Permaculture Designers Manual
CHAPTER 10 –
THE HUMID TROPICS IN PERMACULTURE
Section 10.5 –
Humid Tropics House Design in Permaculture
Optimum comfort levels for people are at dry-bulb temperatures of 20°C (68°F) in still air (winter) and 25°C (77°F) in summer, subject also to individual preferences.
Above relative humidity levels of 40%, we effectively add 1°C to dry bulb temperature for every 4% increase in humidity.
As average summer humidity in wet dry tropics commonly exceeds 50% and long periods of humidity of 70% or so are experienced, there are times when sensible temperature exceeds 30°C (86°F) and heat stress results.
In homes, a useful indicator is a wet-bulb thermometer, where the mercury bulb is kept damped by a cotton wick drawing from a beaker of water. Below 18°C (65°F) wet-bulb temperature, we can remain fairly comfortable.
Factors that accentuate heat are nearby radiant surfaces and lack of air movement.
For a nearby radiant heat source (wall or pavement) that exceeds 38°C (100°F), we can add 1°C (per degree radiated) to the air temperature and conversely, we can subtract 1°C for any air flow above 1m/second.
Evaporative cooling in dry air greatly reduces heat, but as the high humidity periods of the tropics do not enable us to cool by sweating, we must therefore use every strategy available to de-humidify air (mainly by cooling), to cast shade, to develop cool surfaces and above all to induce cool air currents in houses.
Over shading trees, attached shade house, white exterior and interior surfaces and clear path breezeways are essential design strategies both in equatorial and subtropic
In many continental subtropical locations, we are faced with dual problems of quite intense winter cold, with some frosts (and rare snows) and very humid and hot summers.
Thus, the sort of house we need to build has several unusual characteristics and needs perhaps more careful planning than either equatorial houses (where reducing heat is the only problem) or temperate and boreal housing (where providing heat is the only problem).
The subtropical house needs to both heat and cool.
For heating, it needs to have an insulated slab floor or trombe wall and for the cooling system it needs induced or forced cross-ventilation from a cool or shaded area to an up draught area.
The secondary effects or high humidity range from the merely annoying (salt will not pour) to far more serious effects (clothes, food, film and books mildew).
Thus, we face two sorts or problems in house construction:
- Human comfort.
- Safe storages; (Figure 10.19) for these we need both cool and warm storages, but both need to be dry.
Human comfort is greatly aided by these factors:
- SHADE: Light and heat are both excluded as incoming radiation in shade. Shade is particularly critical on massive walls or over water tanks close to homes.
- TRANSPIRATION: Plants can assist cooling by transpiring. Partial shade helps this factor in understory species except in extremely wet conditions.
- COOL BODIES: Large heat reservoirs used as water tanks and relatively cool blocks of (shaded) stone, concrete and mud brick absorb heat from the air and from warm bodies. Conversely, hot radiant bodies adversely affect us.
- AIR FLOWS: Even low air flows from shaded areas greatly aid both transpiration and evaporative cooling. To create such air flows we need to develop both relatively hotter and colder air sources and to provide a cross-flow airway. Even a fan, simply stirring the air, aids in human comfort.
- REMOVING HEAT SOURCES: All massive cook-stoves and hot-water systems are best placed in a semi-detached kitchen in the tropics. Commonly, these are reached via a vine-covered shade area, are themselves shaded by palms or trees and have wide eaves and ceiling vents for hot air escape
Heat can be used via metal roof areas, hot-water storages and attached glass-houses or solar chimneys to vent hot air and create up draught, which in turn provides a heat engine to draw in cool air.
The essentials of good cross-ventilation are that the flowing air has a simple pathway to follow (no unnecessary corners to turn) and that large vents are used to allow a good volume of air through workrooms and storage areas. (Figure 10.16)
Probably the best cooling systems in tropical houses are those which use a hot roof or metal chimney to draw in cold air from earth-cooled underground tunnels or pipes.
As this cool air is dense, it will naturally flow downhill or sink to lower levels; this cool air can be drawn into houses via a positive exhaust system or actively fanned into rooms.
To cool a pipe and lead off the heat continuously, we need to construct a trench 1m deep and 15-20m long cut in the earth, drain off the condensed water (ideally making it self-draining to a lower slope) and provide this trench or pipe with a sloping floor.
The intake end can be box-screened to keep out mice and shaded by plants. Outlets can be floor grills or a louvered “cupboard” opening to the pipe or pipes in the trench. (Figure 10.17)
Natural cross-ventilation can occur if a well-sealed room has a roof vent or chimney to create an up draught. Some forms of air scoop may help this process.
The cold tunnel solution is very effective and can be used together with evaporative cooling in desert housing, but it is also expensive and difficult to fit to an existing house.
For this reason, many homes can be sufficiently cooled by the use of vertical shutters acting as air scoops – a satisfactory solution on subtropical trade wind coasts.
Or a shade house can be added to the pole ward side of a house and cross-ventilated to a well-vented GREENHOUSE on the sun side of the building. (Figure 10.18)
BASIC ESSENTIALS OF THE EQUATORIAL HOUSE
Some essentials of truly tropical housing (no cold season) are:
- Orientation is to prevailing winds, not to the sun. Cooling is by cross-ventilation;
- Shaded valley sites greatly aid cooling and shelter;
- Induced ventilation is essential, achieved by siting in palm groves or over shaded by trees, which should be permeable to wind at ground (house) level. Palms and trees can be pruned up the trunk;
- Site sheltered from hurricanes, tsunamis and volcanoes and sited on stable soils that resist mudflow in heavy rains;
- Walls white or reflective, over shaded by wide eaves and palms or trees;
- Heat sources such as stoves and hot water systems detached from the main structure (e.g. outdoor kitchens);
- Wall-material light, even permeable to wind (woven matting and mosquito screens);
- Mass, if any, internal to rooms, smooth and white-painted. The whole house can be of light construction on the outer walls;
- Vertical louvers and window shutters aid in cross-ventilation;
- In hurricane areas, a strong central core or refuge may be needed or an earth bank shelter raised to protect the house; cellars should be entered above ground due to flooding danger in hurricanes, or well sealed against flooding;
- Very strong cross-bracing, deep ground anchors and strapped timbers may be necessary if powerful winds are known to the area. Large bamboo groves placed to the windward will bend to the wind without breaking, protecting the house;
- Where thatch or tile is impractical, use a vented sheet metal roof. In this case a thin (12mm board) ceiling is necessary, and soffit lining can be of permeable netting or screened to allow an air flow to the roof space and thence to the exterior via high roof vents.