Natural Ventilation and Colour in Hot Climates
How buildings heat up when not ventilated
In a hot climate, the cost to air condition the entire volume of a large building is often prohibitive, and without some precautions a building quickly heats up during the day via radiant heat and heat conduction.
Radiant heat from the sheeting and purlins make the inside feel hotter and a lot more uncomfortable than the building’s ambient air temperature would suggest.
The ambient air temperature is also increasing via conducted heat from the hot sheeting and purlins. This creates a ‘hot zone’ which starts at the apex and spreads downwards during the day, heating more and more of the trapped air mass inside the building. The air temperature at ground level will hit its peak by around mid afternoon.
‘In’ and ‘out’ vents, chimney heights and air flow
In the perfect natural ventilation system, the ‘out’ vent is at the apex (and nowhere else) and the ‘in’ vents are as close to the ground as practical.
This arrangement keeps the ‘chimney’ height as large as possible, and by doing so, not only creates the greatest air flow, but means that there is plenty of air movement near the ground where the people generally are.
Vents should be on all of the walls. The R EIDsteel ridge vent has been designed to sit at the building apex and adds its power to the ventilation by allowing wind from any direction to suck air out of the building. Vents can be added to our buildings.
Our standard vent has a throat of 200mm, it should be the full length of the building except in hurricane/ cyclone areas where the vent should start 5m from each gable end to avoid damage.
The 200mm vent is good for up to a 30m span or so. The area of ‘in’ vent on one side and on one end added together should equal the area of the ridge vent. In buildings with a heat source and in very big buildings, the ridge vent and the matching wall vents should be increased. Mechanical ventilation may also be needed, preferably assisting the natural ventilation.
Internal air temperatures
With perfect natural ventilation the internal air temperatures stay very similar to the external air temperatures, but it is the air movement past the workforce which keeps them cool.
Sheeting colours and radiant heat
The amount of radiant heat within the building will depend on how hot the inside of the roof/wall sheeting becomes. To minimise this heating, the colour of the roof should always be white, or as light a colour as possible.
The colour of the walls should also be as light as possible, though this is less important than the roof. At the hottest part of the day, white sheeting may reach temperatures of up to 50°C. Black sheeting however, may reach temperatures up to 110°C !
If you want to be at 25°C, the black sheeting could be as much as 85°C above the chosen temperature – that’s 3.4 times worse than the equivalent sheeting in white!
Insulation reduces the temperature of the inside of the sheeting. This greatly reduces radiant heat, and reduces the warming of the high level air.
Roof insulation is more effective than wall insulation, however, if the building is to be cooled (or heated) then insulation to the roof, walls and even floors is essential.