76 — Build 133 — December 2012/January 2013

GERMANY’S PASSIVHAUS concept has garnered a lot of interest in New Zealand lately with the formation of a Passive House Institute and the completion of the first certified Passivhaus home in Auckland.

BRANZ supports measures to significantly improve the thermal performance and energy efficiency of New Zealand homes. However, Passivhaus’s almost wholesale use of tools, techniques and products in a climate, culture and environment that is different to Europe’s needs examining, even if its systems-based approach to low energy and high comfort design is rigorous.

Traditional passive solar design

Integrated design, based on a well thought out approach to higher thermal perfor-mance, is a key tenet of good sustainable design. In traditional passive solar design, con struc tion features of the home – the mass, insulation, orientation and size – and their properties – such as insulation characteristics and thermal mass – are integrated so that they can collect, store and distribute solar energy without mechanical means (such as a ventilation system) yet achieve a comfortable temperature all year round.

The primary benefits are indoor comfort with considerably lower energy use

Departments/SustainabilityBy Roman Jaques, BRANZ Building Environment Scientist, and Steve McNeil, BRANZ Materials Scientist

A local comparison

A useful comparison with a Passivhaus home is Beacon’s Waitakere NOW Home®, com -plet ed in 2009 and heavily monitored for 2 years after occupancy. It was built for a similar price as conventional houses in the area, using light timber frame construction, an insu lat ed peri meter concrete slab floor and insulated glaz ing units with aluminium framing. Its occupants found the internal temper atures very comfortable, with near zero space heating required (see Table 1).

Achieving indoor comfort for a house located in the warmer climate of Waitakere City is easy with conventional spec ification, design and construction methods – none of the Passivhaus special internal building wraps, expensive windows or post-build pressure testing were necessary.

Performance targets may be off

From previous BRANZ modelling work, a target considerably less than 15 kWh/m2 (the maximum allowable for Passivhaus) might be more appropriate for space heating demand in warmer New Zealand climates such as Auckland and Northland, given the relatively warm ambient temperatures, clear skies and reasonable amount of harvestable winter sunshine hours.

Passive house or Passivhaus?

Homes based on the Passivhaus system are being built in New Zealand, but are their performance requirements and

design suited to our local environment?

– accou nt ing for about 30% of an average home’s energy use, according to BRANZ studies. Much smaller space heating systems are required for passive solar designs, resulting in a more durable house with a higher intrinsic value than a traditional design built to Building Code minimums.

Concerns about Passivhaus

BRANZ has concerns about some aspects of the formal Passivhaus system in the New Zealand context, specifically that the perfor-mance targets are set at the same levels as in Germany, as the system has a:

● bias against smaller houses ● reliance on sophisticated and expensive systems

● reliance on a ventilation system for fresh air.

Designers may find that the more

traditional approaches to passive solar design

fulfil their needs.

Build 133 — December 2012/January 2013 — 77

Beacon has developed post-occupancy benchmarks for New Zealand homes – the High Standard of Sustainability (HSS). In this system, like the Passivhaus system, there are established benchmarks for heating, hot water and plug loads. For new homes in Auckland of the NOW Home® size, a maximum of 5,800 kWh/yr (or 8,060 kWh primary energy) is set for comfortable indoor temperatures. Direct comparisons with Passivhaus are difficult as Beacon's set comfort temperatures are slightly lower.

While this case study is just one home and the behaviour of the occupants is a criti cal contri butor in determining energy demand, given that occupant comfort is the ultimate goal, it seems that the Passivhaus perform -ance target might need some fine tuning.

Bias against smaller houses

Larger houses can use more energy and still meet the Passivhaus standard, since the energy intensity performance target is based on floor area. This is an on going issue with building rating and assessment tools – how are the resources used within a house fairly examined so as not to disadvantage the smaller home?

In whole-building assessment methods such as the United States LEED and New Zealand’s Homestar, a multiplier effectively makes it harder for those houses that are larger than average and easier for those that are below average to be rewarded. This multiplier recognises that resources, and therefore consumption, have both upfront and on going costs. Applying this type of approach to the Passivhaus system, building on already completed and published studies, would seem fairer.

Sophisticated expensive systems not needed

Are requirements for heat recovery mechanical ventilation systems, interior building wraps and post-construction blower door test ing necessary for New Zealand? With each level of sophistication, extra resource use, installation and maintenance are needed.

The occupants of the heavily monitored Waitakere NOW Home® had none of these sophisticated systems yet were delighted by its performance, suggesting – at least in warmer areas – they’re not needed.

Passive solar design in new homes provides a high comfort level using

Passivhaus concept The formal Passivhaus concept – as opposed to passive solar design in general – is a systems-based approach to low-energy design which originated from a conversation between two European professors.

In 1996, the Passivhaus Institute was founded

in Germany providing a framework on which to

measure, assess and certify domestic buildings.

By 2010, an estimated 25,000 houses built to

the standard had been completed – mostly in

Germany and Austria.

The aim of the concept is to build highly

energy-efficient buildings that require little

energy for year-round comfort, in terms of

internal temperature – space heating or cooling

– and operation, including hot water heating

and electric plug loads. The thermal objective is

met by a highly insulated and airtight thermal

envelope combined with a balanced heat-

recovery ventilation system, rather than relying

on solar gain and storage. Top-up space heating

in winter is usually provided by a small heating

element within the supply air duct of the

ventilation system.

A certified Passivhaus home must meet

performance targets:

An annual heating demand – as determined

by a special calculation tool – of not more

than 15 kWh/m² per year in heating and

15 kWh/m² per year cooling energy OR be

designed with a peak heat load of 10 W/m²

A total primary (that is), gross auxiliary energy

consumption for heating, hot water and elec -

tri city, of not more than 120 kWh/m² per year.

The building must not leak more air than 0.6

times the house volume per hour at an indoor/

outdoor pressure difference of 50 N/m².

In 2012, the first New Zealand-certified home

was completed in Auckland. The building costs

are estimated to be about 20–30% above what

it would have traditionally cost and include

super-insulated glazing units with an average

R-value of 0.66.

PASSIVHAUS PERFORMANCE TARGETFOR NEW ZEALAND FOR 122 M² NOW (ANNUAL ENERGY HOME® INTENSITY) (ANNUAL ENERGY)

THE 122 M² BEACON WAITAKERE NOW HOME® REQUIRED

Maximum 15 kWh/m² for space heating demand

Maximum 1,830 kWh for heating season

Near zero space heating to achieve comfortable winter temperatures. Passivhaus target may be too easy for the warmer zones in New Zealand?

Maximum primary energy of 120 kWh/m² for heating, hot water and plug loads

Maximum of 14,640 kWh

An average of 8,000 kWh for all electric energy end uses. This converts to about 11,100 kWh primary energy, accounting for generation losses.

COMPARISON OF WAITAKERE NOW HOME WITH PASSIVHAUS REQUIREMENTS

Table 1

78 — Build 133 — December 2012/January 2013

Departments/Sustainability

simpler philosophies and design approaches than Passivhaus. Auckland and Northland are two regions where this can be easily achieved.

The installation of heat exchangers to recover heat from air being vented to the outside needs careful consideration at the early design stage. Ducting losses, particularly if ducts are outside the thermal envelope, mean the energy required to run the system can negate the benefits from the heat recovered, especially in milder parts of New Zealand. To get the most from a heat exchanger, a significant difference between indoor and outdoor temperatures is required.

Additionally, blower doors are used by the Passivhaus system to establish airtightness, but there is no regulatory framework around blower door testing use or calibration in New Zealand. Calibration of blower door equipment is crucial when trying to measure envelope airtightness at such low levels of leakage, and care and skill is required to operate this equipment.

Mechanical ventilation for fresh air

The Passivhaus airtightness requirement of 0.6 ACH @ 50 Pa translates to around 0.03 ACH of infiltration under normal condi-tions. As most international guidelines for indoor air quality recommend a minimum ventilation rate in the 0.35–0.5 ACH range, the homeowner is effectively reliant on the ventilation system for fresh air needs.

International experience shows that, where whole-house ventilation systems are mandated, there is often lack of maintenance that can have negative effects on occupants. This needs to be considered carefully in New Zealand where successive house condition surveys show a general lack of home main-tenance. Installation quality is also a serious issue overseas. Poor installation can negate possible energy savings.

BRANZ is supportive, but…

BRANZ supports many aspects of the Passivhaus systems-based approach, including:

● recognising the market importance of low -energy buildings with high utility

● the use of a rigorous thermal modelling tool ● the benefits of a rigorous and integrated approach to building design, specification, construction and post-construction test ing to ensure a high-quality product

● explicit, well defined performance targets that anyone can access.

However, for many clients seeking homes that are both comfortable and thermally efficient, designers may find that the more traditional approaches to passive solar design fulfil their needs, especially when used in conjunction with the latest thermal assessment tools such as SketchUp’s Open Studio, Design Builder, AccuRate or IES-VE Ware.

Certainly, for the warmer climates of New Zealand, we feel that a simpler and less costly design, specification and build approach that is more reflective of our culture and built environment has merit. For more See the Passive House Institute of

New Zealand, visit www.phinz.org.nz.

A typical Passivhaus design.

solar radiation

supply air extract air

supply airextract air

super insulation

fresh air entry

polluted air extraction

air-air heat exchangers (ventilation system with heat recovery)

low -E double glazing

well insulated slab

Departments/Safety