A note on climate considerations
Compact buildings with good thermal protection
All components making up the building envelope must be well insulated. Edges, corners, connections and penetrations must be planned with special care in order to avoid thermal bridges. All opaque building components should be so well-insulated that their heat transfer coefficients (U-values) do not exceed 0.15W/(m²K), meaning that no more than 0.15 watts of heat energy are lost though the external envelope per degree Kelvin and square meter. For free standing, single family homes, these U-values are often under 0.10 W/(m²K). The more compact the building envelope, the easier and more cost-efficient it is to reach the Passive House Standard.
An airtight building will ensure favourable ventilation and temperatures while preventing moisture damage. A Passive House’s airtightness must be demonstrated with a pressure test wherein the allowable air change, as shown by a pressure test, is limited to 50 Pascals and the pressure differential cannot exceed 0.6 times a room’s volume per hour.
Window glazing and frames
The entire window, ie: glazing and frame, should have a U-value of 0.80W/(m²K) or less (this value may have to be more stringent in more extreme climates, whereas milder climates may manage to meet the criteria with higher U-values) and the installed window should have a total U-value of no more than 0.85 W/(m²K). It is therefore essential to use well-insulated window frames with multiple lip packing. Glazings should have a high total solar transmittance (g-value) of at least 50% making a net heat gain possible during the winter (although lower g-values may be appropriate for extremely warm, sunny climates). The windows themselves must be airtight and the spacers in the glass seal edge must be thermically separated, ie: not aluminium. Windows should be installed in a thermal bridge free manner in the insulation layer.
Window orientation and shading
Appropriate window orientation and shading are essential for Passive Houses. To capture as much of the sun’s energy as possible when it is needed most, the largest window surfaces should face the equator it at all possible. When designing the windows, it is important to reduce the amount of window framing to a minimum so as to minimise unwanted heat losses (or gains). To prevent overheating, windows facing East and West should be equipped with shades. Especially in warmer climates, shades are also recommended for windows facing the equator. In order to ensure adequate cross ventilation on warmer days, every outward facing room must have windows that can be opened.
Ventilation with heat recovery for efficiency
Ventilation units with heat recovery are key in terms of energy savings, as they ensure that the warmth carried by the exhaust air is not wasted, but first transferred to the incoming fresh air without the two air streams ever physically mixing. In extremely hot conditions, heat exchangers can also work in reverse so that the heat carried by the incoming air is transferred to the exhaust air and thus pre-cooled before entering the rooms. These systems should also be equipped with automatically controlled bypasses, thus allowing the incoming air to bypass heat exchange, for example, during the night at times when days are warm and nights are cool.
A Passive House can only function with a highly efficient heat recovery, as ventilation systems without heat recovery waste far more energy per year than a Passive House uses for heat (at the same rate of air exchange, a ventilation unit without heat recovery may lose about 24kWh/(m²yr) whereas a Passive House’s maximum space heating demand is only 15kWh/(m²yr).
The ventilation systems used in Passive Houses must thus have heat recovery efficiencies of at least 75% while the electricity consumption for such systems should not exceed 0.45 Wh/m³ of the transport air volume. Additionally, the acoustic load of the ventilation systems for use in Passive Houses should not exceed 25dB. Pipes and values should be planned accordingly, making use of silencers.
Ventilation with heat recovery for comfort
A ventilation system with heat recovery ensures that plentiful, nearly room temperature fresh air enters the building in a controlled manner. Draughts are eliminated and residents need not actively air out the rooms. It is important that the fresh air entering the building not exceed 30m³ per hour per person, so as to avoid overly dry air. Such a ventilation system should not be confused with air conditioning systems; humidifying the air within the ventilation system is to be avoided for reasons of hygiene.
The ventilation systems used in Passive Houses provide unparalleled indoor air quality through the use of a high quality, F7 filter at the suction point (the unit must also be equipped with a drain). During heat recovery, the exhaust air must not mix with the supply air. Due to reasons of hygiene, a humidifier within the ventilation unit is not possible. It is important to remember that Passive Houses utilise ventilation systems, NOT air necessarily conditioning systems.
Protection against mold
In order to avoid the build up of moisture and mould, continual aeration with a mechanical ventilation system, good thermal protection and a thermal bridge free structure, all hallmarks of the Passive House Standard, are a must. Window and door frames must be well insulated. Triple low-e glazed window panes with noble gas filling should be used, although double glazing may be sufficient in hotter climes. Thermically separated, non-aluminium spacers at the glass edge seal are also important.
Domestic hot water
In Passive Houses, the heating demand for domestic hot water is more significant than that for space heating. Therefore, it is extremely important that the system be efficient and that the heat losses incurred through the preparation, storage and allocation of domestic water be minimised by seamless insulation. To reduce fossil fuel consumption, solar thermal, biomass, and/or heat pumps can cover all or a portion of a buildings domestic hot water needs.
Efficient household appliances and lighting
Reducing electricity consumption is not only good for the environment and your wallet, it also reduces internal heat loads, thereby reducing the chance that rooms will overheat during the warmer months. In Passive Houses, unlike in conventional buildings, the small amounts of heat given off by household appliances, lighting and even people (every person gives off around 80 watts of heat) matter. This makes energy efficient household appliances (refrigerators, ovens, lighting, washing machines, dishwashers, etc) and lighting as well as well-insulated domestic water heating systems essential in Passive Houses. As opposed to a dryer, for example, a dying cabinet connected to an extract air valve can provide for fast, energy efficient drying in a Passive House. By reducing internal heating loads, such measures facilitate passive cooling when needed.
Optimising the whole concept and saving
In order to reach the Passive House Standard, all components must be optimised and checked for compatibility. The Passive House Institute developed the Passive House Planning Package (PHPP) to help designers in just this regard. An extremely accurate, Excel-based energy balance tool, the PHPP not only determines the space heating and primary energy demands of a design, but also calculates aspects such as window U-values, the influence of orientation and shading, heating loads and overheating frequencies. With PHPP, designers can optimise the components to be used and their construction plans to come up with the most cost-effective solution. The PHPP thus forms the basis of good Passive House design.