Natural cooling is particularly important as an economical method of keeping people comfortable and safe from extreme heat even in areas where there is no electricity or when the power grid fails. It also helps reduce peak loads and lowers the stress on generation and distribution systems. Natural cooling has been utilized for centuries and has only recently been ignored, most obviously since 1950, as mechanical air conditioning and electricity became artificially cheap. As a result architects, engineers and designers began to ignore the potential value of even the most basic principles of natural cooling.
The traditional cooling methods are:
- Solar control (shade)
- Convective cooling
- Conductive cooling
- Evaporative cooling
- Radiant cooling
These five methods of natural cooling can provide cooling and considerable comfort throughout the summer almost anywhere in the world. A better understanding and application of natural cooling options is needed because some studies suggest a half million people die from heat disorders every year.
Summer heat is a problem even in advanced countries. The monthly cost of running an air conditioner in an older home during the summer months can run as high as $350 in Arizona and people who can’t afford to fix or run their AC system are at risk. In the Arizona desert the Maricopa county medical examiner’s office confirmed 602 heat deaths in 2024 and was still investigating eight others. In Europe a heat wave in 2023 killed an estimated 47,000 people.
This article covers the first three well-proven techniques for natural cooling while the second part of the article will explore powerful cooling with evaporation and radiant cooling.
Shade (solar control)
The most cost-effective step in natural cooling is solar control. One of the most important factors for solar control is proper building orientation with very few windows on the east and west walls, and most windows on the south and north (in the northern hemisphere). Windows should be shaded or covered in summer with overhangs, awnings, exterior shades or panels. Awnings can reduce solar heat gain from windows up to 65% on south-facing windows and 77% on west-facing windows. A roof overhang can provide useful shade at low cost. This also allows the lower winter sun in for heating.
The simplest and cheapest method for exterior shading is often to hang a shade-cloth, canvas, or bamboo shade outside the window or wall supported by the roof overhang or an exterior valance. Shade cloth as an exterior screen wall can also provide shade. Shade cloth can provide low cost shade on all window orientations and is less likely to get blown around. A light colored shade cloth will be less obtrusive, but get a reasonably high density shade (50%). Exterior Venetian blinds are used in Germany, Switzerland and Austria and these can provide flexible solar control and reduce heat gain compared to internal blinds.
A space for venting should be left at the sides or top to prevent buildup of a pocket of hot air. This kind of system provides good protection for a modest investment. Where high winds are a problem, it is necessary to provide a ground anchor, preferably with an elastic shock cord, to prevent flapping and possible damage to the shade. The shade can be rolled up in the fall and stored.
Many traditional designs of outdoor space were effective for improving comfort. From the shaded streets and souks of desert cities to the toldos, fountains and landscaping of courtyards in Italy. Tall, narrow alleyways also provide climate protection. Traditional home designs sometimes had a summer and winter pattern for living to take advantage of cooler spaces in summer. Shade cloth and shade screen (with many tiny louvers) that also serve as an insect screen, can be used on windows. For east and west facing windows aluminum foil can be placed on the window.
Building colors matter also. A white roof in our studies was 25°C degrees cooler than a black roof. White wash could be used. This is a solution of slaked lime and will form a thin layer of calcium carbonate on the walls. If straw is being dried on a flat roof it can help reduce the temperature spread out and held down with a net or shade cloth. If you can afford it add a shade a few inches above the roof. Or if you can afford it add a second roof. Thats what I did to my trailer. Roof temperatures can be lowered even more by shading, sprinkling, misting. Watering adjacent paths and patios can also cool temperatures.
Convective Cooling (wind)
Convective cooling techniques were highly developed in traditional architecture and building. A visit to a plantation home in the Deep South or homes in many desert areas will enable you to see living textbooks for natural ventilation. The tall windows, tall doors, vents, tall ceilings, cupolas and orientation of these buildings were all refined to produce the greatest possible comfort without air conditioning. Cooking was done outside.
The wind or breezes, stack ventilation by thermal density differences (warm air rises), or assisted ventilation with a solar chimney, down draft design, fans, or a whole house fan can all be used. The most basic form of ventilation uses windows for cross flow. Cross ventilation is possible because the pressure on the upwind side of a building increases, and the pressure on the downwind side is reduced. This pressure differential drives the air flow across and through the building. While it is often shown as a simple arrow flowing through the building air movement can be very complex. A building is rarely the one room shown in text books, so the flow paths actually develop across a set of rooms, hallways, stairwells, and openings.
The location and shape of furniture and fixtures, the height of ceilings, door undercuts or slots above doors, transom windows and vents may all play important roles in flow patterns. Subtle changes in shape from hard edges to rounded edges or vanes can also improve flow patterns.
With operable windows people can adjust to different wind speed and directions. They can also shut windows if a dust storm is likely. This is true for homes and offices. People will open windows if given the chance.
For effective cross ventilation there should be large inlets and outlets for air movement. These can be either windows, vents, or screened or louvered doors. For better air flow the openings on the windward wall should connect with flow paths to openings on the downwind wall. Multiple windows can provide better cooling across a room or building than just large windows placed opposite each other.
In traditional designs, windcatchers were often used to augment breezes. They have proven themselves for centuries. These are very effective and can be seen in is use today in Iran. The air flow should contacts the human body to create the desired cooling. Low screened inlets and high exhaust vents are needed. The type of opening and placement of inlets and exits also influences air flow. Taller ceilings with transom windows or vents over the interior doors can help vent cave type rooms. If hallway ceilings are as tall or taller than rooms the flow can be further enhanced and easily assisted with a hallway mounted fan. Doors can be undercut to allow air flow.
Ventilation cooling can be particularly effective if the night air temperature drops significantly. You just open the windows and doors to let breezes cool the inside at night and store the coolness or “coolth” using the thermal mass of the house (adobe, brick, ceramics, wallboard, tile, water walls, etc.). The potential for night ventilation can be estimated from the high and low temperatures, dry and wet bulb temperatures, the prevailing wind patterns, and the diurnal temperature differences. In Davis July daytime high averages 34.4°C (94°F) but only 19.4°C (67°F) at night. A daytime temperature in Tucson, Arizona can be as high as 36.7° (98.6°F) in August, but a tolerable 23.9°C (75°F) at night. In August the daytime temperature in Petra, Jordan is 36.1°C (97°F) but only19.4°C (67°F) at night.
Thermal mass is needed to store nighttime “coolth” for the coming day. Water, stone, bricks or adobe can provide the needed thermal storage. The cool air from vents or windows can be most effective if it flows by the surface of the thermal mass. Thermal mass for night ventilation driven cooling is most effective when it’s widely distributed through walls, ceilings and floors. More surface area helps, because the heat exchange rate is fairly slow between air and mass. Water is more effective than masonry, concrete or stone. A steel tank with water can exchange energy more rapidly thanks to internal circulation of the water. Water walls with larger surface areas, such as corrugated vertical pipes are also a good option.
In recent years phase change materials (PCMs) have made it to market in some areas. These are light weight and effective. They absorb and release latent heat when they melt and solidify. This can enhance thermal energy storage and temperature regulation with less weight than water. Natural ventilation with PCMs has resulted in energy savings of up to 90% in some climates. More often the saving might be just 30%. Installing PCM above suspended ceilings resulted in up to a 25% reduction in peak cooling loads for office buildings in one test. PCMs have also been embedded in wallboard and insulation. By modifying the crystal size and shape, the temperature at which the material changes phases can be adjusted to suit specific building needs.
For example, Dörken, a German-based company, sells Delta Cool 24, a salt hydrate in several forms. Thermal mass has also been ignored in most building codes but was included in building code regulations we developed for the City of Davis to take advantage of cool night temperatures. These were replaced my a very weak state code.
Cross ventilation alone may be sufficient to provide comfort throughout the day in a well insulated building with good solar control. Cross ventilation is more likely to be effective in open terrain for lower buildings. Tall thin buildings were often used in traditional designs for good air flow. Cross ventilation is typically more powerful than stack ventilation, but it varies with wind direction, wind speed and turbulence and building shape. Grills or louvered vents can be used to provide ventilation without a loss in privacy or security. The magnificent grills in the MidEast, Spain and Mexico served this purpose.
Where strong winds or rain are common when cooling is needed careful detailing can reduce the impact of the wind yet still provide air flow. A sill vent allows good ventilation without having drapes and other objects blowing around. It also provides protection from driving rains and allows an uninterrupted view through the glass without the visual interference of louvers. Adding a bay window with a horizontal sill vent can provide excellent ventilation that won’t be adversely affected by high winds or rain.
Allergy problems that might be created by the introduction of mold, fungi spores, pollen, dust or smog are best dealt with by using filters. Free flowing filters may cover the entire window or vent area. These will reduce air flow so vent area may need to be increased.
Cross ventilation can be difficult to model accurately because it depends on wind speed, wind direction, window operation, furniture and fixtures, human actions, and distribution of people and activities in a building. Detailed wind data are not likely to be available for a specific site, and topographic features, landscaping and other buildings may mean that the winds on site are not similar to the nearest weather station data. It is worth thinking about it because cross ventilation works, it is inexpensive, and people like the sense of control they get from being able to open and close windows.
Stack effect ventilation
Hot air is less dense than cool air and will rise, creating air movement in a building with a low intake (cool) and high outlet (hot). Stack ventilation was used in many traditional building designs, and usually included very high ceilings, multiple floors, thin building elements, open stairwells, roof monitors, dormers or cupolas with operable windows. The intake(s) should be as low as possible and from an area with cooler air, perhaps a landscaped area or shaded courtyard with a fountain or from a qanat.
Louvers can be used on intakes to help direct cooling air towards people. The outlet(s) should be placed as high as possible. Performance can be estimated with simple formulas, calculated with software or computational fluid dynamics or tested with models. If well developed design and climate data exist for larger buildings, computational fluid dynamics programs allow for more accurate modeling than in the past.
The stack effect can augment or replace cross ventilation. The inlets and outlets should be as far apart as possible in height, with 2 m (6 feet) as a minimum and 5-7 m (15 or 20 feet) preferable. Multiple stories and tall ceilings help air stratification and air flow and improve occupant comfort. Open plans are generally best. Cubicles and partitions can interrupt the flow of window driven cross ventilation. Floor vents can be useful in some cases. Many tropical homes were built on stilts for better air flow.
Ventilation shafts, stairwells or courtyards are well suited for stack effect cooling in larger buildings. Over door transom windows can help maintain ventilation flow paths between rooms while providing security and privacy. Over door vents can be installed even in most residential buildings. To improve air flow I used a small fan in the wall to get more fresh air in a back room.
Building users or owners should be trained and provided with a simple guide to proper operation. This could be printed, placed in a plastic sleeve and mounted in a secure location where users can see it, perhaps on the inside of a closet door. Clear temperature guidelines might be provided with paired thermometers showing exit and intake temperatures to guide operators. The mechanisms for operable inlets and outlets should be well designed and maintained. Although a long pole with a hook can be used to open and close high windows and vents a permanently mounted crank mechanism is better and won’t be misplaced.
Conductive cooling
Cats and dogs are able to find the cool spots on the floor for conductive cooling. This is direct contact with a cooler surface. Not widely used or studied but effective. In our office we had waterbed as part of the thermal mass for heating and cooling. With night ventilation the water bed was very cool in the morning and made for excellent comfort in an afternoon nap.
Using these three cooling options and the two added in Part 2: Evaporation and Night Sky Radiation make it possible to make much more comfortable, safe and secure buildings. They can maintain comfort and safer temperatures without electricity in most cases. This requires integrated design.
Further Reading
Bainbridge, D. A., Haggard, K. and Aljilani, R. 2009. Passive Solar Architecture: Pocket Reference. International Solar Energy Society/Earthscan/Routledge. 90 p.
Ghamari, M., See, C. H., Hughes, D. et al. 2024. Advancing sustainable building through passive cooling with phase change materials, a comprehensive literature review. Energy and Buildings. Volume 312, 114164.
Haggard, K., Cooper, P. and Rennick, J. 2005. Chapter 3. Natural Conditioning of Buildings in Elizabeth, L. and Adams, C. eds. Alternative Construction: Contemporary Natural Buildings. Wiley.
Neubauer, L. W. 1953. Summer comfort. Small Homes Council Circular G6.0:1-7.
Olgyay, V. and A. 1957. Solar Control and Shading Devices. Princeton University Press.
Rempel, A. R. and A. W. 2013. Rocks, clays, water, and salts: highly durable, infinitely rechargeable, eminently controllable thermal batteries for buildings. Geosciences. 3:63-101.
