Designing an efficient building envelope

Zero Energy House

Auckland’s temperate climate makes it fairly simple to design energy efficient buildings. A house that is comfortable all year round can be achieved with reasonable levels of insulation, reduced thermal bridging, summertime shading features and ventilation.

The building envelope of a house (walls, roof, floor, windows, and doors) not only protects the people living in it from the elements, but it is also a very important element of the design and can have a huge impact on the energy performance of the building.

To understand how these elements would perform in their home, Shay Brazier and Jo Woods, owners of the Zero Energy House, used a thermal model that Jo created during the project’s preliminary design stage. This model showed that with adequately insulated walls and high-performance windows the couple would be able to avoid the need for heating the house for a large part of the year. As the design process evolved so did the thermal model, in which different levels of insulation and shading alternatives were tested to find the best way to achieve the performance levels they were expecting.

Using the thermal model, Shay and Jo also realised that high insulation levels would result in uncomfortably high temperatures in the summer without adequate ventilation and shading. This issue was addressed by including opening windows and eaves that keep the hot summer sun from entering the house.

Benefits of a high-performing building envelope

For Shay and Jo, constructing there building envelope to avoid the need for heating and cooling got them 30% closer to the Zero Energy goal, as the average home utilises this much of its energy consumption in space heating. Eliminating the typical heating loads meant the size of the solar array on the roof could also be 30% smaller.

There are also additional cost and health benefits to living without the need for heating or cooling:

  • There’s no need to worry about paying for heating as electricity prices go up in winter, and year-on-year.
  • No maintenance of mechanical units is required.
  • The entire house is warm rather than having heated living areas and cold pockets in bedrooms.
  • There is always fresh air as windows can be opened in winter and the house will still be warm.

Incorporating passive design

To maximise the effectiveness of the building envelope, it is important to consider not only the products used but also solar gain. Orienting the house so that it can use the heat of the sun to warm up during the winter months is essential for the comfort and health of the people living in it. A high-performing building envelope will help trap the sun’s heat inside the house, keeping it warm after the sun has set.

Solar gain is also one of the most cost-effective ways to make a building perform; moreover, taking this approach during the design phase won't actually cost any more money. Some of the key methods used in the design of the Zero Energy House were:

  • Facing windows towards the north, and designing their size to allow the right amount of passive solar energy to enter the house.
  • Placing living areas like the lounge, dining, and bedrooms on the warmest side of the house (the north) and spaces less lived-in such as the laundry and bathrooms on the south.
  • Placing the kitchen at an eastern corner to take advantage of morning sun.
  • Designing the size and shape of the first-floor overhang to provide shading to living areas in the summer, but to allow the lower winter sun to penetrate and heat the concrete slab.

What makes the envelope of the Zero Energy House special

With the building envelope designed, Shay and Jo used a number of products and methods to help control the temperature inside, most of which are shown in the concept mark-up below.

Concept markup of building envelope features. The section shows the building when viewed from the east

One of the key features that helps Shay and Jo achieve their Zero Energy goal is the wall-framing system used in the house. Adjusting the design of a traditional timber frame, the couple removed the nogs from between the studs and replaced them with battens on the inside of the frame. This reduces thermal bridging and creates room for a second insulation layer. It also results in a 50% increase in the performance of the walls compared to the New Zealand Building Code requirements. The same approach is taken in the upper floor skillion roof.

At ground-floor level, the Zero Energy House uses a concrete slab for thermal mass that has been insulated using pods and edge insulation.

The thermal performance of windows is typically less than that of walls. To have a high-performing envelope, the windows used in the house needed to be high-performing too. Shay and Jo choose timber frames for their window joinery that perform better than aluminium and prevent condensation. Glazing in the house uses double-paned glass, which is argon-filled and has a low-e coating.

In the winter, air infiltration is a major way in which a building loses heat. Shay and Jo paid a lot of attention during the build process to seal up any leaky spots in the frame and around the windows. In this way, ventilation can be controlled to enhance the comfort of the building. When windows are closed, air doesn’t leak into the house.

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