Windows - You better look well into these...
Windows are an essential part of a house, but at the same time, they account for about 25-30% of the heat loss, therefore, the level of insulation for Passive House windows is about three times better than standard double glazing or six times better than single glazing. The maximum Uw-value (heat-transmittance value) is 0.8W/(m2K) and to reach this value, the windows have to be triple-glazed, filled with argon gas and have a low-e coating.
Most important is a well insulated frame as this is the part of a triple glazed window where most heat is lost. Therefore the U-value should take the complete window into account, symbolised by Uw, compared to Ug, which only measure the insulation of the glass.
Triple glazing will also give you better noise reduction and a much higher level of thermal comfort. Even with frosty temperatures outside, the chill factor will not come inside, even close to the windows or patio doors.
Designing your openings
The recommended approach to the design of a Passive House is to avoid excessive areas of north facing glazing and place relatively large windows facing south or due south. This is in order to minimise heat losses through the north facing elevation, which receives no direct sunlight, while maximising ‘free’ solar heat gains on the south. This might not always be possible, so careful design and choice of windows is even more important in these cases. One advantage of large windows is the increase in interior daylight levels which, in turn, reduces the need for artificial lighting and ensures a more pleasant natural light-filled living environment. There is, however, a balance to be achieved between heat losses through the glazing and solar heat gains through the south/east/west facing windows. When designing a passive house, PHPP software can be used to calculate the heat losses and heat gains taking into account building orientation, areas and angles of glazing, as well as specific types of glazing, so the optimum balance of glazing for each passive house design can be reached.
The Passivhaus Institut has certified a range of glazing and door units suitable for use in Passive House buildings. Although it is not obligatory to use certified Passive House products in a Passive House, choosing approved products means the validity of technical data has been tested and verified by an independent certifier. The principle characteristics and advantages of using triple glazed windows in a Passive House are listed below for both glazing and frames:
What you have to watch out for when choosing your windows:
Glazing
• A low emissivity (low-e) coating on the inside of the outer two panes which reduces solar re-radiation back out through the glass. It should be noted that a ‘soft coat’ has slightly better U-value but a ‘hard coat’ glazing has higher solar transmittances
• Insulating gases between the glass panes, typically argon or krypton, which help to reduce heat escaping through the glass
• Three panes of glass separated by special low-conductivity spacers to eliminate the risk of condensation at the bottom of the glass in cold weather (which may lead to rotting of timber frames over time)
• The size of the gap between the panes makes a difference, but only to a certain extent, e.g. there is no advantage having a ¾ inch gap compared to a ½ inch gap
• High solar energy transmittance (g = or more than 0.5) which allows solar radiation to penetrate the glass and contribute towards the heating of the dwelling
Frame
• The frame must be well insulated and also be thermally broken. Even wood conducts heat and a thermally broken timber window frame will result in much lower heat losses than a solid one. Therefore frames for triple glazing are typically much wider and of a stronger construction than their conventional double glazed counterparts
• Two to three weather gaskets help to seal the window properly. The primary function of the outer one is to prevent weathering and the inner ones improves the airtightness
• Quality and form stability of the frame is important. Frames made out of one solid piece of timber are much more likely to deform and loose. their airtighness than laminated timber frames
Optimising insulation and solar gains
Triple glazed windows with low-emissivity coating and insulated window frames will have better U-values than double glazed windows, resulting in less heat loss. However with triple glazing the solar energy transmittance (gs), that is the amount of solar energy entering through that glazing, is somewhat reduced compared to double glazing due to the effect of the additional layer of glass. The requirement of the Passivhaus Standard is to use glazing with minimum solar transmittance of 50% or higher (i.e. g=0.5, see above). The use of larger areas of glazing on the south elevation help to maximising the amount of sunlight available in the short days of winter. It must be remembered however, that highly energy efficient windows allow less daylight (visible light transmittance) into a building than a normal double glazed windows without e-coating. Light transmittance is an optical property that indicates the amount of visible light being transmitted through the glazing. It varies between 0 and 1 (0 to 100% light transmitted) with the higher the light transmittance value the more light is transmitted. A double glazed window with low-e coating will transmit 72% of visible light. A triple glazed energy efficient window will transmit 65% of visible transmittance (these are indicative values only – for actual values check manufacturer’s specification). In a Passive House, locating radiators beneath windows is not required as the heat load is transferred throughout the house via the mechanical ventilation system. This has the added benefit of enabling unobstructed use for placing furniture against all external walls. Nevertheless, you will often find under-floor heating an added comfort which will also help raise the temperature level in the bathroom(s).
Shading
Because of the high level of insulation, the solar gain can, and will, lead to overheating in summer if no adequate provision for shading has been made. Typically, balconies, roof overhangs and/or awnings are used to keep the sun out in summer when it is at it's highest, but still allow using the solar gain when the sun is low in winter.
Doors
For doors, the same U-value limit of 0.8W/(m2K) applies in a Passive House. This again is not only to prevent energy loss, but also to avoid condensation and add to the comfort inside the house.
