Low energy homes are traditionally associated with passive solar homes. Today, there are also other kinds of homes or house designs available: the passive house & the zero energy house. So a good question to ask yourself is:

Passive Solar Design

Passive solar design uses natural energy flows (sun and wind) to maintain the most comfortable indoor environment possible for a building for the climate in which it’s located.

Passive House

Passive House has defined performance parameters to adhere to, which include energy consumption limits to achieve the indoor comfort levels.

Zero Energy House

The Zero Energy House is essentially a Passive House fitted with a zero emission, renewable energy system to generate all the power that the house will consume throughout the year.

Traditionally these passive solar homes were designed to operate without mechanical cooling and often no heating either, depending on location.

The lived experience of these homes very much depends on the quality of design, the quality of construction, and the behaviour patterns of the occupants. If we assume that the designer and builder are skilled and diligent, then the major variable is the occupant.

Many passive solar homes work like a dream. However, they do rely on the occupants running the building well. We often say that ‘passive homes require active users’, as it can be a process and could take a little time for the occupants to work out exactly how to optimise the internal conditions to their needs.

On a cold day windows need to be closed, a task instinctive for almost everyone. On a hot day the windows also need to be closed, this is not instinctive to all and, depending on how the building is dealing with the increased outside temperature this (in)action may be counterintuitive. When the evening temperature drops those windows need to be opened to allow the building’s accumulated heat to escape, replaced with cooler external air. This requires someone to be home or awake, and willing to open the windows. In urban areas where people are not always home at the ‘right’ times and due to security concerns, this free natural cooling is not always happening.

The design considerations required to achieve a passive solar building include:

• Orientation

• Zoning

• Thermal mass

• Shading

• Insulation

• Ventilation

• Glazing

As you can see in the list to the right, many of the elements are the same and none of them contradictory to the solar passive designed house. 

Importantly, in a certified PassivHaus, the air-tightness is guaranteed because it’s tested before completion and certification, with 0.6 air changes per hour at 50 Pascals (air pressure) being the minimum compliance. The cooling requirements only apply if mechanical cooling is installed. Cooling must be used if the 10% discomfort criteria is not met.  In Australia’s warmer climate zones, most passive homes would be expected to incorporate cooling to compensate for the predominant warm weather. Contrary to common belief, the passive house approach can be incorporated throughout Australia irregardless of the climate zone.

Heating/cooling criteria are:

• Heating demand max 15kWh/m2/yr
  OR Heating Load 10W/m2

• Cooling demand max 15kWh/m2/yr
  OR Cooling Load 10W/m2

• Air tightness of 0.6 air changes per hour
  at 50Pa (ACH50)

Additional points to note for the Passive House include the five key elements:

• Orientation

• Appropriate insulation

• Airtightness

• High performance windows

• Balanced ventilation with heat recovery (heat recovery ventilation system -HRV)

The Zero Energy House

The Zero Energy House is more or less a Passive House fitted with a zero emission, renewable energy system that is sized to generate all the power that the house will consume on a day to day basis through-out the year.  

It is an approach that is achievable in both cold and warm climates. 

Most ZEH also incorporate an energy management system (EMS) that assists to manage the distribution of the generated power during the day, and can also be designed to control shading (devices like blinds & awnings), ventilation (opening/closing key fenestration) and the HRV unit.  

In other words, it is no longer a thing of the future having a house that can self-control the living space temperature and the mood – and doing all of this at no external energy cost to the planet – irregardless of the external environment conditions.

A customised ‘solution’

You may decide for various reasons to customise your build design – ie to  pick and choose which elements to consider.  You may decide, for example, to select insulation and thermal mass but ignore ventilation and glazing quality. This is a risky strategy: any design solution that does not adequately consider the building physics, and mitigate risk where necessary, risks failure at some point. Such failures jeopardise the building fabric’s longevity, and pose a serious human health risk.

A project does not have to do all of things listed above for the solar passive house or the Passive House, but it does need to know (not assume) that the building can deliver health and comfort for its occupants at a reasonable energy cost and ensure the longevity of the building envelope.  Failure to do this could lead to eventual structural failure, compromised thermal performance, and sickness or disease to the habitants.

A word of caution…

Recent changes in the Australian building industry have seen increasing levels of insulation going into buildings (a good thing). However, most other building practices have not changed (a bad thing).

More insulation means a bigger temperature difference between inside and outside, which is a good thing most of the time. However, this can increase vapour drive, the push of moisture in the air from inside the building to try to get out (in the tropics this drive can be inwards). If the construction does not allow for this vapour to escape, or provide other means of moisture control, then interstitial condensation (i.e. in walls or roofs) can occur.

These issues have been documented in new construction work (even in buildings 3 months old) in Australia. And it has been and continues to be a huge issue in New Zealand, Canada and the USA.

It is fair to say that Australian building standards are many years behind most other developed countries. We are currently, at an industry scale, replicating the mistakes of other countries instead of learning and avoiding these.  The momentum of a ‘building practise’ can take years to die out.  Learning from our friends would help us leap frog the mistakes that they have made.  With applying a ‘build it tight, ventilate right’ approach would allow us to avoid the large rectification costs that will inevitably flow from ill-considered incremental Construction Code changes.

Key Takeaway Points

Both Passive Solar Design and Certified Passive House can deliver comfortable, affordable and enjoyable buildings in a range of climates, neither one is inherently better than the other.

Passive House allows a wider range of sites to deliver low energy and high comfort.  Just as important are its environmental benefits of ensuring that the household consume far less energy for the entire life of the building (which could be far longer than the original owners occupation).  Further benefits are also especially in denser, noisier and poorly orientated locations.

As with all issues of design there is no single answer, as we are all individuals and usually with varying site parameters. But you can develop a brief setting out your goals and aspirations, and then seek informed advice as to how to achieve them within your budget.  

If you are interested in designing a new house, be it a Passive House or a solar passive designed house, give us a call – with over 20 years of designing energy efficient housing - we have the experience and the expertise to assist  you.