Which Passive Ventilation Strategy Works Best for my Site?
Architecture isn’t just about how buildings look. It’s also about how they work. Part of making a building work for its particular purpose is the plan and section – in other words, the design or layout – but a larger part, mostly unnoticed by non-architects, are the systems that make it work. For example, how does the building stand up? How do you get light, power, and water to all the places they need to be? How do you ensure people can escape if there’s a fire? Then, there is a question that will get you all sorts of bonus points: How can I do this all cheaply, efficiently, and with the smallest cost to the environment?
Until about the 1950’s, it was rare for a building to be one of the sealed boxes we are used to today. Advances in HVAC technology meant that we did our best to control the internal environment, no matter what was going on with the weather outside. This came at a cost, though: it takes a lot of energy to run all of that equipment, and that means a higher energy bill. With the production of electricity and the fumes and refrigerants involved, ventilating buildings also takes a toll on the environment in the form of increased greenhouse gas production.
Passive ventilation is the natural alternative we seem to have forgotten. There are a variety of strategies to get air to move through a building without mechanical assistance. Of course, there are times where we still need to use those machines, but even then an understanding of how air moves will help us to use them more efficiently and effectively.
I started to look at different passive ventilation strategies as part of my terminal studio project at the University of Oregon. The end result – which I will post eventually – was a passively-ventilated mixed-use apartment building in Harlem. Before I decided on a site, I created this chart, categorizing the different passive ventilation types and which would work best depending on the lot type and unit division I ended up with.
The lot types are these:
- Single-exposure: Common in cities, where the building is surrounded on three sides.
- Through: Also common in cities, usually in the middle of a block, where the site is open on two sides.
- Edge: Open on three sides – Christopher Alexander would approve.
- Block: Open on all sides.
Since we’re talking about housing, as well as the need for fresh air to make the strategy work, each unit needs to have at least one exposed side. The various ways to carve units out of the building are shown on the chart above.
So what are the strategies?
Cross-ventilation: This is the one most people have probably noticed. A breeze travels from one opening to another. If it’s a hot day, and you open two windows, one on opposite ends of a room, you will likely get a breeze blowing through. This is partly caused by wind having an opportunity to travel where it couldn’t before, but it can be encouraged with a pressure differential when the first opening is larger than the second. Cross-ventilation can be created with either two wall openings or one wall opening and one roof opening.
Wing Walls: This strategy is very similar to cross-ventilation, but it instead works to capture a prevailing wind that is already present. Small vertical walls stick out from an opening, redirecting the wind into and through the area you wish to ventilate. These can be on either one face or two, as shown to the right. To get this strategy to work, I’d recommend doing a good site study to make sure you are pointing the wing walls in the right direction.
Solar Chimney: The solar chimney is probably my personal favorite, and the strategy I ended up using in my project. This capitalizes on the heat differentials – in other words, hot air rises, and cooler air drops. A solar chimney is a ventilation shaft topped by glass or another material that naturally heats the air at the top. As the heated air escapes from the top of the shaft, cooler air entering from below rises up to replace it, in effect sucking the air through the area you wish to ventilate. Air can enter the area through either a dedicated vent or an open window. The height of the shaft, as well as the dimensions of both openings determine how much air can be moved and how quickly.
Sunspace + Shaft: This strategy is sort of the opposite of the solar chimney. A sunspace, which is a small area (or entire room) where air is heated through glass (think greenhouse effect), is placed to one side of the area you wish to ventilate. Cool air drops through a shaft in the back, travels through the room, and replaces the heated air escaping through the sunspace.
Sunspace + Plenum: I am honestly not sure how well this one would work, but it makes some sense. Again, it relies on the idea of a sunspace, but instead of drawing air inside through a shaft, a plenum space is created below the area you wish to ventilate. Cool air enters here and is drawn to the back, then enters through floor vents. It is then drawn through the space as it replaces the hot air escaping from the sunspace.
The one thing that all of these strategies require is two distinct openings, one to let air in and another to let it out. All of them would work most efficiently when the air entering is from the prevailing wind, which means a careful site study is required. Some of them rely on sunlight to heat the air. Because of this, sunspaces and the tops of solar chimneys should face south to maximize the amount of sun received. Depending on what you are intending to ventilate and how often it is needed, you may need to put in some mechanical assistance or filtration when you implement your chosen strategy, but this will get you started.
Hopefully you find this chart useful. I’ve got it in a handy-dandy pdf document that you are welcome to use. If there’s anything you think I’ve left out, let me know and I’ll make an improved version.