Facilitating Passivhaus

A case study by Ben Walsham, ARKATA, in collaboration with Jackson Digney, Enduro Builders.

Passivhaus Certifier: Marcus Strang, HIP V. HYPE

The Project

Jackson Digney from Enduro Builders engaged ARKATA to provide NatHERS thermal modelling services for his own (to be) Certified Passivhaus in Hahndorf, South Australia.

Hahndorf is a beautiful suburb within the Adelaide Hills, famous for its German history and heritage. Hahndorf is situated in a cold micro-climate that experiences on average temperatures of 11 degrees with lows frequently hitting 1 degree.

Enduro Builders are industry-leading Passivhaus builders, pushing high-performance building to the next level in South Australia.

ARKATA’s Role

Our role in this project was to facilitate Enduro Builders’ project goals and enable them to move forward with minimal friction throughout the compliance process.

A fully integrated and initially verified Passivhaus Planning Package was provided by Jackson, which underpinned the formation of the NatHERS thermal model. With most of the exploration and optimisation having already been completed, our job was to build a NatHERS model that served the project’s intentions, not one that worked against them.

Airtightness as the Bridge

A critical difference between the NatHERS and Passivhaus thermal modelling approaches is found within the air leakage and air infiltration assumptions.

Within the Passivhaus Planning Package, if your intention is certification, you are entering in an N50 air change rate result of 0.6 per hour. This is the pressurisation test result from the building being under 50 Pascals of pressure during a blower door test. This forms a critical part of the Passivhaus criterion that is required to be met before achieving formal Passivhaus Certification and thus is assumed throughout the thermal modelling process.

In NatHERS, the Chenath simulation engine calculates a variable air change rate based on factors such as zones, hourly weather data, site exposure, and air leakage sources from windows, doors, openings, and penetrations.

When we compare the thermal models built by NatHERS and the Passivhaus Planning Package, air change rates become a fundamental area of misalignment.

But they don’t have to be. Within the NatHERS model, we can toggle a setting called “non-regulatory mode”, which allows us to override the NatHERS-calculated air change rate. This allows us to fully investigate the Passivhaus Certification intention within NatHERS, and it can reveal pieces of the puzzle most critical to high-performance outcomes.

In Jackson’s home, the NatHERS-calculated air change rate under pressure was 3.85. The PHPP is assuming an air change rate of 0.6. This is a difference of 540%.

When we input 0.6 air changes per hour at 50 Pascals into the NatHERS mode, the rating moves from 8.4 Stars to 10.0 Stars.

But this significant Star Rating change is only the surface.

What the data reveals is a 55% reduction in heating demand and a 9% increase in cooling demand. This shifts our area of focus from compliance, to cooling.

Designing for Resilience

With the focus shifted from compliance to performance, we can turn attention to resilience.

Does this home maintain healthy and comfortable indoor conditions for its occupants into the future? Is it resilient in a changing climate? Is it independent and self-sufficient?

For Jackson’s Certified Passivhaus, the critical question is overheating. With Passivhaus levels of airtightness alongside the highest level of thermal envelope performance, overheating risks need to be investigated, understood, and addressed.

To uncover these insights, we use predictive weather files developed by CSIRO that extrapolate current NatHERS weather data into the future, based on varying trajectories of atmospheric greenhouse gas emissions. These scenarios, called Representative Concentration Pathways, range from optimistic to continued and increasing fossil-fuel reliance.

When we isolate the results to RCP 8.5, the worst-case scenario, the picture shifts dramatically. By 2070, the loads converge and move from heating-dominated to cooling-dominated, and for a home built to last generations, that matters. A home handed down through a family needs to perform, not just in today’s climate, but throughout tomorrow’s too.

Sitting beautifully in the rolling Hahndorf hills, Jackson’s home has windows framing sweeping views to the west. That western exposure, under future climate scenarios, becomes a vulnerability. Adaptable and flexible external shading was included to these windows as paart of a forward-thinking design strategy.

When we remove the external shading from the model, the cooling loads become significantly more pronounced.