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Dezi et. al (2010)
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Projectile motion
Introduction
Do you know that in Australia, 40% [1] of the energy used in your house is for heating and cooling? In America, this number increases to 51% [2], and in European households, this number is a crazy 79%! [3]. Needless to say, most people around the world aren’t thrilled with their energy bills each month.
But what if there was a way to reduce energy consumption from heating and cooling by up to 90%? Yup, that’s right, energy savings of up to 90% can be achieved with the ‘Passive House’ (or Passivhaus in its native German) design framework. ‘Passive House’ is a global design standard for building designers that helps optimize the design to improve energy efficiency. It’s internationally recognized, scientifically proven, and already widely used in Europe. Some countries have local standards for this, like Switzerland’s, Minergie, but Passivhaus is the first to go global. By November 2020, more than 60,000 households had received Passivhaus accreditation.
What is a ‘Passive House’?
Of course, applying the Passivhaus guidelines goes a bit beyond simply adding solar panels. A ‘Passive House’ distinguishes itself from an ordinary house by taking advantage of ‘passive’ design elements like building orientation and envelope, windows and natural ventilation, and even human body temperature. Utilizing these before rather than resorting to active heating and cooling systems to control thermal comfort.
Key features of a passive house include:
- Effective insulation: Good insulation aims to keep the interior at a comfortable and stable temperature. It prevents heat from escaping and prevents cold air from affecting the inside temperature.
- No thermal bridges: This ensures no thermally weak points in the design. A thermal bridge is a part of the construction with a higher thermal conductivity than the materials around it. The cause could be using thermal conductive materials like steel in the windows. This results in the heat easily escaping or entering the house, reducing the thermal resistance of the house.
- Highly efficient windows: A typical thermal bridge in our homes is the thin, single-glazed window that causes heat loss by allowing heat to seep out. Hence, the windows of the passive houses are usually double or triple-pane glass to avoid such losses.
- Optimized building orientation and opening placement: The position of the sun's rotation relative to a building impacts the angle of sun rays received. Passivhaus considers this and how the sun's angle varies across seasons in its guidelines. The guidelines provide means to analyze the sun’s position relative to local design aspects, like orientations, awnings, and opening positions. This allows designers to optimize heat gain and loss by season for any given house, and leverage the sun to control thermal comfort at zero cost.
- Heat recovery ventilation: is arguably the most important component in a Passive House. Natural ventilation can be unreliable because of natural factors, particularly unpredictable weather conditions. A heat recovery ventilation device can recover heat from the extracted air and transfer it to the supplied air, ensuring minimal heat energy loss and good indoor air quality.
- Airtightness: good airtightness (reducing or eliminating air leaks) can improve thermal comfort and energy efficiency – air leaks can cause up to 30% heat loss in buildings (USI Blog). Hence, the specialist insulation material is required in Passive Houses to provide an airtight layer. Additionally, gaps in windows, doors, balconies, or roofs must be well-designed and constructed to avoid air leaks.
Why are Passive houses so great?
What sets a Passive House apart from a normal low-energy house? A low-energy house is a classification rarely designed to actual target values (heat load or space-heating minimum). A passive house, on the other hand, is designed and built to a rigorous standard with specific recommendations and quantitative targets designed to save energy, for instance:
- Thermal comfort: depends on the specific climate. In winter, it can be achieved at a minimum of 20°C. In summer, the temperature in a passive house is specified to not reach over 25°C for more than 10% of the hours in the year.
- Heating demand: will not exceed 15 kWh per square meter of net living space per year (15 kWh/m2/yr).
- Cooling demand: same as the heating demand of 15 kWh/m2/yr (in humid climates, this allowance increases for dehumidification).
- Humidity: must not exceed 12g/kg for more than 20% of the year.
- ‘Airtightness’: must be below 0.6 air changes per hour at 50 Pascals pressure (0.6ACH50)
- Overall energy use: Primary energy must not exceed 120 kWh/m2/yr, and renewable energy supply must not exceed 60 kWh/m2/yr, including heating and cooling, hot water, electricity…
Compared to normal low-energy building research conducted by the International Passive House Association (iPHA), Passive Houses have it all, with superior performance in heat consumption.
Ecology / Sustainability
Passivhaus's design is a highly sustainable building approach with a low-carbon footprint. These houses use extremely little outside energy, typically produced from natural resources such as oil or gas. The embodied energy (energy required for construction) seems insignificant compared to the operational energy (energy during building usage) saved. Saving this kind of energy in Passive House buildings is a major component of global carbon reduction initiatives.
💡 If you want to find out about how the world is countering embodied carbon, head to our article about 10 ways to reduce embodied carbon in our built environment
Passivhaus principles and design tools are made available for architects around the world. A great example is the Passive House Planning Package, published by the Passive House Institute. Powered by several spreadsheets, it provides reliable performance results to Passivhaus guidelines.
Affordability
Passive Houses typically cost more upfront than conventional counterparts. The biggest driver is the installation of Passive House-certified windows, insulation, and a ventilation system instead of a mass-produced heating and cooling system. On average, Passive Houses in Germany cost anywhere from 3 to 8% more than conventional buildings. At the same time, these costs can increase in some markets when specialist components are not readily available.
Don’t let that worry you, though, as Passive Home design becomes more widely adopted, manufacturers' supply is expected to increase to meet demand thereby reducing material costs. This is a ‘chicken or the egg’ problem, so it’s critical that the longer-term benefits of Passivhaus buildings are promoted!
Similarly to solar panel installation, Passivhaus upfront costs are significant But in most cases, the expense is a positive investment, substantially reducing houses' carbon footprint and energy bills. In a passive house case, your electricity bill can be reduced by 75 - 90% [5]!
The maximum level of comfort
In Passive Houses, quality insulation, a lack of thermal bridges, and building layout work together to drive fantastic thermal comfort levels. A passive house's temperature only fluctuates five degrees regardless of the outside climate.
Attaining great air quality in Passive Houses is not as simple as opening the window to let air in, as buildings near industrial zones or in certain climates can have poor natural air conditions. Passivhaus overcomes this by using a mechanical ventilation system, which allows for 100% fresh air to be provided all year round. Filtered fresh air is constantly supplied to living areas or bedrooms, while exhaust air is simultaneously extracted from areas like the bathrooms and kitchens. This mechanical ventilation system is the same one that enables the heat recovery process mentioned above, further increasing the building’s design efficiency.
How to design a Passive House?
If you’ve made it this far, you’re convinced that Passive houses are fantastic. So, how do you design them? Well, there are five principles of Passive House design that you need to consider simultaneously.
- Building envelope and insulation:
As mentioned above, one of the selling points of the Passive House is its thermal comfort, where good thermal insulation retains the temperature from foundations and walls to the roofs (thick yellow line in the below image). The building also has to account for another layer of air leakage, as most insulation materials are not airtight (red line).
The effectiveness of the insulation is determined by the level of heat loss W/m2K or the ‘U-value (table below). The U-value indicates the heat transfer rate through a specific component (wall, floor, etc.) over a given area (Unit: W/m2K). Hence, the smaller the U-value, the better the insulation performance. To better understand the U-value effect on heat loss and annual costs, refer to the example below of a small, single-family house with external walls surface area of 100 m2, using the heating system output of 1000 W (equal to a hair-dryer’s energy output).
Surprisingly, the U-value of a typical house in Australia is 1.6 W/m2K (this number might be lower in cooler regions)! And according to the Passive House Institute, the U-value of a Passive House is typically about 0.15 to 0.1 W/m2K!
So that means that a Passive House’s heating bill is less than 10% of an equivalent non-passive house.
That is why choosing insulation materials is so important, as they determine the wall's U-value and thickness. Take a typical Passive House with a U-value of 0.13 W/m2K and calculate the wall thickness needed.
All the materials above are appropriate for the insulation, and the chosen materials should consider local availability and the construction techniques required. Price is also a key consideration. Some typical insulated wall types for Passive Houses include:
- Integrated structural walls: consist of a concrete wall insulated on the outside or a monolithic wall composed of porous concrete and mineral foam insulation panels.
- Straw-bale wall: a minimum of 500mm thick walls made up of straw is typically the minimum requirement for a Passive House.
- Conventional insulation: conventional insulation materials can be used in Passive houses too. Mineral wool, polystyrene, and cellulose require layers about 300mm thick.
- Vacuum-sealed walls: these wall types are state-of-the-art and consist of a vacuum between the inner and outer walls. This approach allows for super thin walls with high-performing thermal properties.
2. Windows:
Windows are also a key part of the insulation system. Windows are crucial for providing light and visibility but typically can’t be insulated to the same degree as walls (a window’s U-value of 0.8 W/m2K or below is a good value for a certified Passive House). This means that most buildings' windows are the weakest point for heat transfer.
Passive House-certified windows are not like traditional single-glazing windows, however. The window frame materials are insulated and connected directly to other airtight building layers and the wall’s insulation layers with airtight tape. This eliminates any potential for thermal bridges to occur at the openings. The windows themselves are also double or triple-glazed. The gaps between glass panels will be filled with a low-conductivity gas like Argon, further increasing insulation performance.
3. Thermal bridge-free design:
When conducting passive building design, ensuring certain design elements introduce no thermal bridges is essential. Particular care needs to be taken at junctions between key architectural features.
Some actions that can be taken to avoid thermal bridges forming include:
- Reducing the number of cantilevered balconies - cantilever structures must pass through thermal envelopes to be supported. Avoiding them can improve thermal performance.
- Simplify the building’s geometry - structures with simple geometry and fewer angles and junctions reduce the risk of thermal bridges occurring at junctions.
- Use low-conductivity external elements like porous concrete blocks or solid bricks.
- Install intermittent connections for shelf angles - extra insulation in front of certain connections around the foundations reduce the risk of thermal bridges occurring at these locations.
- Prefabricated materials - constructions using prefabricated lightweight concrete elements limit the risk of construction errors causing thermal bridge development.
- Window layout - positioning windows to line up with the insulation layers reduces the complexity of connections and limits thermal bridge development.
- Insulating blocks - installing insulating blocks at the corners or junctions or extending insulation below slab level can reduce the thermal bridge coefficient of the element.
4. Airtightness:
Insulation materials usually cannot achieve airtightness; inversely, airtight layers usually cannot achieve effective insulation. Hence, the Passive House’s airtight layers are usually separated from the insulation layer. An example of a good building envelope that achieves both airtightness and insulation is the Ecology Building System.
They applied a simple yet effective racking board with a sheen inside and an acrylic coating on the outside. This has helped the wall to achieve vapor control, structural strength, and airtightness simultaneously. The racking boards are then covered with Tesconvana tape (a type of sustainable airtight tape) to achieve airtightness and seal all junctions in the vapour block. The board was prefabricated to leave holes and space for insulation filled by wood fibre glutex.
Other airtight features could include a polyethylene foil strip, which has been used in a Passive House in Hannover-Kronsberg, Germany.
However, there is a strict Passive House standard for a building to be considered airtight, using a Pressure test. Quantitatively, allowable air change cannot exceed 0.6 times a room’s volume per hour, and the pressure differential is limited to 50 Pascals [4].
5. Mechanical Ventilation (with Heat recovery system):
Given the high airtightness requirements, Passive Houses need an effective ventilation system that brings fresh air in and pushes exhaust air out. This is critical to avoid the build-up of pollutants, odours, CO2, and moisture. It is a bit different from normal ventilation, as Passive House uses a Heat Recovery Ventilator (HRV) system to achieve both thermal saving and good air quality. The system extracts heat from the exhaust air while continuously delivering fresh air from outside and removing moist air and humidity. It puts heat into the delivered air without directly mixing the airstreams. This way, the heat in the exhaust air can be utilized to the fullest. For a Passive House’s HRV to work effectively, at least 75% of heat from the exhaust air needs to be recovered, according to the International Passive House Association.
What does the future of Passive House look like?
The superior performance of Passive Houses is clear. But you might also wonder: how can Passive House be applied globally when primarily designed for European climates? Even normal building codes must change depending on different climates, so engineers in different regions must follow codes like US standards, Australian Standards, or European Standards.
However, the dispute might not last long as a mega Passive House project in China has been Certified Passive House development. The Gaobeidian Bahnstadt only consists of certified Passive House buildings - 8 high rises, 12 multifamily buildings, and six villas over 330,000 m2 of development [6]. The project has stunned the whole Passive House community and proves that the Passive House standard can indeed be international and scalable.
The Passive House Standards have already gained a diverse database of Passivhaus-certified buildings. These range from residential homes, hospitals, schools, and even retail facilities proving that designers can apply the standard in more than just the residential sector. And best of all, you don’t need new builds to own a Passive House! The PassivHaus standard is even starting to be applied in refurbishment projects.
With the broader adoption of the Passivhaus standards, designers are starting to create a future with standardized energy-efficient design and construction approaches. This begins to form a basis for all engineers, designers, and builders to find common ground in their definitions of energy-efficient buildings.
CalcTree
📚 Resources
- [1] Wyndham. J, Geoff. M., Ryan. P & Pavia. M. (2020). Australian’s Guide to Environmentally Sustainable Homes: Energy. Australian Government
- [2] U.S Energy Information Administration. (2015). Use of energy explained: Energy use in homes
- [3] Directorate-General for Energy. (2016). Mapping and analyses of the current and future (2020 - 2030) heating/cooling fuel deployment (fossil/renewables)
- [4] Ambrose. M, & Syme. M. (2015). House Energy Efficiency Inspections Project. Energy Flagship
- [5] Matyas. (2020). Passive house magic: How 3% higher initial costs can lower energy bills by up to 90%?. Ecokit.
- [6] International Passive House Association. (2019). 23rd International Passive House Conference in Gaobeidian, China.