Roof Design for Hot and Humid Climate
If you have a house with both floors and walls being concrete or masonry material, there are two scenarios to consider your roof design:-
- the situation where you have operable fenestration (openable windows, doors) on your wall
- the situation where you have no operable fenestration on your wall (as metal and glass have different property attributes like thermal-mass, in this case, we will just assume all your walls are in concrete only)
In a hot and humid climate, the way its climatic attributes cause the human body to feel hot is due to:-
- direct thermal transfer from the hot sun
- direct thermal transfer from the hot air
- indirect prevention of heat dissipation from the human body due to humid air when the moist and dust particle stay on the human skin
For both situations, you need to prevent hot direct sunlight from hitting on the interior and the human body for too long.
A non-transparent roof such as the concrete flat roof, pitch roof, a non-concrete flat roof like a metal roof is able to shade the sunlight.
The situation where you have openable windows and doors on your wall:-
When you are able to have ventilation inside your house, cool air from the outside that hits the occupant’s body makes him feel cool.
However, this also means that, when the surrounding area has hot air (think urban heat-island), hot air from the outside that hits the occupant’s body can make him feel hot.
Given that the hot air having a lower density than the cooler air, hotter air tends to rise and stay above cooler air.
This is where a roof without a vent can trap heat from the top, and hot air slowly accumulates until it reaches the occupant at the occupant level.
Our ancestral design suggests that a traditional jack roof can solve the problem with stack effect (hot air rises, cooler air remains at a lower level).
However, when the floor area is big, it often requires a high roof space and the light intensity in the indoor environment is too dim to undergo any daylight-required activities.
When the outside air is hot, the air movement (wind) serve no purpose for cooling.
Having skylight would defeat the purpose as it permits direct sunlight to hit into the indoor environment.
A sawtooth roof can solve the above problem with its vent-able roof design at the same time permits daylight from desire angle (facing early morning sun angle, the blue sky, overcast sky instead of direct sunlight).
When you have an operable wall and roof fenestration for ventilation, you get a cooler room in the afternoon if the outside air is cooler.
But again, permitting air exchange means allowing moist and dust particles to take up the air quality and humidity inside your house to an optimal level, which eventually, makes you feel hotter (skin becomes harder to dissipate heat).
In terms of thermal-mass, a concrete roof, concrete roof with earth (soil), pitch roof with concrete tiles has a high thermal-mass - can absorb and store heat better, while a pitched roof with timber tiles, clay tiles have a relatively low thermal-mass - stores heat and then dissipate the heat in a shorter period of time, in comparison.
This does not immediately implies that one material is greater than the other in cooling the house, and you need to study your home-space usage pattern to weight the advantages and disadvantages in them.
A timber, metal or clay roof tiles stores and dissipate heat in a short period. This implies that your room immediately below the roof feels hot in a shorter period of time when the roof gets hotter (usually in the afternoon).
Having a ceiling underneath the roof would delay the heat from air-transferring to the occupant’s body, but again it depends on the thermal-mass value of the ceiling material.
On the contrary, as the roof heat dissipates fast, your room feels cool in the evening and at night when the air temperature is relatively cool.
A concrete roof and concrete tiles roof can store heat longer, which means that they would store heat from the sunlight during the daytime and only dissipate heat during the cooler night.
This makes you feel hotter in the room at night even though the outside air temperature is actually lower and cooler. A roof with leafy vegetation does differently than the conventional roof.
When the sunlight hits the vegetation, its solar radiation that hits the leaves turns into food for the plant through photosynthesis instead of heat.
This is why standing under the tree feels cooler than under the direct hot sun (when it feels hotter even you stay under the tree, it is due to the hot air around you).
A slightly taller plant on your roof actually makes the roof cooler, as the plant leaves act as a primary “roof”, while your roof as a second roof and wind in between them as a ventilator to delay heat transfer through the air.
So what do all these mean? An architectural synthesis (combined alternative) is needed considering climate, thermal-mass, air, wind, and usage altogether:-
1. Limit the air exchange between indoor and the outside during the hot period
2. Double roof strategy: roof and ceiling, roof and leafy plants on it, or air gap between roofs
3. Increase the operability of the doors and windows (not increasing the number of doors and windows)
4. Clay tiles on a sawtooth roof with operable mechanical vents
5. Water elements in the indoor spaces