Passive design for energy efficiency

Good passive design considers:

• house orientation – positioning to allow solar access, wind and temperature-induced breezes when and where needed, year-round
• shading elements – for example, wide eaves shade when sun is high in the sky, as well as providing increased weather protection
• glazing placement and size – to ensure solar energy gets to where it’s needed most
• ventilation – for example, window joinery that allows ventilation, such as security catches so windows can securely be left partially open
• insulation – to reduce heat flows both into and out of the building’s “shell”, which assists with heating and cooling
• thermal mass – using heavy building materials in areas that receive direct sunlight (during the colder months, this solar energy is then released overnight to provide free space heating)

Passive design is a building approach with no formal targets. A passive house is a building that achieves a performance standard, rather than meeting individual specifications. This means the whole house is considered, rather than just steps that are ticked off during the design-and-build process. The goal is a house with an airtight shell that only requires low energy inputs and provides its occupants with even temperatures throughout the year.

Each house is uniquely designed and the building’s performance is modelled, taking into account the local climate, before it’s even built. This computer modelling determines the exact levels of insulation, glazing and shading required to achieve the desired environment.

So, say your site is in the deep south, in shade for most of the day, the glazing and insulation requirements will be very high (think triple glazing). Up in the far north, in some scenarios you could get away with double-glazing, but the priority might be on shading to keep the house cool, rather than maximising the heat the house absorbs from the sun.

Because passive homes are airtight, they usually require beefy ventilation systems that force stale air out and replace it with fresh, filtered air from outside. You can still open windows to take advantage of a breeze but, unlike in many of our stuffy houses, it’s not a requirement for ventilation.

To achieve passive house certification, a house is assessed during both the design and construction phases. Part of the construction assessment is fitting a blower door to pressurise the building to measure airtightness. To make the grade, the number of air changes per hour needs to be below 0.6. In comparison, a newly built house might have three to five air changes an hour, while air changes an old villa with more holes than a chunk of Swiss cheese might be as high as 20.

Passive houses are fairly new here, and putting one together is probably beyond the average DIYer. However, there are companies that specialise in creating them. To ensure the project goes smoothly, get an experienced passive house designer and builder to take on the project, as they’ll have dealt with the teething issues this type of structure can encounter.

If you decide to go all in and get your house certified, the design needs to be put through modelling software to determine what’s required to bring the home up to the standard. A passive house relies heavily on being airtight, so the house would need to be stripped back to its bare bones and then built back up again.

Even if your budget doesn’t stretch to getting the house certified, you should incorporate as many passive design elements as your budget will allow. Even if you can’t achieve all of the criteria, your remodelled home will be much warmer, more comfortable and cheaper to run.

Our houses don’t need to be terrible. Some builders are leading the charge to build the best homes possible. Their creations put the Building Code to shame.

Specs

Walls: R 4.5 (Building code requirement R 1.9)
Ceiling: R 7.8 (Building code requirement R 2.9)
Floor: insulated concrete pad
Glazing: double-glazed, argon-filled, wood-framed windows

Why is this house better?

This home, located on a lifestyle block in Taupiri, was built by eHaus – a design and construction company specialising in passive houses. It’s spacious (252m²), with four bedrooms, two bathrooms and north-facing living areas.

The modelling showed the house will never overheat (no mean feat in a Waikato summer) and, with its super-insulated walls and roof, it’ll cost next to nothing to keep warm over winter. Contributing to the lower running costs are 5kW of solar panels on the roof and a heat pump hot-water system.

What does it cost to go passive?

Building a passive house is more expensive, but the flipside is lower running costs and a comfortable, healthy environment year-round.

Shelley Cresswell, eHaus marketing manager, puts the numbers for building one of its certified passive homes, based on recent builds, from $2950/m² for a simple design that’ll deliver very low running costs.

That puts the starting build cost for a 200m² passive house at $600k, and that’s before you start adding in extra bells and whistles like granite benchtops.

It’s always difficult to put a number on the true cost of building per square metre, as costs can vary considerably depending on what finishings and materials are used. Last year, a rough estimate to the average price in New Zealand was about $2000/m².