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  • Alun Carter

What is Home Performance and why as Kiwis we should be putting more effort into this. (Part 2)

In the previous post we introduced the concept of our homes constantly evolving with times as needs, materials and technology has changed. We also covered the four main reasons why in today's age we should be constantly looking at ways to improve our homes performance.

In Part 2 of this series we are going to:

1) give a general overview as to why our housing stock in New Zealand compares poorly with regards performance to houses in other OECD countries with a similar climate to us.

2) brief overview of how we can improve our house performance?

What is causing poor house performance

New Zealand in general has a climate of extremes ranging seasonally from hot and dry to cold and wet with a relatively large daily fluctuation in temperatures.

Most of the housing stock in existence today was/is designed, orientated and built based on housing from other countries with a completely different climate. In addition, many of these original designs, and build methodology, were based on imported kitsets which by their purpose were designed as cheap and transportable as possible. It should then be no surprise that our housing performs poorly in New Zealand.

It is well regarded that change happens most willingly and quickly when under pressure. Historically cheap energy costs in New Zealand meant there was little to no financial pressure to change the quality of housing. The relationship between building quality and performance has only relatively recently been linked to occupant health. Added to this the culture of “she’ll be right” and “just toughen up” has further dampened any motivation for change. In recent times increased energy costs along with a more cosmopolitan and traveled population is challenging the status quo of New Zealand housing.

New Zealand Building Code reflects our historical construction thoughts and practices. Whereas in many other OECD countries their building code is reviewed every 2-3 years our last proper review was completed in 2011. Thankfully the initial consultation phase for a review of NZ3604 (NZ Building Code) was started at the end of 2019. It is no surprise then that our current Building Code is significantly behind those in other developed nations with a similar climate, as are the recently legislated “Healthy home standards” for rental properties. An example of this is whilst our Zone 3 Ceiling insulation requires R3.6 insulation to be installed the United States for a similar climate zone requires R5.28.

NZ3604 gives minimum requirements for building component performance. This ideally should be exceeded but rarely is. Older buildings were constructed using old Building Code requiring even lower performance requirements than the current Code (e.g. prior to 1978 no insulation requirement). BRANZ HCS 2015 determined 47% of all houses had sub optimal insulation (<120mm thick and <80% coverage) based on the current Building Code. Other factors that affect insulation effectiveness is the material they are made from, the R-Value, quality of installation (2mm gap between insulation can decrease effective insulation R-Value by 50%) and standards they are tested to, and lifespan of product (e.g. may settle/compress with time).

Because our building code is so poor, house temperatures fluctuate from high to low daily resulting in increased condensation on internal walls and furnishings. Little to no consideration is in the current code toward requirement for active ventilation and regular controlled trickle ventilation (preferably balanced pressure with heat exchanger) to ensure control of internal moisture and air quality.

The result is our current housing stock (including most currently being built) is significantly below where it could and should be.

How we can improve house performance

The goal of any work is:

- To minimize the variation in house temperature and maintaining it within WHO recommendations.

- To minimize free moisture production and build up within the house and provide enough ventilation to provide fresh air and keep the humidity within WHO guidelines.

The ultimate goal is to get all houses to a level which could achieve Passive House Certification. Although this may not be practicable for existing houses, by using considered step wise improvements one can vastly improve the standard, comfort and performance of New Zealand housing stock.

There are two ways we can improve house performance:

1) Physically changing or installing better structure, systems and appliances within a house.

2) Behaviorally, how we use the existing and upgraded structures, systems and appliances.

It is important to consider temperature, humidity and ventilation together as they are interrelated and changing one often will have a knock-on effect on the others. For example, increasing temperature would decrease relative humidity, or decreasing draughts without use of some form of controlled ventilation may increase house humidity and moisture levels.

To explain we first need to understand a couple of terms commonly used.

Law of thermodynamics: Heat energy moves by conduction, convection and radiation from high energy to low energy. Also, the greater the difference in heat energy the greater the rate of transfer of energy. i.e. a warm house in a cold environment (winter) will lose heat faster than if it was in a warm environment (summer).

Relative Humidity: the amount of moisture in air compared with the maximum amount of moisture the air could hold at that temperature, measured as a percentage. i.e. as the temperature of an air sample increases, it can hold more water and thus its RH% will decrease.

Properties of moisture movement: Moisture will move from a high moisture content to a lower moisture content.

Moisture Scientific Correlation: that compared to when dry, a material (including air) with a higher moisture content requires more energy to heat it by 1°C. Thus, it can take longer, is harder and hence more expensive to heat something with a higher moisture content.

Conditioned Space: is the area in a house where the occupants most often ‘live’ and do most day to day household activities. Generally, excludes garages, and the roof space or loft. This area is often heated or cooled.

Envelope: Commonly also called “Thermal envelope”. This is the boundary structure between the internal ‘conditioned’ space and the outside. Includes floor, walls, ceiling, windows and doors.

R Value: A material's resistance to conduct heat. Measured or rated in terms of its thermal resistance or R-value -- the higher the R-value, the greater the insulating effectiveness.

The diagram below of an uninsulated house (Beacon Pathway) shows the amount of heat lost through its different components. It is important to bear in mind that as you improve the insulation of one component whilst the overall heat energy for the house would decrease these percentages for heat loss will change, and hence would change the area of focus for further improvements.

The aim is to have a continuous envelope around the conditioned space which is both airtight and as insulated as practicable. Importantly there also needs to be enough controlled ventilation to provide fresh air for the occupants and to regulate the humidity of the conditioned space.

It is easier and more cost effective to do this in new build houses as the systems can be considered at the design stage and incorporated as the house is built. Existing houses can also be improved, though this takes a bit more thought and may be limited in final overall performance due to existing construction methods, limited access and the cost to implement.

With proper thought and consultation existing housing can be improved in stages. For example, when repairs, maintenance, improvements are made to a house and/or as and when finances are available. Importantly the impact of each stage of improvement on the other existing systems needs to be considered. For example, reducing the passive ventilation of a house by sealing holes in the envelope to keep the warmth in without consideration for providing controlled ventilation can result in increased humidity of the conditioned space could result in mould growth and ironically increased heating costs. Worryingly this can often be found in new build houses, which through better build materials and techniques, are now more airtight but where proper ventilation has not been considered.

It is also important to appreciate that it may be easier, quicker and cheaper to improve the insulation of one component compared to another. It is also important to consider that the cost of implementing any work is largely made up of labour costs. Thus, if an improvement is to be made, it is often more cost-effective long term to install higher performing product the first time. For example, install R4.2 batts compared to R3.3 batts where both would require similar installation labour time.

Often the first step to improving house performance is prevention. For example, preventing excessive heat gain or loss, heat or moisture production within the conditioned space (cooking, shower), excessive movement of moisture into conditioned space from external sources, and excessive uncontrolled ventilation (draughts). The second step is then installing and using correct suitable energy efficient products, appliances and systems to maintain temperature, humidity and air quality within recommended guidelines. For example, lighting, heating, ventilation, and smart appliance controllers. The final step is consideration for onsite energy generation and/or storage systems to reduce overall and peak energy demands on the national supply chain. Depending on individual houses, their location and occupant priorities some tasks may be done in another order.

It is critical that each improvement is made in consideration of the current situation and future improvements to minimize repeat work, purchase of materials/appliances which would become excessive or obsolete in future, and in some cases even making the situation worse.


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