(Revised March 20, 2024)
The best method to analyse the comfort of your new home is becoming increasingly questioned, so we thought we’d delve into the differences, advantages, and disadvantages, of these two thermal modelling methods from the perspective of building a new home.
Understanding energy usage is key to creating eco-friendly buildings that use less energy and emit fewer carbon emissions. Two widely used tools for this are NatHERS (Star ratings) and the Passivhaus Planning Package (PHPP). Although both methods focus on thermal comfort, they have distinct approaches and inputs, resulting in varying levels of accuracy, and therefore, value.
What are NatHERS & PHPP?
NatHERS stands for the Nationwide House Energy Rating Scheme. It’s a scheme implemented by the Australian Government which aims to uphold design standards and make new homes more energy efficient. A NatHERS star rating gauges a home’s thermal performance (heating and cooling) using a 10-star rating system. NatHERS is assessed through a process of thermal modelling where the proposed dwelling is modelled using NatHERS approved software (such as Firstrate 5, Hero, BersPro, or AccuRate) These are typically a graphic interface software, where plans are traced over, and data is entered (except for AccuRate which is only a data entry package).
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 occupation, the building fabric, orientation, location etc.. PHPP requires projects to achieve set benchmarks for energy required to retain occupant comfort, as well as whole of home energy usage. It is widely used in Europe and is steadily gaining popularity not only in Australia, but worldwide.
Both packages use some standardised assumptions to anchor their calculations. These assumptions can be adjusted to test ideas or alternative lifestyles & comfort expectations in some software packages. There are even some add-on tools to further enhance the functionality and outputs of various software packages.
Differences between PHPP & NatHERS
Many professionals suggest that comparing the PHPP and NatHERS is like comparing apples and oranges. We tend to agree, and to explain this in more detail, we have the prepared the following table of differences:
Thermal Comfort Assumptions
PHPP
Passivhaus considers the building as a single zone that needs to be between 20-25 degrees Celsius. It accepts a frequency of overheating of up to 10% for buildings that do not have active cooling systems (i.e. if no cooling system is installed, it can be above 25 degrees for up to 876 hours in a year (24hrs x 365days = 8760 x 0.1 = 876).
NatHERS
NatHERS works with variable cooling and heating thermostat settings and separates the dwelling in multiple zones that have different temperature setpoints depending on the time of day and the climate zone where the building is located.
NatHERS software also has an allowance of 2.5°C from the set-point, which means that the internal temperature in Melbourne, for example, can exceed 26.5°C before cooling is required. If ceiling fans are present, the indoor air temperature allowed can be even higher (around 29°C)
NATHERS thermostat settings:
Heating thermostat (same across Australia):
Cooling thermostat: ranges from 22.5 degrees Celsius to 28 degrees Celsius according to the Climate Zone where the building is located.
Examples:
Airtightness Assumptions
PHPP
The Passivhaus standard requires that a building must have a maximum of 0.6 air changes per hour when pressurised/de-pressurised to 50 pascals. The airtightness is tested with a blower door test, usually twice during the build process.
NatHERS
NatHERS assumes the airtightness to be between 8-12 air changes per hour, dependent on door and window inputs. This is not verified after the project is built. In ‘Non-Regulatory’ mode the airtightness can be adjusted in some software packages.
Post Design Verification
For a home to be certified as a Passivhaus, it must go through a strict certification process, where airtightness, ventilation system and construction details are verified by an independent ‘Certifier’ throughout the documentation and construction processes.
The specifications of the NatHERS certificate should be verified by the building certifier prior to construction. Airtightness and ventilation are not tested, and the construction details and specifications of the assessment are not required to be checked by anyone during or after construction.
Thermal Bridging
Passivhaus modelling considers the effect of thermal bridges of all building components and materials in a project, and their effect on the heating and cooling loads of the building.
NatHERS considers the effect of thermal bridges from metal stud frames in walls only, and their effect on the heating and cooling loads of the building. In ‘Non-Regulatory’ mode Hero software can model the impact of timber stud wall framing.
Thermal Mass
In PHPP the thermal mass of a building is considered under the specific heat capacity of the building, used as measure for heat storage in the building components. However its impact on the thermal performance analysis is less prominent than thermal performance analysis done in the Chenath Engine/Nathers software. PHPP and PHI primary focus is on insulation and airtightness, as those have been proved to be the most important factors in the thermal performance of buildings. According to the Passive House Institute "the heat capacity of the interior building components facing towards the interior has a perceptible influence on the temperature stability and thus on summer comfort" however "the influence of the thermal storage capability of the external walls is extremely small (0.5%)".
NaTHERS appears to favour higher thermal mass homes, most commonly accounted for in floor materials and some wall types. The technical workings of how the calculation engine (Chenath) works is beyond our reach. What we suspect is that the higher internal temperature swings and greater air infiltration inside NaTHERS homes is complimented well by high mass construction.
Indoor Air Quality
The Passivhaus standard considers the indoor air quality by incorporating Heat Recovery Ventilation systems, which allows for the internal air to be replaced with fresh air with minimal energy losses. PHPP also takes in to consideration the internal surface temperature in potential weak points of the structure and has climate specific requirements to make sure that there is no mould growth on these areas.
NatHERS does not consider internal air quality.
Moisture Management
PHPP calculates the energy demand associated with cooling and dehumidification and the frequency of excessively high humidity. There are specific targets to be achieved if the building is to be certified as a Passivhaus project.
NatHERS does not consider moisture management in the building.
Building Longevity
Certified Passivhaus projects are likely to be more durable than houses built to the minimum construction standard in Australia because its superior focus on continuous insulation, air tightness, thermal bridge mitigation and moisture management. The certification process is also likely to improve the building durability as it makes sure the building is designed adequately and, at the same time, enforces that the building is built exactly how it was designed.
NatHERS does not consider building longevity.
Windows
With Passivhaus window dimensions are accurately modelled, including dimensions and thermal characteristics of frames and glass independent of each other.
With NatHERS window frames and glass performance values are combined into single performance values based on a set window size. Therefore the frame/glass ratio is fixed and can’t be adjusted to suit window size.
Solar Energy Transmittance
PHPP uses 'g-value' to calculate the amount of energy transmitted through glazing. The g-value is a unit specified by the European Standard EN410. There is no linear relationship between g-values and SHGC values, and g-values tend to be slightly higher than SHGC values.
Nathers uses Solar Heat Gain Coefficient (SHGC) to calculate the amount of energy transmitted through glazing. The SHGC is defined by the National Fenestration Research Council (NFRC) of the United States. Because of the differences in these calculation methods, SHGC values tend to be lower than g-values.
Ceiling Fans
PHPP does not consider the potential cooling impact of ceiling fans when analysing the thermal performance of the building.
Ceiling fans have a considerable impact in the cooling demand calculations in NatHERS by considering lower perceived temperatures, instead of the absolute air temperature, when assessing the cooling temperature thresholds.
Shading
Shading is modelled and calculated accurately with PHPP (and DesignPH). A 3D model considers the real shapes of all shading elements, including mountains, trees, and any other obstruction. These must be accurate for project certification.
NatHERS has a simplified shading calculation in 2D form. Some software packages limit the quantity of shading elements per wall, so that you need to choose the most relevant ones to model. A NaTHERS assessment must exclude shading of vegetation unless it is covered by a ‘Tree Protection Order’. All trees can however be ‘proxied’ in ‘Non-Regulatory’ mode.
Microclimate Flexibility
PHPP is more flexible to account for variations in local climate. The impact of the altitude of the project is precisely taken into consideration by the software and it is also possible to take into account the impact of urban island effect. PHPP will also allow for the simulation of future predicted climates, and includes a summer ‘stress test’ assuming an increase of 2 degrees Celsius in the summer months.
In the NatHERS scheme, there is no room for an accurate account of the altitude of the project. There can be significant variations in altitude in the same climate zone that are not taken into account, unless the assessor identifies another climate zone that is more compatible with the project location, given the difference in altitude.
Simulation method
PHHP uses a steady state building thermal performance simulation. It assumes that the building reaches a stable of equilibrium, where temperatures and heat flows remain constant over time. It is very accurate determining the heat flows of a building over time (a year), but it is not as accurate to predict when exactly the overheating will happen, however does provide monthly data.
NaTHERS uses a dynamic thermal performance simulation, which considers time-dependent conditions that could influence the thermal performance of a building and then aggregates the total heating and cooling loads based on the time-specific results. NaTHERS certificates report annual calculations, however monthly data can be provided in some software packages.
Comparability
The Passivhaus standard cooling and heating demand and load limits are the same across the globe. This way the results achieved by a building located in Melbourne can be compared to a building built in Darwin (or Siberia).
The NatHERS star system has varying loads and ‘starbands’ for different climate zones. For example, for a house in Melbourne to achieve 7 stars, it must have a maximum conditioning load of 81MJ/m2a, whereas in Darwin, the 7 stars can be achieved with a total conditioning load of 298 MJ/m2a.
In Summary
What are the advantages of Passivhaus Modelling over NatHERS?
More accurate estimation of cooling, heating and total energy needs (considering window and thermal bridge detailing)
It allows to estimate internal air humidity and need for dehumidification
It assesses the internal surface temperatures so that there is no mould growth. Beneficial to internal air quality and building longevity.
It allows accurate estimation of cooling and heating systems as well as the size of a Photovoltaic solar array.
Alternative air-tightness results can be modelled.
More accurate window modelling
More detailed and accurate modelling of shadows over the building (including the the building’s photovoltaic array).
What are the advantages of NatHERS over Passivhaus?
It is a faster and more user-friendly process than Passivhaus certification.
It allows a zone-by-zone (i.e. room by room) assessment of the project’s thermal comfort.
It is accepted as a compliance pathway by the National Construction Code (PHPP accepted in NSW only)
The cooling effect of ceiling fans is modelled.
What’s missing from both?
Neither of these packages will tell you how to design passively, they will merely measure the design and report on its performance. Therefore using these tools during the design of a project can assist a designer. Of course, having a solid understanding of passive solar design (relative to the climate of course) is going to make the design - thermal modelling - redesign - process much smoother and reduce the time and expense in resolving a comfortable and energy efficient home design.
It’s also worth noting that some design nuances can be difficult/impossible to model in these packages. For example, a large eave or roof over an alfresco dining area, will trap warm air below it and increase the heat load on the windows that separate the outdoor from the indoor environments. In warmer climates this can actually be quite significant, and neither package will model this out of the box (although an experienced designer will understand these scenarios and work around them).
Neither of these packages currently model the associated carbon emissions from operational or embodied energy of the home and its construction, but we understand it’s coming!
Therefore an experienced expert in sustainable and passive solar design will understand climate related challenges and design them out of your home to start, and they will save you time and money in the design process.
What do Maxa Design recommend for thermal modelling?
Well, it might sound like we’re sitting on the fence with this one, but to some extent both methods will garner the best results for your future comfort levels and energy modelling. We are firm believers in the accuracy and thoroughness of the PHPP, however we do like NaTHERS for helping determine which rooms of the home require more or less conditioning. Therefore we believe in PHPP modelling for the entire project, including certification, and we like to see the home modelled in NaTHERS to allow better guidance on locations of heating and cooling appliances.
It’s important to note that whilst NatHERS & PHPP aren’t perfect, they are incredibly valuable tools for designing and modelling energy-efficient buildings. They both rely on quality data entry however, therefore the true value of these tools can be completely undermined if they are not completed accurately and thoroughly.
As sustainable design experts, we aim to deliver the environmental credentials and performance excellence for the home you’ve set out to achieve. For us, sustainability isn’t an optional add-on but an integral part of the reason we do business.
We have developed a free Passive House video series to share our knowledge of Passive House principles, and the wonderful benefits that building a Passive House can bring to you and your family. This is a must watch if you are thinking about building a Passive House sign up here to access our free video series.