A building envelope design process for Zero Energy

Camp Glenorchy Eco Retreat

Outside the summer holiday period, Glenorchy is a quiet town at the head of Lake Wakatipu with few visitors. However, Camp Glenorchy Eco Retreat is changing that by offering an accommodation alternative that attracts tourists and corporate groups year-round. Comfortable buildings that are cool in the summer and warm in the winter are key to making this happen.

An approach to creating comfortable buildings was incorporated into the integrated design process from its early stages, as finding a balance between guest comfort and energy use was essential to achieving the goal of Zero Energy. The team decided to focus on a holistic solution where mechanical systems and the building envelope evolved in parallel. By designing a high-performing envelope for buildings that could retain heat and warmth, reliance on mechanical systems could be reduced, and in turn, the amount of energy required to maintain comfortable indoor temperatures.

Design team meetings held from the early stages of the project allowed the team to work collaboratively and make decisions that took into account multiple points of view.

Establishing a baseline for the building envelope design

Concept design drawings created by Mason and Wales, the project’s architects, were the first step in the design of Camp Glenorchy. This set of plans translated initial goals and aspirations into a basic layout of the buildings on the site. In an effort to incorporate performance from the beginning, Camp Glenorchy’s concept design included proposed materials and R-values for the different elements of the envelope, something typically done in later stages of the design process.

Having an initial layout of the project outlining the size, use and requirements of each building allowed Tricia Love Consultants to develop an energy budget for Camp Glenorchy. The budget, which estimated in detail how much energy would be required by buildings on an annual basis, was a key component of the project’s Zero Energy design. Understanding how buildings would be used and how much energy the systems, lights and appliances needed to run, allowed the team to determine how much energy had to be produced on-site to achieve a balance between generation and consumption.

In addition to providing estimates for energy generation and use, the energy budget became a tool for decision-making. As the design progressed, each major piece of equipment that was chosen and each significant change in the layout of buildings would have an impact on the budget. This enabled everyday choices made by the design team to be assessed in terms of their impact on the project’s ability to achieve the Zero Energy goal.

Preliminary designs in the early stages of the project included proposed materials and R-values for the different elements of the envelope.

Testing the performance of Camp Glenorchy’s cabins

To achieve Zero Energy, the energy budget estimated the heating demand for Camp Glernochy’s cabins had to be around 30 kilowatt-hours per square metre per year (kWh/㎡/yr), a particularly challenging benchmark for non-residential buildings. Thermal modelling experts were brought into the design process to help determine the feasibility of meeting this target based on the initial concept. What followed was an iterative process involving them and the architects to refine the design so it could meet the stringent heating demand target.

Round one of thermal modelling

By creating a 3D model of one of the cabins using specialised thermal modelling software, the team was able to determine:

  • The energy required to keep rooms at comfortable temperatures, i.e., the heating demand.
  • Peak indoor temperatures for the summer, which needed to be within a healthy range as the rooms have no mechanical cooling system.

This first version of the model used the plans, high-performing materials and R-values provided by the architects in the concept design, which were at least two and a half times higher than those required by the Building Code. Results from this round of modelling, however, estimated an average heating demand of 51-53 kWh/㎡/yr, nearly double what was allowable in the energy budget.  

Round two of thermal modelling

For the second iteration of the model, thermal modelling experts used the same set of drawings as before but increased the R-value of all the elements of the building envelope. The objective was to understand how high the R-values of the different elements of the building envelope had to be to reach the 30 kWh/㎡/yr heating demand target.

Having the results of the thermal model at hand allowed the design team to make an informed decision regarding which changes to the building envelope were worth keeping and which ones could be discarded.

Through this exercise, the modellers were able to determine that higher R-values for walls and windows could be easily achieved without having much impact on the project’s architecture, or its cost. Others, like the R-values used for roofs and floors, required significant modifications, including an enhanced slab, 70mm thicker than the original and with more insulation.

The design team evaluated these results, their implications for the design and their practicality in terms of cost/benefit. Having this information at hand allowed them to make an informed decision regarding which changes to the building envelope were worth keeping and which ones could be discarded.

Final round of thermal modelling 

The third and final round of modelling for the cabins used an updated preliminary design provided by the architects. This version included tweaks to the architecture and a more compact footprint for the cabins. It kept the thicker walls with higher R-values that were recommended in the second version of the thermal model and added even higher-performing windows and doors. Insulation levels for the other elements of the envelope remained similar to those modelled in the first round. Through these changes, Camp Glenorchy’s design team was able to get closer to the heating demand benchmark, now estimated at 33.3 kWh/㎡/yr.

In this last iteration, the thermal modelling team ran some additional scenarios to determine if some minor changes in the specification could further reduce the heating demand. One of them, for example, increased the g-value of windows (a coefficient that measures solar energy transmittance). This brought the demand down to 25.4 kWh/㎡/yr but increased the temperature of the rooms in the summer to 35℃, which meant it wasn't viable unless the additional complexity, cost and maintenance of mechanical cooling systems were introduced.

Camp Glenorchy's building envelope
Through thermal modelling, Camp Glenorchy’s design team identified the benefits of seemingly small changes, such as adding a Low-E coating and argon filling to already high-performing windows.

Incorporating the results into the final design 

Through thermal modelling, Camp Glenorchy’s design team was able to find the point of diminishing return for the cabins’ insulation, where adding more no longer had significant benefits in terms of economy or performance. They could also identify the benefits of seemingly small changes, such as adding a Low-E coating and argon filling to already high-performing windows.

In the final version of the architectural design, recommendations from the thermal modelling team were followed closely. However, a decision was made to keep the R-value of ground slabs lower than originally proposed in the concept design. To achieve the level of insulation recommended in the third round of thermal modelling, a layer of foam insulation under the finishing layer of concrete was required. This would have created a conflict with the proposed underfloor heating system and the pipes that circulate hot water under the rooms to keep them warm. In the end, the design team opted to remove the extra layer of insulation in favour of the proposed heating solution, which was extremely efficient and made the most of the free energy provided by the sun.

Evolution of R-values through the design process

The outcomes of each thermal modelling stage , in terms of R-values and heating demand, are summarised in the table below. These values are also compared to minimum R-values defined by the Building Code for the climate zone where Glenorchy is located: 

<table>
<tbody>
<tr>
<th>Building envelope element</th>
<th>New Zealand Building Code R-values (climate zone 3)</th>
<th>Cabin R-values for round 1 of modelling</th>
<th>Cabin R-values for round 2 of modelling</th>
<th>Cabin R-values for round 3 of modelling</th>
<th>As-built cabin R-values</th>
</tr>
<tr>
<td>Roofs</td>
<td>R 2.9</td>
<td>R 8.1</td>
<td>R 9.4</td>
<td>R 7.9</td>
<td>R 7.8</td>
</tr>
<tr>
<td>Walls</td>
<td>R 2.0</td>
<td>R 6.9</td>
<td>R 8.0</td>
<td>R 7.8</td>
<td>R 7.8</td>
</tr>
<tr>
<td>Floors</td>
<td>R 1.3</td>
<td>R 6.7</td>
<td>R 7.6</td>
<td>R 7.0</td>
<td>R 4.7</td>
</tr>
<tr>
<td>Windows</td>
<td>R 0.26</td>
<td>R 1.1</td>
<td>R 1.1</td>
<td>R 1.2</td>
<td>R 1.2</td>
</tr>
<tr>
<td>Doors</td>
<td>-</td>
<td>R 0.8</td>
<td>R 0.8</td>
<td>R 1.2</td>
<td>R 1.2</td>
</tr>
<tr>
<td>Expected heating load (target 30 kWh/㎡/yr)</td>
<td>n/a</td>
<td>51-53 kWh/㎡/yr</td>
<td>~30 kWh/㎡/yr</td>
<td>33.3 kWh/㎡/yr</td>
<td>See footnote</td>
</tr>
</tbody>
</table>
<figcaption>Outcomes of each thermal modelling stage, in terms of R-values and heating demand.</figcaption>

Footnotes
  1. Camp Glenorchy’s sustainability team has been analysing performance data during operation and will be making this information available in the near future.

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