Glazing, Solar Gain & Shading

 Solar Gain

• Glazing must be optimized on the south façade with reduced glazing in the north.
• Position Habitable rooms such as the living room, dining room, children's bedroom in the south as large windows will provide a pleasant ambiance with good daylighting factors. Conversely, rooms where a view out and good daylight factors are not so important such as WCs, bathrooms, store rooms and building services can be placed on the north façade.

Triple Glazing and Doors

In more temperate climates such as parts of Southern European it is possible to achieve the Passivhaus standard using good quality double glazing. In the Ireland however Passivhaus buildings must use triple glazed windows; there are two main reasons for this:

 1. To reduce unwanted heat losses through the window 

 2. To increase the surface temperature of the inner pane thereby reducing radiant the sensation of cold “draughts” from the glass and the possibility of mould growth. 

Glazing suitable for use in a Passivhaus building should have been independently certified by the Passivhaus institute in order to verify that a standard glazing unit (1.24 x 1.48m) has a whole window UW value of ≤ 0.80 W/m²K and can achieve U value ≤ 0.85 W/m²K once installed. Glazed components of doors must achieve a similar glazing specification and the installed U value of a Passivhaus door should be ≤ 0.80 W/m²K. Airtightness is a critical aspect of all Passivhaus glazing and doors and is often overlooked. It is critical that multiple continuous airtight seals are used in conjunction with a robust gearing system to ensure that air leakage when tested at Q(100 Pa) ≤ 2.25 m3/hm

Solar gains make up a significant component of the free heat gains available to a Passivhaus during the heating season.  To make optimum use of the useful solar gains in winter in addition to good orientation the glazing must have low installed U values (≤ 0.85 W/m²K) to reduce heat losses and good solar transmittance (g-values ≥ 0.5). Conversely too much glazing can lead to an overheating risk in summer, so good seasonal shading is important in Passivhaus design. Careful attention should be given to shading from the high summer sun angle particularly on South, West and East facades. The efficacy of seasonal and permanent shading devices can be tested in the Passivhaus Planning Package as part of an overheating reduction strategy. For maximum performance the glazing and shading specification should be fine-tuned on each façade of a Passivhaus building.

Summer overheating and shading 

A main part of the Passivhaus principle is to make use of solar gains in winter to reduce the heating demand. This means that there is a potential risk for overheating in the summer. To prove this from happening it is important to implement some external shading to reduce the amount of solar heat in the summertime.  shows a simple external shading system which utilises extended eaves for the first floor and a thermally broken brise-soleil for the ground floor in order to reduce high angled solar gains in summer. Correct positioning of fixed shading devices will allow maximum use of direct solar gains from the lower angled winter sun when it is most needed. It is a requirement for Passivhaus certification that temperatures exceeding 25°C cannot occur in a building for more than 10% of the occupied year. For a dwelling the occupied year is considered to be 365 days a year but for a school this period might be much shorter. In the light of climate change predictions designers are recommended to achieve a figure of 5% overheating frequency or less (using current day data) and to make provision for additional seasonal shading devices to combat future overheating risks. A number of further strategies are available to reduce overheating risk in Passivhaus design. These include the use of conventional cross ventilation and night purge ventilation. Mechanical options include using the Heat Recovery Ventilation system in by pass mode with or without additional ground or brine loop pre-cooling options. Thermal mass may also be used where appropriate to attenuate some of the diurnal temperature variations induced by unwanted solar gains however attention should be given avoid over reliance on this concept since it may be contra indicated during periods of prolonged overheating.