Introduction to Passive House

Construction cost

The construction cost of any Passive House or high-performance building is largely determined by the maturity of the market i.e. the local availability of quality products and the level of experience of the designer and builder. Another driving cost factor is the complexity of the building design. The simpler the form and selection of materials, the easier it is to achieve a well-insulated and airtight building envelope. In comparison, elaborate geometries and a large array of different materials and build ups, lead to greater complexities and the need for typically expensive engineering fixes. The last consideration when assessing the additional cost for a Passive House building, is the baseline standard the construction cost is compared against. Whether the baseline is minimum building code compliance, or whether it is a building standard that already considers occupancy comfort and energy performance will largely influence the overall cost uplift to achieve the Passive House standard.

While it is difficult to put an exact number on the Passive House premium, a frequently referenced study is the European CEPHEUS (Cost Efficient Passive Houses as European Standards) research program from 2001, which investigated more than 200 apartments and single dwellings upon various performance indicators, of which one was the additional investment cost. The Passive House dwellings were compared against architecturally identical dwellings built in accordance with the minimum legal standard at the time. The typical results ranged from 7 to 17 %. The lowest values were attained for apartment buildings, the highest for detached single-family house. The average additional investment costs indicated an 8 % uplift, when compared to the minimum building standard.

A very similar and more recent study comes from the UK Passivhaus Trust report from October 2019 that shows an 8% premium with a projection of just 4% at scale. An even more recent and local case study demonstrates that for a custom design home in Sydney, the cost uplift can even be negative. Please refer to this detailed case study from Architectural Firm Envirotecture and interesting comparison of different construction methodologies and performance targets.


Passive House versus NatHERS

All new buildings and major extensions and refurbishments must demonstrate a minimum level of energy efficiency by complying with part 2.6 (Class 1 buildings) or part J (all other building classes) of the National Construction Code (NCC).

Class 1 and 2 buildings (single houses, townhouses, and apartments) demonstrate compliance via the Deemed to Satisfy (DtS) approach or a NatHERS rating software, such as FirstRate5 or Basix. Using the software, a dwelling’s energy performance is rated on a scale from 0 to 10, where the minimum performance is currently (NCC 2019) 5 or 6 Stars.

A typical Passive House building would rate at the upper scale, close to 8 or 9 Stars. Although, it should be noted that the Passive House standard is far more encompassing than what the NatHERS software captures. The Passive House standard considers building attributes such as airtightness, thermal bridging, and efficiency of equipment and plug loads which all have a significant impact on a building’s overall energy performance and occupancy comfort.

While currently Passive House certification is not a confirmed performance solution, we understand that some building surveyors have accepted Passive House modelling results to demonstrate compliance with NCC Section J or Part 2.6. APHA is working with the Australian Buildings Codes Board to make Passive House certification a means of NCC Energy Efficiency compliance in the future.

A certified Passive House Consultant is not the same as an Energy Assessor who has obtained accreditation to undertake NatHERS ratings. A certified Passive House designer typically undergoes an intensive 2-week full time training course and requires a successful pass result in the end of course exam. The two accreditations are not interchangeable; however, an energy assessment background is a good skillset to have when aiming to become a Passive House Consultant.


Airtightness and Ventilation

Passive House buildings are built in an airtight manner and must demonstrate an airtightness results of no more than 0.6ACH50 (0.6 air changes per hour at 50 pascal pressure) to achieve certification. Airtightness of buildings is important for various reasons, but these are the main ones:

  1. Avoidance of uncomfortable draughts
  2. Reduced energy losses
  3. Eliminating the risk for interstices condensation, such as within wall and roof build-ups
  4. Avoiding egress of pollutants
  5. Increased acoustic separation
  6. Ensuring fire compartments are working effectively

A common misunderstanding is the need for ventilation (via windows, doors, and/or a mechanical ventilation system), and the avoidance of infiltration, which is uncontrolled air exchange via the building envelope. In Australia, we a very lucky that for a significant part of the year we can comfortably ventilate via our windows. But nevertheless, there are also times when it is either too hot, too cold, too noisy, too smoky, or simply not safe or comfortable to rely on fresh air supply via façade openings. In a Passive House building, that is the time when we are relying on the provision of fresh filtered air via the mechanical ventilation system. The truth is, the mechanical ventilation system typically runs 24/7, as it not only runs very energy efficient, but also recovers heat (or cold) via the mechanical heat recovery unit (MHRU). Overall, this way the energy losses that are typically occurred through ventilation via windows and doors are reduced by up to 90%. In short, a Passive House building should always allow occupants to open windows and doors, but at the same time make this not the only means of providing fresh air.

But how do we achieve an airtight building envelope? Two questions to ask are:

  1. Is the material I am using airtight?
  2. How can junctions be sealed in an airtight manner?

Some materials are inherently airtight, such as glass, concrete and most timbers. In Australia, most construction is however done in a lightweight manner i.e. via a timer or steel framing system. In those instances, it is recommended to introduce an airtight, but vapour permeable membrane. These smart membranes are always airtight, however also have the capability to adjust their vapour transmission properties in response to the varying humidity content in the air. In winter for example, moisture is not able to pass the membrane and enter the structure, eliminating the risk for mould growth and corrosion within walls and roofs. In summer however, the vapour permeability increases, allowing any moisture that had entered the building envelope to dry out.


Fire resistance of airtight membranes

We are often being asked whether membranes are safe to use from a fire resistance perspective. While this information is of general nature and cannot replace a fire engineer’s project specific advice, we would like to highlight that with the introduction of NCC 2019, membranes may be used as part of a non-combustible construction, as long as these do not exceed 1mm in thickness and have a flammability index not greater than 5 (NCC 2019 Volume One, Section C1.9 Non-combustible building elements). Most airtight membranes fall into this category.


Windows

Glazing represents a critical part in any successful Passive House project, forming a thermal barrier from the inside to the outside, controlling solar heat gains while being part of the airtight envelope. Glazing systems typically comprise fixed, awnings, tilt and turn and sliding doors.

The minimum performance of a glazing system is primarily driven by the project’s location (climate zone), the extent of glazing (window to wall ratio), the type of glazing and whether a building can make use of passive solar gains. As a rule of thumb in Australia, the Passive House standard can be achieved with double glazed windows, always argon filled and a high-performing, thermally broken frames. Thermally broken frames are usually made out of timber, UPVC, or thermally broken aluminium. Passive House Certified systems are typically preferred as very few local suppliers providing typical products to the Australian market are likely to be meet the thermal performance requirements.

In a warmer climate like Melbourne, a typical residential glazing performance would range from 1.25 to 1.8 W/m2K. In greater Victoria, glazing needs to be specified at the lower end of this range and are likely to be required to be triple glazed, particularly when considering site elevation and extended winter periods. Within Sydney and other warmer climates, the glazing would perform at the lower end of this range (1.8 W/m2K ) and typically be double glazed. Commercial buildings can usually accommodate slightly higher U-values, which is due to their greater volume to envelope ratio, as long as the risk for condensation is ruled out and hygiene requirements are met.

Glazing typically forms the weakest link in a building’s envelope overall performance, with the frame representing the weakest link. Therefore, it is recommended to choose a frame with a relatively low thermal conductivity and fewer larger windows over smaller windows. Warm-edge spacers, replacing typical aluminium glazing bars, will also improve the glazing further.

In Australia, we use the WERS (Window Energy Rating Scheme) rating scheme for windows and glazed doors, which has been plugged into NatHERS energy ratings. While the WERS system is based on the NFRC (National Fenestration Rating Council) scheme, the Passive House standard is based on the CEN (European Committee for Standardisation) standard. When it comes to high performing glazing, the two standards do not differ significantly. Nevertheless, a performance certification in accordance with the CEN standard is required for formal Passive House certification. The CEN standard tends to provide slightly higher performance values than those assessed against the NFRC standard. Suppliers of high-performance windows are typically able to provide the data on request.

It should also be noted that WERS ratings are based on ‘standard window sizes’. If a selected window, however, deviates from this standard size, the overall thermal performance will differ. Furthermore, WERS does not include all elements of the framing system, omitting subframes and potentially other aspects of the frame that will impact the performance of a glazing system. While in a building that barely exceeds minimum code compliance, these differences may be small, in a Passive House building, these differences must be accurately accounted for.


Passive House in Warm and Hot Climates

While the Passive House standard originates in Germany’s colder climate, the science, building physics, and stringent benchmarks behind the standard apply to every climate around the world. The targets are clear: heating demand/load, cooling demand/load, primary energy renewable (PER) demand and airtightness. The only thing that changes for different climate zones is the strategy how to get there.

The one extreme is the arctic climate. Here, high levels of insulation, high performance windows, significantly reduced thermal bridging, reduced ventilation losses, airtightness and maximum solar gains are key to success. The other extreme, in warm and hot climates, a Passive House strategy looks slightly different. All the particulars mentioned for the artic climate remain important, however often not as critical. Instead, shading and ventilations strategies become critical to avoid overheating. Solar gains must be carefully controlled, and internal loads are strategically managed via a combination of mechanical heat (cold) recover ventilation and night purging when temperatures and humidity levels allow.

We are often being asked whether in a Passive House one can still enjoy the Australian indoor-outdoor lifestyle. Absolutely yes. If the weather is right, a Passive House like any other building should be opened up to provide the connection to the outdoors and make use of natural ventilation. But even in warm and hot climates, there are times when it is just too hot or too humid to keep windows and doors open. That is the time when Passive House buildings stay comfortably cool with significantly reduced reliance on active systems.

One more thing worth mentioning is that there is one exception to the aforementioned Passive House benchmarks. In humid climates, the cooling demand allowance significantly increases due to the energy demand associated with active dehumidification.

There are now many reports and case studies from warm and hot climates around the world, showcasing how the Passive House standard can be successfully applied to buildings in Australia.


EnerPHit

The EnerPHit standard is the Passive House standard for retrofit projects. Not only does it set the certification bar a little lower in regard to airtightness, and in some climate zones also in relation to energy demands and loads, but most importantly, we think it is a wonderful tool to guide any retrofit project to the best possible outcome, taking investment cost and long term savings into consideration.

When embarking on an EnerPHit project, the existing building is first modelled as accurately as possible in PHPP. This model describes the baseline i.e. the existing conditions. With the use of the Variants tab in PHPP, different retrofit stages are than being modelled, tested and compared. The Energy balance chart in the Heating tab is always a good starting point to understand the weak points and low hanging fruits of a retrofit projects. Where is the majority of heat being lost or gained? Is it the uninsulated roof, the single glazed windows, or the lack of shading to the western windows? All this information is useful when deciding where to invest, and if renovating in stages, where to invest first. PHPP then further provides the opportunity to consider and compare different initiatives, their investment costs and long-term savings via the Comparison and PHeco tabs. As an EnerPHit project output, clients receive an EnerPHit Retrofit Plan, which documents the different retrofit stages, their requirements and outcomes, including a schedule that explains the interrelations that take place between the different initiatives and building elements.

While there are significantly less certified EnerPHit than Passive House projects, various projects around the world have demonstrated that it is applicable to a diverse range of building typologies, climates and construction methods.


Embodied Energy

While the Passive House Standard does not calculate embodied energy, there have been numerous studies conducted in Europe on the relative uplift on construction (insulation, additional membranes, etc.) to reach PH certification over and above code. It has been found that this additional energy expenditure for manufacturing is fairly negligible compared to the operational emissions which can be saved over a building’s life-cycle. More information can be found here.