The question often arises whether roof ventilation is a good way to help cool a home. I generally don’t recommend them. This post explains my reasoning.
By Richard Keech
Roof ventilation – how does it work?
Roof ventilating fans are a common and fairly simple device. They can be mains powered, solar powered, or simply operate by natural convection. They have a simple principle of operation:
- The sun heats the roof, which in turn makes the air inside a roof cavity very hot;
- Heat transfers through the ceiling and into the living spaces, reducing thermal comfort in summer;
- Sucking out the hot air lowers the air temperature, which in turn lessens the flow of heat into the living spaces.
So at face value, this would seem like a simple and good thing to do, right? After all, the physics are very simple. The rate of heat conduction through the ceiling changes in direct proportion with the temperature difference.
Why it’s not all it’s cracked up to be
However, I think there’s a lot more to it than that, because the benefits are often exaggerated, and there may be unintended consequences.
If you have a summer thermal-comfort problem, then active roof ventilation, in most cases, should not be the first or primary method of dealing with it because:
1. You don’t live in your roof. Trying to cool your roof cavity is focussing on the wrong problem. It’s a second-order issue;
2. Focus first on the thermal envelope. You can most effectively control the flow of heat into the living spaces by improving the insulation and dealing with draughts;
3. Seasonality. It doesn’t help in winter, whereas insulation helps in both hot and cold weather;
4. Draughts. Actively extracting air from the roof cavity can exacerbate pre-existing draughts within the house;
5. Moving heat. The capacity of the fan to move heat is small compared to the direct solar radiation falling on the roof. So the capacity of the fan to offset the solar heating effect is small too. See the example below for more information;
6. Not so squeaky. As fans age their bearings can become noisy;
7. Minimise penetrations. As a general proposition, penetrations to the roof and ceiling should be avoided if possible. In the case of roof fans, aside from draughts, they can become pathways for ingress of wind-blown water, insects, dust, smoke and noise (including wind noise);
8. Visual impact. Roof ventilators become a visible artefact on the roof that some might not find appealing;
9. Fire risk. In the case of a house fire, a roof ventilator fan might allow fire to spread more quickly. In the case of bushfire, the rooftop penetration might make the house more vulnerable to ember attack. If you live in a bushfire-prone area, then you’ll want to be sure that any penetrations comply with requirements of AS3959 and use a non-combustible mesh material with gaps no larger than 3mm.
The colour and material of the roof surface can also make a big difference. A light-coloured surface can significantly reduce the direct solar heating. See the specific example below for a sense of how big an impact this can make.
When would it makes sense?
Fitting roof ventilation fans might make sense after you’ve fixed other things, especially if you have ducting that passes through the ceiling cavity.
Maths of moving heat
Extracting heat with moving air
Just how quickly can a fan extract heat? Lets look at an example.
Air flow rate. Let’s assume that the ventilator fan can move 1000m3/h. This is 0.28m3/s (F) This rate is in the middle of the claimed range of capacity of one solar-powered unit I saw described.
Temperature difference. Let’s assume a heat difference of 20degrees C between ambient and within the roof cavity (dT).
Heat capacity of air. Air at constant pressure can hold 1.0kJ per kg per degree. This is called the specific heat capacity (Cp).
Density of air. One cubic meter of air weighs about 1.1kg at 50degrees C (rho).
So the net flow of heat associated with the air flow in this situation is simply:
= 20 * 0.28 * 1.0 * 1.1
= 6.2 kW
Heat added to the roof by the sun
At the same time as heat is being removed from the roof cavity by the fan, heat is being added by direct solar heating of the roof. It’s well known that direct solar radiation that reaches the earth carries 1kW per square meter. In summer the sun angle is very high, so most of a roof will be exposed to the sun in the middle of the day. Common dark roofs are very effective at directly converting that solar energy to heat because very little is reflected.
Let’s assume a house has a roof area of 250m2, and it’s midday in mid summer in southern Australia. The angle of the sun is about 75degrees above the horizon, so by simple trigonometry, the 1.0kW is scaled by about 0.96 on average. Let’s assume the roof is Colorbond Monument, with a specific absorbtion of 0.73.
In this scenario the thermal power into the roof surface is a massive 250*0.96*0.73 = 176kW.
In this example, sun is heating the dark roof at 176kW. Since the roof doesn’t keep heating up indefinitely, clearly the incoming heat energy is balanced by an equivalent heat flow out. The heat will be lost from the roof normally and mostly by thermal emission (i.e. glowing of hot objects) and conduction to the air. An additional 6kW of heat loss represents only a very small part of the overall heat balance of a dark roof in summer. So it follows that extracting 6kW won’t make a very big difference to the air temperature inside the roof cavity, all other things being equal.
What if we substitute the Monument (dark) roof for Colorbond Surfmist, which has a specific absorbtion of 0.32. In the above example, the 176kW of heat is reduced to 77kW. That’s a reduction of 99kW just by changing the color of the roof. Clearly changing to a lighter colour has much greater potential to reduce overheating than does extracting air from the roof.