Cooling a House in the Tropics
For renters (and most buyers) keeping a house cool has very few options, usually just running fans and air conditioning. This is because most cooling has to do with the architecture and construction of the building, which can be difficult or expensive to change. However, if renovation or building new is possible, then there are many additional approaches to lowering both perceived and actual cooling.
The apparent temperature is how the temperature appears to humans. This can include a variety of factors including:
- Wind speed
- Radiation (sun)
Combining all four of these is the Wet Bulb Globe Temperature (WBGT), a not common but a measurement used by military and work environment researches.
In the tropics, there is increased temperature, radiation (sun intensity), and humidity, all contributing to a higher apparent temperature.
Heat Index - Humidity and Evaporation
Heat Index is a measurement which provides a relative temperature due to humidity over 20%. The body is cooled by evaporation, but high humidity reduces the evaporation rate, and therefore the rate at which a body can cool itself both actually and perceptual.
Heat Stroke is a serious risk in high temperatures, especially if not acclimated to them, so visitors to the tropics who come from a cooler climate need to be cautious.
Perceived Cooling - Wind on a Body
Cooling is generally about lowering the ambient temperature. However, running a fan or facilitating airflow through a house can both increase evaporation on a body, and decrease the perceived temperature by quite a bit. There is an upper limit at which room temperature air, in a hot room, would tend to feel hot rather than cool. Certainly this is the case when approaching 40 degrees Celsius.
Airflow in and out of Structures
Having a gap at the top of the roof is one architectural method for cooling a space, allowing for hot air to rise and escape. However, places that have significant air pollution for extended periods require a sealed structure or at least a less permeable one. Therefore cooling should not depend solely on airflow from the outside to the inside, or the inside to the outside of a structure.
Ceiling insulation is a signficant advantage in keeping a building cool. The use of low cost, high R-value insulation such as rice husks can make this practical. Insulation tends to prevent both warming and cooling, so ensure that no other passive or active cooling systems might override the base temperature (such as a thermal mass heating up a building and insulation keeping it discomfortingly warm at night).
The use of earth building techniques to create a thermal mass can help in places where there is a signficant difference between daytime and nighttime temperatures. A thermal mass generally stays at the average temperature in a location. That is, during the day and night the mass is slow to warm and slow to cool. Where it is cold, there needs to be exposure to the sun (and little cloud cover), and where it is warm, there should be shade to help keep the thermal mass temperature lower.
Certain parts of Thailand would have little temperature differential and therefore thermal mass would be minimally effective (or negatively effective), such as the farther south one is. However, at higher elevations and lattitudes, there is enough temperature differential that thermal mass would be effective, when combined with shade and other techniques.
Shade - Lessen Radiation
Shade is meant to lower the effect of ambient radiation on a structure, including infrared (heat) and other forms of radiation (such as UV) which also tends to damage structure surfaces.
Roofs, sun screens, verandahs, and vegetation are the common means to increase shade. The vernacular architecture in Rajasthan, India has Jali, lattice screens which are highly decorative and made from soft stone. Aesthetically interesting and intricate, they create shade without blocking wind.
In areas of seasonal high air pollution, ensuring that shade does not interfere with airflow is useful in the low-pollution seasons, but should not be the sole means of cooling.
Passive Cooling with Ground Contact
Ground contact flooring (slab-on-ground) acts as a heat sink and cools a structure (and also lowers the apparent heat through contact with the ground. In places where barefoot, socks, or very thin sandles are in use in the house, this can have an added cooling impact on perceptual temperature.
Courtyards (and Wind)
Courtyards are another well-used aspect of passive cooling, where shade is present from the surrounding structures, and wind is encouraged to flow through buildings and courtyards with temperature differentials. See more vernacular architecture in Rajasthan and related climates.
Misting / Evaporative Cooling
One technology which can work in high heat, low humidity situations is a misting/evaportaive cooling system. These pump fine mist directly into the air surrounding the openings of a structure. I've seen people run a hose up over a roof to get the same cooling effect of an entire building, though there should be some way of recapturing the water. That said, if there is decent insulation in a ceiling, this is not as effective.
What can be useful in times of air pollution, is that small water vapor can combine with particulates to render them heavier/inert. Note that pm10 appears to be a better target for water vapor produced by misting (generally 30 microns but possible to do with special equipment down to 10 microns) rather than pm2.5 which would be affected but less dramatically.
Stack Ventillation, Venturi, Bernoulli
A more architectural option would be having a dome-like roof with a small opening which would provide a Venturi effect / Bernoulli principle mechanism creating a breeze. Apparently the Pantheon occulus has this form of passive cooling (along with thermal mass). The cupola is a more weatherproof adaptation of an occulus.
Cross-ventillation where low openings on cooler sides of a structure bring in air that then exits at higher openings near the top helps warmer air exit. Air can be directed via windscoops from the sources of wind, and even landscapes can help direct wind (the placement of trees and other objects, along with removing barriers in front of the structure. Special air passageways could be included to help circulate air along cooler/shaded areas to warmer areas.
There are a variety of considerations when designing airflow, cross-ventillation, and the like, including steering breezes, wing walls, and night-purge ventillation. For non-passive interventions, the use of ceiling fans, counter-clockwise, helps pull hotter air up.
Unfortunately, this doesn't help very well when there is air pollution problems, as well as if there are noise issues in the external environment.
Air conditioners do dry a room but not very efficiently. Chemical or electrical dehumidifiers (though not a passive technology) may be a low ecological footprint option (instead of air conditioners)