Passive House vs NatHERs

Energy modelling is crucial in designing sustainable buildings that consume less energy and produce fewer carbon emissions. In Melbourne, there are two popular methods for energy modelling: NatHERS and PHPP. While both ways aim to achieve energy efficiency, there are significant differences in their approach and accuracy. This blog will explore these differences and explain why PHPP is a better option than NatHERS.

What is NatHERS?

NatHERS (Nationwide House Energy Rating Scheme) is an energy rating system widely used in Australia. It is a prescriptive method that calculates the energy performance of a building based on its design, materials, and location. The software used for NatHERS modelling is AccuRate, which is an approved tool by the Australian government.

What is PHPP?

PHPP (Passive House Planning Package) is an energy modelling software developed by the Passive House Institute in Germany. It is a performance-based method that calculates the energy performance of a building based on its actual energy consumption data. PHPP is widely used in Europe and is gaining popularity in Australia.

Differences between NatHERS and PHPP

Prescriptive vs. Performance-based

The main difference between NatHERS and PHPP is their approach to calculating energy performance. NatHERS is a prescriptive method that uses a set of standard assumptions and formulas to estimate the thermal performance of a building based on its design and location. PHPP, on the other hand, is a performance-based method that uses actual energy consumption data to calculate energy performance.

Building orientation

NatHERS only considers the orientation of the building in terms of the four cardinal points (north, south, east, and west). PHPP, on the other hand, considers the exact orientation of the building and the surrounding landscape to determine the best design for passive solar heating and cooling. Currently, NatHERS does have some limited controls regarding external shading influences.

Thermal bridging

NatHERS does not consider the impact of thermal bridging, which is heat transfer through building elements such as walls, roofs, and floors. PHPP considers thermal bridging and calculates its impact on the overall energy performance of the building.

Air leakage

NatHERS assumes a fixed air leakage rate for all buildings. PHPP, on the other hand, calculates the air leakage rate based on the design and construction of the building.

Climate data

NatHERS uses generic climate data for the entire country. PHPP, on the other hand, uses actual climate data for the specific location of the building.

Thermal mass

Thermal mass is the ability of a material to absorb, store and release energy. NatHERS has the ability to factor this in, whereas PHPP doesn’t.

What makes PHPP better?

Accuracy

PHPP provides a more accurate estimation of energy performance than NatHERS. PHPP uses actual energy consumption data, while NatHERS uses a set of standard assumptions and formulas. PHPP takes into account the specific orientation, thermal bridging, air leakage, and climate data of the building, which results in a more accurate energy performance estimation. PHPP uses a dynamic simulation method that models the energy performance of a building over time, while NatHERS uses a steady-state calculation method that models the energy performance of a building at a single point in time. PHPP's dynamic simulation method considers the variability of external conditions, internal loads, and interactions between the building elements, resulting in more accurate energy performance predictions.

Better Design

PHPP provides better design recommendations than NatHERS. PHPP takes into account the exact orientation of the building and the surrounding landscape to determine the best design for passive solar heating and cooling. PHPP also considers thermal bridging and calculates its impact on the overall energy performance of the building. This results in better design recommendations that can significantly reduce energy consumption and carbon emissions.

Health and Comfort criteria

PHPP considers a broader range of comfort criteria, such as indoor air quality, radiant temperature asymmetry, and daylighting, while NatHERS primarily focuses on thermal comfort. This means that PHPP can provide a more comprehensive evaluation of the building's overall comfort, resulting in a better indoor environment for occupants.

International Standard

PHPP is an internationally recognised standard for energy modelling. It is widely used in Europe and is gaining popularity in Australia. PHPP is a performance-based method that provides accurate energy performance estimation and better design recommendations, which makes it a superior option to NatHERS.

PHPP considers a broader range of factors related to the building envelope, such as thermal bridging, airtightness, and optimised solar gains. These factors play a critical role in the energy performance of a building and can have a significant impact on its overall energy efficiency.

Thermal bridging, for example, occurs when heat is transferred through a material with a higher thermal conductivity than the materials around it. This can happen at junctions between different building elements, such as where a wall meets a roof or a window frame meets a wall. If not correctly accounted for in the design, thermal bridging can lead to significant heat loss and reduce the energy efficiency of the building.

Airtightness is also a critical factor in building energy performance. If a building is not airtight, warm air can leak out, and cold air can leak in, leading to energy waste and discomfort for the building occupants. PHPP considers the level of airtightness in the building design and calculates the energy savings that can be achieved by improving it.

Optimised solar gains refer to the amount of solar energy allowed into the building through windows and other openings. This can be a beneficial heat source in the winter, but if not properly managed, it can lead to overheating in the summer. PHPP considers the orientation of the building, the size and placement of windows, and the use of shading devices to optimise solar gains in a way that maximises energy efficiency and occupant comfort.

Considering these and other factors related to the building envelope, PHPP can provide a more comprehensive and accurate analysis of a building's energy performance, resulting in a more energy-efficient and comfortable living space.

What makes NatHERS better?

Broad applicability: 

NatHERS is designed to be applicable to all types of buildings, not just passive houses, making it a more versatile tool.

Simplicity: 

NatHERS is generally considered simpler and easier to use than PHPP, with a more user-friendly interface.

Standardisation: 

NatHERS uses standardised weather data and calculation methods, which makes it easier to compare the energy performance of different buildings.

Availability: 

NatHERS is widely used and recognised in Australia, which means that it is easier to find trained professionals who can use it to model and certify buildings.

Government recognition:

NatHERS is recognised by the Australian government and is used to assign energy ratings to buildings as part of the National Construction Code.

It’s also worth noting that there are some big changes coming to NatHERS, which will include a new whole-of-home rating tool to measure the energy use of the whole home, including appliances, solar and batteries. This is a huge step forward for this program which has the ability to improve the energy efficiency of many Australian homes.

Where do both tools fail?

While both NatHERS and PHPP are comprehensive energy modelling tools, there are some limitations to what they can address.

For example, both tools do not explicitly address embodied energy, which refers to the energy consumed in the manufacturing and transportation of building materials and the energy consumed during construction. Embodied energy can be a significant component of a building's overall energy footprint, especially for buildings with energy-efficient designs that reduce operational energy consumption.

Additionally, neither tool fully addresses the occupant’s behaviour and the way they use the building. While both tools consider some variables related to occupant behaviour, such as set-point temperatures and usage schedules, they do not account for changes in behaviour over time, such as the addition of new appliances or changes in occupancy patterns.

Furthermore, both tools do not address the social and economic factors that affect a building's energy consumption. For example, a household's income and access to energy-efficient technologies and services can significantly impact energy consumption. Still, these factors are not explicitly considered in either tool.

It is important to note that while both NatHERS and PHPP have limitations, they are still valuable tools for designing and modelling energy-efficient buildings. Their comprehensive approach to energy modelling can provide a solid foundation for optimising building performance and reducing energy consumption.

PHPP and NatHERS play an important role in prioritising sustainable homes. As Passive House builders in Melbourne, we prefer PHPP as we feel it is more precise and hones in on the finer detail. Shortly, NatHERs will be able to control and input the blower door test results, which is a huge step forward. But no matter what method you choose, the people who are passionate about these models have one goal: to build a more energy-efficient home.

We are Passive House and High-Performance builders in Melbourne who have a passion for sustainable homes. We want our homes to be built to a higher standard and demand more from our industry. If you're looking to build or want to know more about sustainable design, architecture or construction, please don't hesitate to reach out to us.