Sterner Design: Daylighting an Underground House & Achieving Net Zero Energy Status
Sterner Design was commissioned to design a single family residence nestled into the hillside on a rural site in southeast Iowa. Two clear goals had been set for the Nest Residence:
- To set the project on track to achieve Net Zero Energy status.
- To ensure that the primary living spaces achieved great daylight so that the home would be warm and light-filled, despite being partially underground.
Project type: Net Zero Energy target single-family residential
Location: Iowa, USA
Lead Designer: Carl Sterner
Building type / program: Single family residence
Floor area/Occupancy: 2 stories, 2400ft2 living area (plus unconditioned storage and garage)
Project stage & completion date: In design
Performance Baseline: IECC 2012
Performance Goal: Net Zero Energy, good daylighting
Approach 1: Delivering Design Options with Good Daylight in an Underground House
In this project, daylight was approached from an experiential point-of-view. Carl Sterner, lead designer and Principal of Sterner Design, set out two benchmarks to evaluate the suitability of design options:
- “Enough daylight”: The first goal was to achieve 200 lux (typical light level for a residence) through daylight alone for at least 60% of year in the primary living areas: living room, dining room, kitchen, and study.
- Even distribution: The second goal was to ensure good daylight distribution across these spaces to avoid uncomfortably bright and dim areas, despite having windows primarily on one orientation.
Analysis was used from the very beginning of the project. Using Sefaira’s daylight analysis, Sterner investigated the impact of different conceptual design options on daylight penetration to help understand the daylighting potential of each option.
Designs were not necessarily ruled out due to poor performance; instead, the analysis was used to understand what mitigating steps would need to be taken if these designs were pursued. Options with a narrow floor plate (20 ft or less) were ideal from a daylighting standpoint; deeper floor plates would require more creative (and perhaps expensive) solutions such as skylights. The parallel energy analysis showed that designs with a more compact shape tended to have better energy performance.
Following several design iterations, the team arrived at a concept that combined daylight and energy performance with the client’s requirements and budget. At this stage, Sefaira analysis was used to further refine the design and address potential problem areas. One such area was the northwest corner of the main living area, which was initially dim.
To tackle this, Sterner tested the impact of taller windows, light shelves, and ways of “borrowing” light from the well-lit level above. To improve daylight in the NW corner of the lower level, a light well will be built into cabinetry in the study above, achieving even daylight distribution below without the energy and waterproofing drawbacks that accompany conventional skylights.
Approach 2: Achieving Net Zero Energy in a Mixed Climate
On the energy side, the plan was to reduce overall energy use to the point where the remaining needs could be reasonably generated on-site. Initial analysis put the site energy budget at 30,000 kBTU/year — which meant that a lot needed to be done to get Annual Energy Use down from the initial code baseline of 118,738 kBTU.
Iowa, like much of the American Midwest, poses a difficult climatic challenge. A mixed climate means it is very cold in the winter but quite hot and humid in the summer. To reduce energy use, proposed strategies needed to keep both cooling and heating requirements in balance.
Resolving the Design & Material Palette
Sefaira analysis helped Sterner target the specific causes of heat loss and heat gains (namely, conduction, infiltration and solar gains), and these results impacted the material specifications a great deal. Insulated Concrete Formwork (ICF) was selected to deliver multiple benefits: the poured concrete adds thermal mass to the design, and air tightness is simultaneously improved because of the unbroken poured concrete structure. ICFs also provided a cost-effective way to achieve the required insulation and air-tightness levels without adding significantly to construction cost or time.
Sefaira analysis also showed that in the summer, strategies like shading would be critical for cutting down cooling loads, but that any shades would block beneficial heat gains in the winter. In this scenario, flexibility was crucial. In response, Sterner used a combination of small fixed overhangs and operable shading screens over the South windows.
By combining both passive strategies (natural ventilation, an optimised building fabric, and shading devices) and efficient mechanical systems (Ground Source Heat Pump and Energy Recovery Ventilation), Sterner was able to achieve a 79% reduction in energy use from code baseline to meet the target site energy budget. To offset the remaining energy use and achieve Net Zero Energy status, the design incorporates roof-mounted solar photovoltaic panels on the large south-facing roof.
About Sterner Design
Sterner Design is an architectural design and consulting practice dedicated to creating a truly sustainable built environment. Their experience includes ambitious LEED Platinum, net positive energy, and zero-waste projects, and encompasses a wide range of project types, from single-family residential to urban planning. Practice director Carl Sterner is also the Director of Product Marketing at Sefaira.