Design for passive heating and cooling

Zero Energy House

In any building, comfort and energy goals are often intertwined. Having a healthy and comfortable indoor environment during the winter usually requires the use of mechanical heating systems, which in turn result in high electricity bills during this time of year.

When Shay Brazier and Jo Woods started thinking about building a home for them and their family, they knew they wanted a house that was comfortable year-round and warm throughout in winter, with no cold pockets. They also knew they wanted to do this without the use of heating or cooling systems, something that would also help them achieve their Zero Energy goal.

Because of their backgrounds in the construction industry, Shay and Jo were aware that this could be accomplished with good design and the use of the right products and materials, especially in a temperate climate such as Auckland’s. This is how they did it.

Passive heating

The Zero Energy House was designed to be warm without the need for heating. For Shay and Jo, this had two key benefits:

  • Financial. While some of the construction costs a little more up front, eliminating heating systems and the need to operate, maintain, and replace them over the next 25 years makes passive heating a cheaper option in the long-run.
  • Comfort. One issue with relying on mechanical heating systems for a home is that temperatures can vary inside. For example, heating systems might keep the living areas warm but then at night people retreat to bedrooms that are cold. Designing a house to be warm throughout without heating eliminates these cold spots.

Passive heating has been achieved through the following methods which are explained in detail on other pages of the website:

  • Solar access. The house has been located and oriented to make the most of the site-specific sun angles. The internal layout has also been planned around which rooms need sun in the morning (kitchen), throughout the day (living areas), and evening (bedrooms). Additionally, heat from the sun is captured during the day in the concrete slab on the ground floor and, thanks to its thermal mass, released overnight.
  • Windows. Most of the glazing is on the north side of the house to optimise daylight and solar gain, while the number and size of windows on the south side has been minimised to reduce heat loss.
  • Walls. Heat is retained inside the house through both high-performance windows and a double layer of insulation. Walls, floors and roofs in the Zero Energy House use up to 50% more insulation than required by New Zealand’s Building Code.

Shading

In the summertime and also in the shoulder seasons, shading is used to control how much heat from the sun enters into a building so that internal temperatures don’t reach uncomfortable levels. Shading, along with ventilation, are essential for optimum temperature control on warm days.

Typical shading features come in the following forms:

  • Overhangs
  • Fins
  • Louvres
  • Vegetation
  • Screens

During summer, the mid-afternoon western sun is the most warming and therefore, in the Zero Energy House, western glazing has been kept to a minimum. In winter, the early afternoon western sun is the most warming and enters the house through the northern glazing (as the sun’s trajectory in winter is further to the north).

Shay and Jo included the following shading options on the design of their house:

  • Fixed overhangs to reduce midday sun on the northern side.
  • Corner fins to reduce some of the eastern sun on the northern elevation.
  • Vegetation on the eastern side to help reduce low-level sun during the summer months.
Cross section illustrating the use of an overhang on the north side of the house to control solar gain in winter (lower sun angles allowed to penetrate to heat the internal concrete slab) and summer (high sun angles blocked by the overhang).

Ventilation

As mentioned above, ventilation is also essential for optimum temperature control on warm days and is important for the following reasons:

  • It removes unwanted heat from internal spaces on warm days. The movement of air over the skin also helps to keep people cool.
  • It provides fresh air to a space and removes stale air.
  • It helps remove chemicals emitted by building and furniture materials into the air, which can be harmful to human health.
  • It also helps remove moisture from cooking, showers and people from the house, preventing the build-up of mould that could lead to respiratory issues.

Shay and Jo decided to keep ventilation in the Zero Energy House simple but effective. They use natural ventilation throughout with mechanical ventilation in high-moisture areas such as bathrooms (extractor fan), laundry (decentralised heat recovery ventilation) and kitchen (rangehood).

The design of the house included the following features to provide good natural ventilation:

  • Windows that can be left securely open throughout the day, even when the house is unoccupied.
  • Opening windows in every room.
  • Opening windows on opposite sides of the room where possible to allow for cross-ventilation.
Windows to on the north and south elevations allow cross ventilation through the main spaces (image shows ground floor).

For natural ventilation to be effective, it’s important for Shay and Jo to use their house mindfully. While keeping good ventilation levels is fairly easy for most of the year, it needs a little more consideration in the winter to maintain reasonable internal temperatures. It is a good idea, for example, to open the windows when there are high internal gains (like when there’s lots of people around or when someone is cooking a big dinner), or on a sunny winter morning when there will be sufficient solar gain throughout the day to reheat the house.

In order to remove moisture, the following practises need to be considered:

  • Turning on the mechanical ventilation system as soon as moisture is produced, e.g., turning on the kitchen rangehood when cooking or the bathroom extractor fan when having a shower.
  • Keeping steaming cooking pots near the kitchen exhaust.
  • Opening kitchen windows when steam is being produced away from the kitchen exhaust, for example when pouring hot cooking water into the sink or washing up.

Through the use of passive heating, shading and ventilation the Zero Energy House remains at comfortable indoor temperatures throughout the year without relying on heating or cooling systems. Equally important though is the mindful use of the house by the people who live there. Opening the windows at the right times and turning on exhaust fans when moisture is being produced, among other small actions, keep indoor air healthy and help Shay and Jo to keep warm in winter and cool in summer.

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