Stack effect can be an effective natural ventilating system for tropical homes. It was once a popular strategy for vernacular homes in tropical regions. However, the stack effect strategy has lost a lot of its popularity in recent decades.
What Is Stack Effect?
Stack Effect is a buoyancy-driven ventilation system. It uses the density of air to facilitate air movement in a building. Air has different densities according to its temperature.
Hot air is lighter than cool air. When the air outside is cool, it enters home. The air inside the home warms up from activities and heat sources such as cooking, electrical appliances and even body heat. As this interior air warms up it rises to the upper parts of the room.
During the day the air immediately under the roof can also tend to be warm due to the radiating heat from the roof.
If the warm air at the top of the room or house is allowed to escape, the air inside the house has to be replaced with other air. Hence cooler, heavier air from the exterior is pulled in from openings at lower levels of the house.
Strategies For Stack Effect In Tropical Homes
The stack effect works best when there is a significant difference between indoor and outdoor air temperature.
In temperate regions, the stack effect can be a problem in a home during winter months.
Since the air on the exterior is very cold, efforts are made to heat the interior space. However, in their situation, it is extremely important not to let that warm air escape at the top of the house. This will encourage unwelcomed cold air to enter their homes.
Therefore, while homes in temperate countries are designed to keep the heat in, tropical homes should try to let the warm air out. This allows cooler air to enter.
Cross ventilation is still the preferred method of passive ventilation in a tropical home. However, if your home is located on a site where breezes are limited, the stack effect can be used to encourage some air movement.
They are two main factors that impact the flow of air in your home using the stack effect.
Temperature Difference Between Interior and Exterior Air
The stack effect is most effective when the indoor and outdoor temperatures are very different.
However, since the temperature range in some tropical countries do not fluctuate much, sometimes there is not a great difference between the interior and exterior air temperature.
Ideally, you should ensure that the air entering your home on the windward side is as cool as possible.
Try planting shady trees and plants on that side of the house. This creates a cooler temperature outside your home. Unshaded paved area absorbs and radiates heat collected from the sun. This generally makes the area around hotter.
The upper-level window where the heat escapes should be located in an area where it is likely to get warm.
This could be right under the roof which tends to be hot, as mentioned before.
It could also be a glazed area that receives direct sunlight. This one of the few times you may hear me say that letting heat into your tropical home is okay.
The hotter that air is, will mean the faster it will escape. Hence, pulling in cool air faster as well.
That hot air or direct sunlight should still be kept far away from habitable spaces as much as possible.
Difference In Height Between Windows
The height distance between the windows that let in cool air and those that let out warm air should be as much as possible. The further these are apart from each other the better.
This, like the temperature difference, increases the flow rate of air movement in your home.
Hence the top windows should be as close to the roof or ceiling as possible.
By extension, having spaces with high ceilings would be ideal in this situation. Not only will it increase the effectiveness of the stack effect but it will also ensure hot air is always high above the area where you and your family are using.
If you are considering introducing stack effect strategies into your home, here are a couple of things you can consider.
Roof Vents
Roof and gable vents were a common element in traditional, vernacular architecture in tropical regions.
In Barbados, the chattel house had louvred vents at the top of the gable ends of the roof.
Masonry houses used breeze blocks to allow hot air to escape from the interior.
However, most homes no longer have a way of venting hot air from the roof space.
I think this should still be part of designing and building a house in the tropics.
There are concerns such as security, easy access to close them when necessary and the availability of well-designed vent windows for contemporary applications. However, I still think it is worth exploring.
Clerestories
Clerestory windows are high-level vertical windows. They are located at the upper level of a wall. It is also normal to see clerestory windows located over part of the roof of the building.
Because of the height that these windows are located, they can be an excellent way to allow hot air to exit a building. They are also a good way to introduce natural, indirect light into interior spaces.
I tend to see clerestories in more institutional applications such as schools and churches. In our region clerestories are not as common in residential applications.
Jalousie windows are popular for clerestories. These jalousies are good at keeping out the rain while open.
Perhaps this is for similar concerns as the roof vents. They are not easy to access when they need to be closed. However, with the growing popularity of motorised, automated louvre and other windows, this could see an increase in them being used.
Conclusion
The stack effect in tropical homes is not as popular a strategy as before. However, I think it can be an effective one. In the tropics it is important to allow rising hot air in your home to have a place to escape, making room for cooler outdoor air to enter.
They are two main factors that promote increased airflow rate. The first is to increase, as much as possible, the temperature difference between the indoor and outdoor air. Secondly, create as much distance between the low-level windows that let in cool air and the upper-level windows that let out warm air.
Consider introducing roof or gable-end vents and clerestories, to provide a way for hot air to escape you home.
Having cool indoor temperatures is critical for any home in the tropics. The stack effect is one way to assist you in creating a comfortable indoor environment in your home.