Ventilation – Design Tips

magazine spread from original article
Note: This article appeared originally in Green Magazine as in the image above. The text below may differ slightly from the original.

Tip 1 – Build in passive ventilation

There are three reasons you want ventilation in your home: cooling, drying and stale air flushing. We are all aware that a good breeze inside the house will graciously fulfil all three purposes without breaking the budget. With passive ventilation however, there are no running costs but the occupants have to be the active ones! Opening windows and skylights when there is a breeze, closing them when it’s too hot or cold outside, the running is up to you and so is the quality of the air. Many homes are very poorly ventilated and only rely on the front door opening now and then and air leakage from building cracks and window gaps to keep the air somewhat breathable! Passive ventilation can be helped with good design, just read on.

Tip 2 – Design for cross-ventilation

Passive or natural ventilation is to be encouraged in summer to create air movement inside the house and reap its desired cooling effect. In winter, keeping the air fresh and flushing vapour and air  pollutants also warrant regular opening of doors and windows. Passive ventilation takes two forms: it is either cross or stack ventilation. Cross ventilation is the better known form and is the one that may slam shut your doors on a windy day. The way to set it up is to start by identify the direction of the prevailing summer cooling breezes in your area. In Australia, these principally originate from the south, ranging from south east to south west (check the Bureau Of Meteorology website). The idea is to ensure that some opening window sashes are located to face the prevailing breezes to let them in, while other windows or doors, typically of larger dimensions, are located on the opposite side. Perfect alignment of the openings in the north south direction is not critical, however one must ensure by working on the floor plan that the wind path is not blocked or too convoluted. This may mean that you locate a window in a room to face the door rather than the opposite corner, or that you create an opening or breezeway within an internal wall to allow for free air flow. A couple of strategically placed openings on the south side of the house can sometimes be enough for breezes to effectively reach most rooms inside.

Tip 3 – Mind your window type for cross-ventilation

Window size on the breeze (south) side is not so as critical as the way it opens. The most popular window type in many parts of Australia is the awning, where the opening sash is hinged at the top. While awnings can prove useful in wet Britain to protect rooms from incoming rain, they do a poor job at ventilation as the wind typically hits the glass instead of moving behind it. Sliding windows, louvres or the so-called sash or double-hung windows are a lot more competent at providing effective ventilation. Unfortunately, they are more difficult to draft proof effectively and might give you a breeze around the clock even when you don’t want it! The winning window is the casement where the sash is hinged on the side much like a door. Much like awnings, they can be made airtight quite easily but their design allows for breezes to flow uninterrupted. The trick is to hinge them on the side opposite the direction of cooling summer reezes so that the open sash can help funnel the air inside. 

Tip 4 – Stack ventilation 101

Stack ventilation happens when hot air rises at ceiling level as it does and finds its way out through an opening at high level, whether it be an open window located high on the wall, an open skylight, a cooling tower or a clerestory window with one or more opened sashes. The upper air layers escaping causes the cooler lower layers to rise in turn which creates a natural cooling draft within the house. This will work well when windows or vents, preferably facing the cooler side of the house are left open to allow for fresh air intake. If the fresh air is allowed to travel through greenery or over a body of water before it enters the house, it will have an additional cooling effect. The great advantage of stack ventilation is that you don’t need to wait for a breeze, you can create one naturally. This is perfect when nothing moves outside or when a house gets poor exposure to cooling winds. Another advantage is that at night, high level windows can remain open and secure while allowing the cool night air to literally fall into the room, due to its greater density. A word of warning however: windows are a much greater source of heat loss than insulated walls and this goes for double-glazed windows as well – although they perform much better than single-glazed ones. It is imperative that high level windows face north so at least some of that heat loss can be recouped with passive solar gains. It is also essential that they be at least double-glazed if not triple-glazed, as they are exposed to the warmest layers of air in the house with much more potential for heat loss than the cooler air beneath.  

Tip 5 – Consider ceiling fans and exhaust fans

Ceiling fans are an inexpensive way to move the air around the house which will suffice in many climes to keep an energy-efficient house comfortable in summer heat. Select a silent model combining little energy consumption and high air movement. Ensure it can be operate in reverse too as this will be useful in winter on very low speed to de-stack the heated air, especially if you have high ceilings. Exhaust fans are compulsory in windowless wet areas but are also useful for those who are not in the habit of opening their window after a bath or shower. Without a window open, these fans should not be left to operate for too long as they exhaust the air from the room, which in turn creates a negative pressure in the house, forcing outside air through unsealed gaps and cracks in the building envelope. Be sure they are fitted with a damper to minimize winter heat loss when not in use. Last but not least, ensure where possible that exhaust fans be flued to the outside rather than to the roof cavity, and select rangehoods to do the same rather than merely recirculate the air inside.

Tip 6 – Know when mechanical ventilation becomes necessary  

Europe and North America started sealing their buildings in the seventies in order to prevent heat loss in winter and cool energy loss in summer. They were soon enough faced with developing issues of mould and air pollution as the indoor air renewal rate dropped to an all time low. In contrast, the average 1960s brick veneer house in Australia leaks air at a rate of over one and a half time its interior air volume every hour. Indoor air pollution is generally not a problem then as long as windows are opened regularly in bathrooms and there are no issues of rising damp or defective roof flashing. It remains that at a time where homes are better sealed and designed to be more energy efficient, many of the new ones are dropping to or under an air renewal rate of half their air volume per hour. This is the threshold for serious indoor air pollution to start developing, says Bernard Deshormeaux from air2energy, a company specializing in heat and recovery energy ventilation. Passive ventilation with outside air intake (read: opening windows) is one solution but it defeats the purpose of sealing homes, particularly where heating or air conditioning systems are running. There are now regulations in the majority of European countries as well as Canada and many American states to have apartments, units and single dwellings equipped with mechanical ventilation fitted with energy recovery system so that the energy efficiency of the building envelope is not unduly compromised. Mechanical ventilation systems are also useful for those with asthma, pollen and other allergies, as they are equipped with a filter. For once, you’ll find the air cleaner indoors than outdoors!  There is no doubt these ventilation systems will become more mainstream in the future in Australia, as levels of mould, fumes and radon pollution become increasingly problematic in newer dwellings. In the meantime, open your windows at least briefly every day, operate your rangehood when your gas cooktop is in use and use balanced flues for gas and wood heaters (they conceal two flues inside, one for air intake and the other for exhaust).