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

The 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 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 temperature within 4.2ºC of that of the surrounding space, preventing cold draughts and minimising ‘temperature asymmetry.’ This design approach not only enhances comfort, but also allows for the creation of architectural features like window seats – popular in Passivhaus properties – where occupants can enjoy views even on cold days without experiencing radiant cold or draughts.

Passivhaus – hygiene & health

Conventional windows, apart from the discomfort they create, can also impact health. The colder surface temperature means that the relative humidity on the window surfaces can be high enough for mould growth or condensation to occur.

Windows with exterior insulation properties keep the surface temperature above the mould growth threshold and dew point. Notably, even timber-aluminium windows – timber is more susceptible to mould growth problems, incorporating insulation between the aluminium
cladding and the timber frame. This ensures that even the timber layer is kept comfortably above the mould growth temperature threshold; Mould develops when the relative humidity of the surface exceeds 80%.

Longevity & airtightness

The high energy efficiency of modern glazing units results in warm surface temperatures, low relative humidity levels of the surface, and absence of mould. These are prerequisites for the component’s longevity.

There are no required airtightness performance criterion stipulated by the Passivhaus standard for windows, as it focuses on the airtightness of the whole building. However, Passivhaus windows need to be as airtight as possible, minimising the infiltration heat losses.

Additionally, when we consider a window’s airtightness, in windy conditions, if the most deformed part of the window not able to cope with the prevailing wind load, the airtightness no longer exists. Glazing units that are manufactured using glass bonding technology can withstand wind pressure up to 2000 Pa, which is equivalent to a wind speed of 205 Km/h.

Vasilis Giannopoulos is specifications manager at Internorm