1] It’s okay to shape-shift.

Designers are very familiar with the concept of iterating on design ideas, and this project reinforces the value of testing and analyzing multiple building forms before locking in a selection.

The original proposal turned out to be two times over budget. By simplifying its structure and slightly reducing the floor area, Sterner Design was able to cut construction costs in half, bring the design within budget, and achieve a 30% reduction in heating & cooling loads.

Comparing heating and cooling loads across design options. Design alternative B achieved a 30% reduction in EUI. Image courtesy of Sterner Design.

Reflecting on the journey to the final built form, Sterner recommends that “..carrying out a form study that compared compact vs. elongated forms would have made for a more efficient way to identify the ideal building form.”

Quickly modeling and analyzing these simple Whitebox models in SketchUp and Sefaira allow for the efficient comparison of buildings based on form. Image courtesy of Sterner Design.

The learning: Quickly model and analyze multiple forms to find energy and cost-saving opportunities.

2] Where possible, refine and simplify.

Changing the form and resulting complexity resulted in a 10% decrease in energy use.

One of the hallmarks of good architecture is that creativity and function merge to create a refined, almost unquestionable built form. The benefits of ‘simplifying’ to crystallize your design go beyond simply delivering great architecture and increasing buildability; it can also drive energy use down. By reducing the number of junctions, corners, and connections in his original proposal while still delivering his client’s spatial requirements, Sterner Design achieved a considerable 10% reduction in energy use. According to Sterner, “This experience has shown me how tightly intertwined design and performance are. By reconsidering some of the basic design moves, we delivered a more energy-efficient design with a 50% reduction in capital cost compared to the original design. Net Zero or Passive House design is not something that can be easily tacked on to a design at the end — it needs to be integral to the design’s DNA.” Analyzing design options early and often created the opportunity for more meaningful client conversations: hard data gave the clients the confidence to approve changes that still delivered on the brief and saved them capital and operating cost.

The lesson: It’s certainly worth keeping things (relatively) simple. Crystallize your design to deliver great architecture, increase buildability, and drive energy use down.

3] Choose the right active HVAC systems.

Designers will agree that specifying the right HVAC system can elevate or tank your passive efforts. Not only can it lead to space savings and a better internal environment, but your HVAC choice can also spike capital and running costs for your client.

Again, leveraging Sefaira’s HVAC analysis early and often to test system combinations that could work for your specific site and project could be the difference between your client spending or saving thousands of dollars.

The Iowa Nest ended up with HVAC systems that were cheaper than conventional, but that provided better comfort and indoor air quality and contributed to it achieving net-zero. How? Firstly, by eliminating air conditioning, which is not an easy feat in Iowa’s mixed climate.

HVAC system cost comparison for the design. The middle option was the original proposal and the third option reflects the final design. Image courtesy of Sterner Design.

The project incorporates passive design strategies such as earth berming, shading, natural ventilation, and minimization of internal loads. This reduced cooling loads to such an extent that the designer could avoid air conditioning altogether. Similarly, high insulation, good air-tightness, and passive solar design reduced heating loads to such an extent that he could specify a small, cheap, electric radiant heating system.

While the inefficiency of electric radiant heat is typically a poor fit for a net-zero home, in this case, loads were so small that the energy penalty was marginal and could be easily made up by some additional PV panels.

The lesson: Look at HVAC and envelope strategies in combination.

4] Draw the details.

Detailed specification notes for the home’s window insulation. Modeled in SketchUp with comprehensive drawing sets created in LayOut. Images courtesy of Sterner Design.

Sterner leveraged SketchUp and LayOut to draw and produce a comprehensive set of typical and bespoke details to achieve airtightness and a stable internal environment. He specified how typical wall details, bespoke wall junctions, windows, and doors needed to be air sealed and insulated.

The home’s air-tightness layer highlighted (left). The home’s thermal insulation highlighted (right). Image courtesy of Sterner Design.

This level of rigor resulted in an airtight interior with no draughts and little to no internal temperature fluctuations, despite a recorded annual temperature delta of 70oF on the site.

Temperature comparison data showing interior temperatures (blue, pink, and red lines) against. outside temperature (black line) with no active heating on during the polar vortex of February 2021. The design delivers steady, comfortable internal temperatures despite exterior fluctuations. Image courtesy of iowanest.com.

The Lesson: Drill down to the details and specifications to ensure an excellent build and occupant comfort.

5] Beyond EUI: Passive Strategies & Metrics.

Although EUI is great for benchmarking and understanding how good a design is overall, it is not a great design metric. It can hide potential problems, particularly around thermal comfort and high load issues. Further along the design process, an engineer observed that the original proposal, which would have cost twice the budget, would likely have had cold bedrooms in the winter and a sweltering office in the summer. This uneven distribution of heat may also have required a more complex, expensive, and/or larger HVAC system to alleviate.

By performing thermal comfort analysis using the operative temperature metric, the team could eliminate the need for air conditioning, specify a smaller, cheaper HVAC system, and avoid uncomfortable rooms within the home.

Once a reasonable EUI baseline is established, it pays to assess other metrics such as peak loads, operative temperature, thermal autonomy (the percentage of time a space maintains comfort passively), providing more granular & meaningful insight during design.

The power of passive design strategies – With final project cost, including valuing the owner’s self-build time at standard labor rates, the build still cost 9% less than a conventional home.

More information about the project, including thermal comfort and energy data, is available at the home’s website, www.iowanest.com.