional thermal comfort and eliminate radiant cold from the glass surface as well as draughts, as well as being long lasting.
Heat losses – thermal insulation Heat losses through the window primarily depend on the heat transfer coefficient (the Uw value) which quantifies the heat losses from the whole window, not just the glass. The thermal insulation depends on the performance of the glazing unit, the frame, the glass spacers and the interface between the unit and frame, and finally the installation detail. However, the latter factor depends more on the window design and architectural details rather than the window itself. Windows for Passivhaus projects in the
UK, as well as most of Europe, need to have Uw value for the installed window less than 0.85 W/(m²K). Standard high performing triple glazed units with argon gas infill and low emissivity coating (Low-E) are ideal for Passivhaus and EnerPHit projects. The U-value achieved for the glazing unit is in the range of 0.5-0.6 W/(m²K). Units with highly thermal insulated frame profiles are typically in the range of 0.8 – 0.9 W/(m²K). In addition, warm edge spacers work by separating the panes of glass in double or triple glazing and are made from low conductivity materials. This significantly reduces the thermal bridge heat losses across the glazing unit. Glass-bonding technology further reduces thermal bridge heat loss as the glazing unit is bonded to the frame. The bonding layer blocks the path between the frame and the
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glazing unit, preventing high convection heat losses that would otherwise occur.
Solar gain
The solar gains primarily depend on the solar heat gain coefficient of the glass, the g-value, and the total surface of the glazing unit. The building design and how the window is located, orientated, and shaded also significantly affect the solar gains. In the winter, we try to maximise the solar gains utilising the free energy from the sun.
Thermal comfort
When the glazing surface of a window is colder than surrounding areas – exceeding a temperature difference of 4.2ºC – people tend to seek warmer spots away from the window. Additionally, heat generated by human bodies is often lost to the cooler window surface, making occupants feel colder. Cold draughts further exacerbate discomfort as warm air in the room cools upon contact with the window, causing it to sink and create unpleasant draughts. However, Passivhaus windows mitigate these issues by maintaining a surface
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