If you are seeking to build your own home, there are lots of decisions to make, but the one thing you want to get right at the planning stage is that it will be warm, dry, comfortable and easy to operate.
Unfortunately, in New Zealand it’s a fact that even houses built to the current minimum standards are prone to cold and unpredictable energy costs. Using sustainable building standards such as Passive House can bring a home up to scratch but questions are sometimes raised about whether that will mean extra cost in the build process.
It is actually possible to design buildings that you know will be warm, dry and comfortable, and where you know how much the energy bills will be - so how do you specify your expectations in order to know the true cost of comfort in your home?
It starts with design. Having the building designed to be healthy and comfortable to live in, with optimal energy efficiency is the main thing to get right. Talking to your designer to make sure the design will take the “building envelope” into consideration is vital. It should include all possible measures to limit heat escaping through the walls, windows and roof areas of the building. Those components, also known as the building enclosure or building fabric, are the parts of the building exposed to the outside temperature.
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Building envelope thermal modelling is a way to achieve the goal of providing a healthy living environment for the occupants. This means:
1. Ensuring a comfortable temperature distribution in the home
There should be as little difference as possible between the surface temperatures of the walls, windows and floors and the air temperature in the rooms.
2. Hygienic air conditions
Mould on the wall surfaces and in the indoor air can be avoided by having sufficiently high internal surface temperatures.
3. A building design that protects the final structure against moisture damage
This includes all aspects of protection against water in liquid form - from driving rain and leaks, to condensation within the building fabric and on surfaces. This also helps avoid future deterioration of the building structure and degradation of its thermal insulation properties - which can save you considerable money in the long term.
The thermal, or energy, modelling calculates how much insulation will be needed, how much heat may be lost or gained through windows, and how much heat could be lost through draughts.
It is straightforward to calculate how much power is required for a home per year in order to keep it healthy and comfortable. This will allow you to set an affordable energy cost for the home before it’s even built. It is commonly expressed as Total Power (kWh) per floor area (m2) per year (a). Recent research indicates that Kiwis can afford about 15-20kWh/m2/a to heat their homes.
To get an idea of what you’re currently spending, you can calculate total energy demand for your existing home by dividing the total energy used over a year (check your power bill) by the floor area.
For instance, 12000kwh total power used in a year divided by a floor area of 150m2 gives 80kWh/m2/a.
Usually, the most energy is used to heat water, followed by heating, then plug loads such as lighting, TV, kettle, computers and appliances.
The affordability of energy is a major concern in New Zealand, and current energy bills often are much more than $450 per month. Combine the high cost of power with a poorly performing building envelope, and the result is unhealthy living conditions.
Designing to the building code minimums does not result in an energy efficient building. Buildings designed to code minimum standards would require about two to three times more heating energy than people can afford. Perhaps this is why studies show that even newly built homes in New Zealand are too cold (less than 18 degrees celsius) and poorly ventilated most of the time.
So if you are having a new home built, it pays to look further than the minimum code standards that currently exist.
- James Powers is director of Oculus Architectural Engineering
