In the first blog in this series we discussed heat transfer via Conduction, Convection, and Radiation. We used some examples specific to your home. We learned that your home’s seal (which defines your conditioned air space) needs both a Pressure and Thermal boundary to work efficiently. We need to build on this now and discuss air movement and natural ventilation.
Your home has many crooks and crevices that allow air to pass in and out of your home. That may sound a little creepy but just think if it was air tight then you would be living in a refrigerator. That is not realistic either because you would run out of fresh air. The sum of all of those crooks and crevices is your natural ventilation.
Realistically, unless your home was recently built and explicitly advertised and certified to specific energy-efficient standards, then your natural ventilation is probably, well, too natural. Crooks and crevices are fine and expected. However, when the sum of those little entry points gets too large then it turns inefficient. You can determine how O’natural you really are by conducting a blower door test as a part of an energy audit.
How much is too much? That is a great question. Think of your home in the summer time with the air conditioner on. What if your child opened your window a couple of inches? Would you think that is okay? Intuitively, we understand that a 60 in2 opening is not good for our efficiency. Your AC unit will work overtime to keep that same temperature. Once it gets to that setting it won’t stay off long because it is going right out the window. The analogy works just as well in the winter time.
Now, the official answer is really determined by calculating the Air Changes per Hour (ACH) in your home. Going back to the refrigerator analogy, living in an air tight home would not allow for fresh air. Hopefully, you can see that natural ventilation is a good thing and the overwhelming majority of homes rely on it to allow sufficient ACH for Indoor Air Quality.
Why does this matter? If you are just a little too natural (most homes are) then there is probably a big opportunity to lower your power bill. As you tighten the seal of your home you are taking control of and minimizing the natural ventilation. However, there is a limit to tightening the natural ventilation with regards to Indoor Air Quality. Once you hit that limit then, as a matter of safety, you will have to augment your remaining natural ventilation with mechanical ventilation.
Energy auditors/ professionals can easily measure your home and determine how close you are to that limit. Also, when you call them to come in make sure you ask them about a second blower door test after you fix whatever it is you end of fixing. This will give you a specific measure of progress, determine if you need mechanical ventilation, and qualify you for rebates. Earthcents, an energy conservation program, offered by Southern Company, is one example with many rebate opportunities. However, like a good program should, they insist on actual measurements to determine progress.