Air Flow in Green Building

Airflow through a houseI have titled this website “Airflow and Shade” because I personally consider them to be the two most important factors for tropical green building.

It seems obvious that you’d like to have a nice breeze flowing through your home, but so many people, including most architects, build houses that don’t allow for an appreciable amount of fresh air, so I thought it was important to revisit the obvious so that when you’re planning your home, you can build it right, and insist that your architect focus on this aspect.

Great airflow isn’t about machines or purchasing any special equipment. It’s just about correct design. Trying to find the right balance of privacy, both visually and acoustically, can be challenging.

The house with the best airflow is of course going to be one with a roof and no walls, which for most people isn’t the best living arrangement. We must add walls for:

To keep bugs out
So we can sleep in the dark
To stop critters from ransacking your kitchen
To keep out dust if you’re near a road
To stop the wind from blowing in rain during storms
So you have somewhere to hang a painting

You can create artificial airflow by using ceiling fans or standing fans, which you will probably have in most rooms in a tropical house, and are much more energy efficient than air conditioning. But that’s not what we’re talking about. What we want is natural, free air flow that comes from the design itself by taking advantage of ambient wind.

The first step

The first step in this is also the first step in permaculture: “prolonged observation.” You should understand in which direction the air naturally flows, and during what time of day? For example, in the afternoon when it’s hottest, is there a usual direction of airflow that you can take advantage of? From which side do storms tend to blow? Where is the sun during dry season when it’s the hottest?

The most important thing is to understand where the breeze is usually coming from during the hottest part of the day. Then, you can simply orient your house so that you have windows on both sides that can be opened to allow air through.

The Power of Evaporation

Now, consider what type of air is going to be passing through. Is that air going to be heated on a dark-tiled patio? Or, will it possibly be blowing first over a swimming pool or fishpond? Any pool of water will have constant evaporation, which is an endothermic reaction, meaning that it absorbs heat. So, a pool can help keep the whole house cool.

Plants, especially bamboo, constantly absorb water and evaporate it through their leaves. They do this in order to keep from burning up in the hot sun, and this is also why so many trees on the Pacific side of Costa Rica lose their leaves in the dry season… they must go into hibernation once they’ve used up all the water their roots can find. You can take advantage of this by planting small bamboo or other plant with a high evaporation rate on the wind side of your house. Of course, planting too much will create a wall that blocks the wind, so you have to get the balance right. An ideal situation is to plant a hillside with bamboo that the wind sweeps over before hitting your house. A design like this can help make your home seem magically cool and save you a lot of money in electric bills.

Flow Through

When most people build a house, including most architects, they don’t think so much about how the air is going to flow through the house. The simple way of course is to put in lots of windows. One option that’s used frequently here in Montezuma is to simply make many windows with folding wood shutters that open while you’re in the house, and closed when you leave. They are locked with a long piece of wood or metal latches. This type is inexpensive and fairly secure, but not ideal for buggy or stormy days, since when they’re open, they’re just large holes and anything can fly in, from mosquitos to rain.

Another option is aluminum windows, which is what I always use when designing eco-houses. You can purchase a type with three or four tracks, so you can have both glass and screens. The screens seem to block about 70% of the wind, but keep out most bugs. If you have them, it becomes part of your lifestyle to try to remember to close them before dusk, so the bugs that come out around sunset aren’t flying into your house, attracted by your lights inside.

Your doors have similar options, and another option is to make sliding “pocket doors” that go into compartments inside the walls, so you can have them both fully open. Sliding glass doors also have the advantage that they don’t swing open, which saves a lot of useful space in your house. More about them here: pocket doors in green building

You can add all the windows you want to a house, but if the design isn’t right, they can only help so much. For example, if the house is bisected by a hallway or design features that include internal walls, then air can only blow into individual rooms, but not through it. Of course, more efficient cooling is obtained if air can easily flow all the way through the house.

Since heat rises, you can improve cooling efficiency by having clerestory windows around the top of the house. This is great for adding natural light too. So imagine that you take a normal house, and raise the roof half a meter up, and between the walls and the roof, you add open bars with screening. Then air can flow through the entire house, blowing out hot air that would normally be trapped in the rooms. While this is a great trick, the disadvantages are that you will have no sound privacy anywhere, and also you can’t air condition any rooms like this. More about this here: clerestory windows.

Site location

It should go without saying that picking the right location for your house is crucial when it comes to airflow. Homes that are located within dense jungle may get a lot of shade, but they will overall be hotter because of the extra humidity and the lack of air flow.

Generally speaking, the higher you go, the cooler, and drier the air will be. An ideal location then, from the point of view of maximizing air flow, is a hill or ridge, in a clearing and perhaps one or two tall trees overhanging the house. A couple of trees won’t impede the wind, and will provide much-needed shade, which is perhaps even more important for keeping your eco house cool. More on this great resource: Trees in Green Building