Surviving Stack Effect in a Hot High Rise

During my junior year of undergrad, I found myself glued to my books – and chair – in an 18^th floor apartment with no AC. The first few months were so hot that my roommate eventually dragged her mattress out onto the balcony in attempts to get a good night sleep. But when December rolled around and the heat still wasn’t on, we began to miss those sweltering nights. We were on a tight student budget, so I was never really fazed by the uncomfortable temperatures. I just assumed that you have to live in a nice building to have a nice indoor climate. As it turns out, I was wrong.

In my fourth year of university, I spent countless hours at my friend’s upscale apartment, as we hammered out the details of our final design project. He owned the lower penthouse of one of Toronto’s nicer apartment buildings; so naturally it became our design team’s headquarters. When I arrived for our first work session however, something didn’t seem right. It was January, and the windows were open, the desks were lined with fans and air conditioners, and my teammates were in shorts. The sun wasn’t even out, and still the apartment was like a sauna. We were in the lower penthouse, not my budget apartment; so why was it uncomfortable? And then it hit me: we were victims of stack effect.

Stack effect depends on varying air pressures. A key factor in this phenomenon is a building’s neutral pressure plane, which is the elevation where the inside air pressure is equal to the outside air pressure. In the winter months, the outside pressure is greater than the indoor pressure for every floor below the neutral pressure plane.  On these levels, the cold outside air seeps into the building and pushes the lighter warm air up. This warm air rises to floors above the neutral pressure plane, where the inside pressure is greater than the outside pressure. In this area, the warm inside air is pushed out of the building. The result: tenants shivering on the lower floors and sweating on the upper floors, not to mention all that heated air being pushed out of the building. Tenants can then put this cycle into overdrive if the lower floor residents crank the heat, while the upper floor tenants open the windows. In the summer, the whole process is reversed.

Stack effect essentially makes tenants uncomfortable, and wastes great amounts of energy in the process. So what can be done to stop this? One of the best strategies is known as compartmentalisation, where floors are decoupled from one another. Separating the floors creates a neutral pressure plane for each level, as opposed to the entire building, and makes infiltration and exfiltration easier to control. This is really in the hands of building designers to implement, who should have this in mind when trying to make buildings more comfortable and sustainable. Hopefully, we’ll see happier tenants and lower energy bills as a result of this technique in the future. But until then, put on you shorts; Christmas is coming!