Author: Charlotte Lee

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.

Incidentally, these sustainability considerations help us to deliver comfortable indoor climates whilst reducing energy needs. Additionally, it is vital to consider cultural and contextual sustainability. In the harsh climate of Norway, we have to deliver robust, long-lasting interiors and exteriors that do not require excessive maintenance.