A different kind of envelope
Oftentimes I refer to the term “building envelope” or “thermal envelope.” Both are interchangeable and synonymous. So, just what is a building envelope? Well, in simple terms, it is the building components that separate the building interior from the exterior environment. And since this column is about energy efficiency, let’s call it the “thermal envelope.” On the simplest of buildings, the thermal envelope consists of the floor, the four walls and the ceiling or roof.
There are two things that can affect the energy efficiency of the envelope. One is the insulation value of the components and the other relates to the amount of air that can transfer to and from the envelope.
For the sake of teaching, picture a normal ice chest. It’s a miniature ultra-efficient structure where the floor, walls and the roof are super-insulated, and there is only one doorway leading in and out. Some have a capped plumbing penetration – the drain plug. This structure requires very little energy to fulfill its job of keeping its contents chilled. However, lifting the lid introduces air infiltration and will affect the energy consumption (melting ice), energy cost (buying more ice) and comfort (warm soft drinks). Keep the lid closed, and the content stays cold and the ice lasts much longer. You may not be aware that many of today’s new homes are insulated with similar foam used in ice chests. Plus, existing homes can be retrofitted as well.
So how does an ice chest relate to a dwelling? After all, nobody wants to live in a big insulated box. Most people prefer to live in a building that contains amenities, comfort and improves our quality of life. Of course, we also want affordable utility bills. Well, the good news is building an efficient new home or retrofitting an existing home is doable. However, the bad news is many desired amenities can adversely affect the energy efficiency of the thermal envelope.
For example, the installation of windows into a well-insulated wall will almost always decrease the energy efficiency of the thermal envelope. This is because the windows may have some air leakage, and the insulation value of the window is less than the well-insulated wall. Also, the glass will allow more heat rays from the sun to enter the house. We all want windows in our house, so what can we do to enjoy the benefits of windows without dramatically affecting the envelope? The answer is that we must analyze the efficiency of each component and its direct impact on the envelope.
In this particular case, we could install the fewest number of windows necessary to make us happy, and choose the most feasible energy-efficient windows. Keep this in mind as a general rule of thumb; if the energy efficiency of the thermal envelope goes down, then the energy bill usually increases. We will continue this subject in next month’s column.
Learn about the thermal envelope while visiting www. smartenergytips.org and your local electric co-op’s website.
Please call my office at 501-653-7931 if you have questions, and I will be happy to help you. Or you may attend one of our seminars sponsored by your local electric co-op.