Posted by: lcowie | November 16, 2010

Understanding the Stack Effect

As discussed before, infiltration and exfiltration will allow uncontrolled air movement through the house. This movement is enabled by gaps in your home. To add a little more to this concept, you need to understand pressure differences, as exhibited by the Stack Effect. The BPI Building Analyst exam focuses heavily on the Stack Effect, maybe not directly, but your understanding of this effect will help you understand concepts that occur later in the training course.

The Stack Effect is the most fundamental concept in your BPI training course. If you understand the Stack Effect, which isn’t too difficult, you will understand everything else in the course.

Everyone has heard the phrase that “hot air rises,” and it’s true. It’s called the Stack Effect! Hot air rises because it is less dense, and therefore more buoyant. Think of children’s swimmies. The water wings are not very dense; their buoyancy allows them to rest at the top of the water, keeping your child at the top of the water. Hot air will rise to the top, just as children’s swimmies.

It’s important to also note that the taller the building, the greater the Stack Effect. That makes sense. You have hot air rising, and it collects more hot air as it goes through multiple levels, or stories. A three-story house will have a more intense Stack Effect than a one-story house.

So you have hot air rising to the top of the house. This creates imbalance within the home, in terms of pressure. The bottom of your home has less heat (which is a negative pressure) and the top of your home has more heat (which is a positive pressure). As I wrote about before, energy likes to balance itself out, hence moving from high levels of concentration to low levels of concentration. Eventually, there should be a balance.

If your pressure and thermal boundaries are not continuous or aligned, there will likely be a gap in your insulation. This gap will enable air movement. Now you have all this hot air in the top of your house – in your attic – and there is a hole in the insulation. The air is going to move through that hole (exfiltration). When 1 cubic foot of air leaves the home, one cubic foot of air must replace it – your home seeks a balance. The air will likely come from the bottom of the home, say, in the basement (infiltration). At this point, the air will heat up and rise to the top again. It’s an endless circle, unless someone takes care of it. Once you have passed your BPI Building Analyst exam, YOU will be able to diagnose this problem and possibly repair the insulation (if you are in that business).

The Stack Effect is very important to understand. Leave a comment if you have any questions about it!


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