Where is that draught coming from?

A draughty house can be the scourge of toasty toes or a be welcome relief for suffocating summer heat, but is there more we can do to control the flow of air in our houses?

Air conditioning is considered the go-to solution for making our homes, cars and places of work more comfortable, but the widespread use of this technology is causing concern over surging power demands during heat waves. Fortunately, novel ways of keeping our air fresh are on the rise.

A good breeze

It might not be something you think much about, but a well-ventilated room is important for your health. “There’s a lot of studies that have been done over the last 20-30 years that say people who work or study in naturally ventilated buildings perform better,” says Ben Hughes, a Lecturer in Building Physics from the University of Leeds. Free-flowing air removes dust and dirt as well as reducing the amount of CO2 and excess moisture in your immediate atmosphere and makes for a much more pleasant environment. “As air passes over your skin it gives you a sensation of being comfortable,” says Ben. 

But moving air also removes heat, so a well-ventilated building needs good heating – and that comes at a price. Most commercial buildings use mechanical heating ventilation and air conditioning (HVAC) systems to control the temperature and airflow in a room. But “depending on where in the world you are, between 40-60% of a building’s energy consumption is from the HVAC systems,” says Ben.

Airtight

One way to tackle this intensive energy use is to design and build super-economical airtight houses. This is the idea behind the Passivhaus standard; a house that doesn’t need heating but relies instead on heat from the people living in the house and items such as the cooker and lights. Passivhaus buildings are super airtight and airflow in and out of the house is automatically controlled: heat is recovered using a heat exchanger, sucking stale warm air out and passing this past fresh cold air, helping to raise the temperature of the fresh air.

Unfortunately this isn’t a solution for everyone. The degree of airtightness required is hard to reach in the old homes which form the majority of the housing stock in the UK. The system also relies on people abdicating their comfort to an automatic system – something we’re not very good at. “Studies show if people have control of their space they’ll tolerate a wider range of temperatures,” says Carrie Behar, a PhD student at the UCL Energy Institute.

So designers are now taking inspiration from the old way of ventilating houses and buildings, so-called natural ventilation.

A good wind

“Temperature control and ventilation are closely linked, so providing proper ventilation and keeping people comfortable can have a significant knock-on effect on energy consumption from heating and air conditioning,” says Andy Acred, a PhD student at Imperial College. “This is the idea behind properly designed natural ventilation - if people are comfortable, there will be fewer demands on the heating and air conditioning, thereby reducing the HVAC energy consumption figure as a whole.”

Windcatcher in Iran.

Credit:Flickr/33449025@N00

Natural ventilation is well established in areas such as the Middle East, where buildings use windcatchers to drive air through houses. The air is driven by prevailing winds, buoyancy and differences in temperature between inside and outside. “You need a temperature difference between indoor and outdoor of around about six degrees to keep a nice even airflow inside the building. That’s a good rule of thumb,” says Ben.

The problem with natural ventilation in more temperate climates is that it’s hard to know exactly how a building will perform in real life. Different external temperatures, varying wind conditions,how much the Sun is shining, people moving around and heat from computers all have an effect and they all change at different rates during the day.

Given the right initial conditions computer models can predict how air will flow around a room. “But it can only analyse the system once it’s designed, not suggest design solutions,” says Doug King, consulting engineer for King Shaw Associates.  

And that’s why is important for all areas that influence the design of a building, from architects who think about the space and how people live, through civil engineers who make sure the building remains standing, to building service engineers who design heating and cooling systems, to understand the physics behind air movement that keeps you feeling comfortable.

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