Net zero buildings are designed to generate as much energy as they consume, creating a comfortable space while keeping operational costs low. Passive design strategies play a key role, using natural resources like daylight, thermal mass, and shading to cut energy needs before adding renewables. Lower energy loads mean smaller systems and greater cost savings. The Iowa Nest project illustrates how these principles work in practice.
Understanding Building Loads: Heating and Cooling
Every building has energy loads that boil down to two basics:
- Heating load: Heat needed to keep a space warm during cold weather.
- Cooling load: Heat that must be removed to keep things comfortable in warmer months.
Comfort ranges (comfort bands) depend on local climate and the building’s use. Schools, homes, and offices often have different needs.
How Heat Moves In and Out of a Building
A building gains heat in several ways:
- Solar gain: Sunlight streaming through windows warms the space.
- Conduction: Heat moves through walls, roofs, and floors.
- Infiltration: Outdoor air sneaks in through gaps.
- Ventilation: Fresh air is brought into the building.
- Appliances and lighting add internal heat.
- People generate body heat.
Heat escapes mainly by:
- Conduction: Out through the building envelope.
- Leakage: Warm air escapes through cracks.
- Ventilation: Outflow of conditioned air.
Cutting Energy Loads With Passive Strategies
Reducing loads lowers both costs and system sizes. Applying low-cost, high-impact strategies early brings the most value. Some standout moves:
- Glazing ratios (right-sized windows)
- Natural ventilation
- High thermal mass (like concrete floors or walls)
- Cutting infiltration with better seals
Below is a sample comparison for a 2,000 sq. ft. residence:
| Strategy |
Relative Cost |
Energy Impact |
| Glazing Ratios |
Low ($) |
High |
| Natural Ventilation |
Low ($) |
High |
| Thermal Mass |
Low ($) |
Moderate |
| Reducing Infiltration |
Low ($) |
High |
Lowering loads early means you can use smaller and simpler heating, cooling, and renewable systems.
SketchUp and Sefaira: Tools for Efficient Design
SketchUp’s pre-design tool runs climate analysis before drawing a single line. With Sefaira, designers test:
- Different building massings and compare results
- Daylight distribution for each layout
- Assembly effectiveness (walls, windows, insulation)
- Annual energy costs and energy use intensity (EUI)
SketchUp LayOut helps produce clear design documentation for the build.
Iowa Nest: A Practical Passive Design Case
Located in rural Iowa on a former quarry, the Iowa Nest aimed for:
- 100+ year lifespan
- High passive performance
- Low maintenance
- Minimal environmental impact
Sterner Design used Sefaira to drive decisions.
High Impact Design Decisions
Using Sefaira, the team ranked strategies:
- Glazing ratios had strong effect at low cost.
- Natural ventilation and thermal mass followed close.
- Reducing infiltration made a clear difference.
A color-coded bar chart displayed how each move affected energy use versus baseline code, making it easy to spot the best moves early. Design choices were tested iteratively for best fit.
Smart Glazing and Seasonal Shading
Glass was positioned to:
- Allow solar gain for heating in winter.
- Block direct sun with shading in summer while letting in diffuse light.
Daylight analysis visuals highlighted well-lit areas, flagging potential glare and identifying underlit rooms.
Lessons From the Iowa Nest Design
- Simplicity matters: Fewer corners cut cost and energy (less reinforcement, simpler bracing, lower waterproofing needs).
- Simple envelope: Insulating concrete forms (ICF) made air sealing easier and costs lower.
- Right-sizing HVAC: Smaller systems (like radiant floors and ERVs) proved enough—no bulky central systems needed.
- Cost control: Less spent on finishes and bathrooms, a compact layout helped stay on budget.
Lower Costs, Better Performance
The Iowa Nest cost about 9% less than conventional builds. Savings came from:
- Decreased finishes (less area, higher impact per dollar)
- Minimal piping (bathrooms near mechanical core)
- Substantial enclosure value due to underground placement
A simplified cost breakdown revealed most savings in finishes, plumbing, and mechanical systems thanks to good planning.
Tracking Real-World Performance
The E-gauge energy monitor tracked every circuit in real time. After nine years, data showed:
- Winter: Inside stayed above 50°F (without heating), often 8–10°F warmer when occupied.
- Summer: Thermally stable without air conditioning, with interiors holding between 73–77°F on hot days.
- Power bills under $30 per year—proof that passive design reliably delivers savings.
Passive Principles for Renovations
Sefaira and passive strategies also fit renovation projects. Constraints like historic exteriors or fixed windows shape options. Interior strategies and selective insulation let remodels reach modern efficiency, with modeling guiding each step.
Evolving Passive Design Tools
Development now aims for seamless, early-stage integration so every SketchUp user can test concepts fast. Early and frequent modeling brings the biggest benefits, with less rework and better results.
Start passive design early for top results. Focus on load reduction first, then size your systems right, and finally add renewables. Iterative testing and simplicity—especially in geometry—pay off with lower costs and long-term efficiency. The Iowa Nest shows what’s possible for any project that puts these tools and principles to work.
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