Passive design strategies by climate

The bioclimatic chart: strategies as zones

Bioclimatic charts plot a place's temperature and humidity against a comfort zone and show how far each design strategy can extend it. Victor and Aladar Olgyay developed the first one in the 1950s, based on outdoor conditions, and Victor Olgyay presented it in Design with Climate (1963).

Baruch Givoni's building bioclimatic chart, later expanded with Milne, moved the analysis onto the psychrometric chart and based it on the indoor temperatures that a building designed for its climate can reach without air conditioning. It marks the outdoor conditions under which comfort ventilation, high thermal mass with or without night ventilation, direct evaporative cooling and indirect evaporative cooling can keep the interior comfortable. Heating strategies, such as passive solar heating and the use of internal gains, occupy the cool side of the chart.

The main strategies and the conditions each one needs

• Air movement (comfort ventilation and fans): moving air cools people, not the air, so it only helps while the outdoor air is close to comfortable. Givoni based his ventilation zone on indoor air speeds of about 2 m/s, and higher air speed is the most common remedy for heat combined with humidity.
• Thermal mass with night ventilation: a heavy building kept closed and shaded by day and flushed with outdoor air at night. The benefit grows with the day-night temperature range, which is larger in dry air. Givoni found it applicable mainly in arid regions with daytime maximums below about 36 °C, where a high-mass, insulated and shaded building can keep its indoor peak below the outdoor maximum by about 45 to 55% of the outdoor daily range.
• Direct evaporative cooling: evaporating water into the supply air lowers its temperature by about 70 to 80% of the wet-bulb depression, but adds humidity and needs high air flow. Givoni recommended it where summer wet-bulb maximums stay around 22 °C, with dry-bulb maximums up to about 42 °C (limits he gives for developed countries; for acclimatised occupants in hot developing regions he gives 24 °C and 44 °C).
• Internal heat gains: people, lighting and equipment warm a building, so it needs heating only below a balance temperature that sits under the comfort range. Hours between that balance temperature and the comfort zone can be covered by those gains.
• Passive solar heating: equator-facing glazing to collect winter sun, thermal mass to store it and shading to exclude summer sun. US Department of Energy guidance for the northern hemisphere is to face collecting windows within 30 degrees of true south and keep them unshaded from 9 a.m. to 3 p.m. in winter.
• Shading: not a zone of its own on most charts, but a precondition for the cooling strategies; Givoni notes that large indoor temperature reductions need high mass together with effective solar control.

Matching strategies to Köppen climates

Köppen-Geiger classes are defined from monthly temperature and precipitation. Read with the physical conditions above, they suggest where to start:

• Tropical rainforest and monsoon (Af, Am; every month averages 18 °C or more): warm all year and humid, so day-night swings and the wet-bulb depression are small. Shading and air movement lead; night-flushed mass and direct evaporative cooling have little to work with.
• Hot deserts and steppes (BWh, BSh): hot, dry air with large day-night swings. Shading, thermal mass with night ventilation and direct evaporative cooling are the classic combination, within the limits above; in the hottest places they reduce the need for cooling rather than remove it.
• Cold deserts and steppes (BWk, BSk): dry like hot deserts but cooler on average, with a mean annual temperature below 18 °C, so heating strategies such as passive solar heating often matter alongside the summer ones.
• Mediterranean (Csa, Csb): dry summers and mild, wet winters. Shading and night ventilation of thermal mass suit the summers; passive solar heating and internal gains help in winter.
• Humid subtropical (Cfa, Cwa): hot, humid summers and cool winters. Summers rely on shading, air movement and fans; winters on passive solar heating.
• Oceanic (Cfb, Cfc): mild summers and cool winters. Heating strategies dominate: internal gains and passive solar heating, with shading and ventilation to prevent summer overheating.
• Continental and subarctic (D; coldest month at 0 °C or below): heating dominates, and passive solar heating and internal gains cover part of it. Hot-summer types (Dfa, Dwa) still need shading and ventilation.

Why the Köppen letters are only a starting point

Monthly averages say nothing directly about the humidity at a given hour, the day-night swing, the wind or the sun, which are what passive strategies depend on. Two sites with the same letters can behave differently: temperature swings between day and night are much larger over land than over water, so a coastal site and an inland one in the same class can suit different strategies.

The reliable approach is to plot the site's own hourly data on the chart, count the hours each strategy covers and check when they occur. Published Köppen-Geiger maps for 2071–2099 (Beck et al., 2023) show how the classes are projected to change under several emission scenarios, so for long-lived buildings repeat the check with a future weather file.

Applying it in a design

• Rank strategies by the uncomfortable hours they recover, then check what each one demands: night ventilation needs secure openings and exposed mass, evaporative cooling needs water and high air flow, air movement needs operable windows or ceiling fans.
• Shape the building to enable the chosen strategies: cross-ventilation paths, exposed thermal mass, and equator-facing glazing with summer shading.
• Combine with the other analyses: the sun path sets the shading geometry, the wind rose shows which winds can drive ventilation, and an energy model checks the result.

How C4B shows passive strategies

In projects on paid plans, C4B's passive design strategies panel plots the site's hours on the psychrometric chart with five zones (comfort range, use of fans, thermal mass with night ventilation, capture of internal heat and passive solar heating) and gives the share of the year's hours each one covers. Evaporative cooling is not one of the five zones, so assess it separately in dry climates.