The stack effect is the movement of air in and out of buildings on account of buoyancy. This buoyancy is due to indoor-outdoor density contrasts caused by temperature, and moisture differences.
A chimney stack increases the airflow by channeling it, as do vents in roofs. The flow may be either way, depending on which force is more powerful. This post is a brief summary of how the stack flow principle applies to weather barriers in houses.
How the Stack Flow Effect Manifests in Houses
Warm air in a house rises, especially in winter when it is cold outside. As the warm air finds its way upwards, and eventually outside it sucks cold air in at the bottom. This is why it is so important to have a well-insulated home.
The opposite happens in summer when the air-conditioner is running. The cool air falls to the bottom of the space and finds a way outside, perhaps through a badly-fitting door. Warm air then enters through the top of the building to fill the void.
Factors Affecting the Power of the Stack Effect
A tall chimney causes a fire to roar in the hearth. The hotter the fire, the more the smoke billows. In a similar way, tall buildings create more powerful stack effects. The upward breeze can be surprisingly strong in an elevator shaft on a hot day.
The Grenfell Tower fire in London was a powerful example of this force of nature. There was a cavity, open at the top and the bottom, between the outer aluminium cladding and the inner insulation. This drew the heat from burning apartments upwards that was impossible to control.
How the Slack Effect Applies to Your Weather Barrier
There’s a space between your siding and your exterior wall. This may be caused by the attachment method, or it may be a deliberate rain screen creating a cavity. Whatever the case, you can bet the stack effect would like to find a way through. There are three reasons why we need to prevent this:
# Water vapour from the air may penetrate, accumulate and cause damp problems
# A weak building envelope significantly increases our heating and air con bills
# Cold drafts in winter and humid ones in summer can negatively affect our health
Get More Advice from Valiant Exteriors Limited
If your HVAC is not performing as well as it should, or your HVAC bill is higher than your neighbours’ then the stack effect may be finding a way through your weather barrier.
If this is happening, then there is also a possibility water is accumulating in the cavity, instead of draining harmlessly away. Call us on (403)829-1661) or send us a message from our contact page. We provide helpful advice and deliver jobs well done.
Air Pressure and the Building Envelope: Pennsylvania State University