What Do Heating Degree Days Mean to Me?

How Heating Degree Days in St. Louis Can Influence Our Home Improvements

Heating Degree Days help us understand that in the St. Louis area, we spend more time and money keeping warm in winter than we do keeping cool in summer. Knowing that, in turn, helps us make the most important home improvements in energy efficiency and comfort.

A Heating Degree Day (HDD) can be calculated in different ways, and it may sound complicated but an HDD is really just the number of degrees the average daily temperature is below 65 degrees. For example, if we have a day in January when the average daily temperature is 20 degrees, then that is equivalent to 45 heating degree days.

Over the last five years in St. Louis we have had an average of 4,482 heating degree days per year. This is a measure of how much heat we have used to keep our homes warm during winter. But, how does this compare to what happens during the summer?

For the summer, we use Cooling Degree Days (CDD). A CDD is just the number of degrees the average daily temperature is above 65 degrees. Over the last five years we have had an average of 1,939 CDD in the St. Louis area. So the number of Cooling Degree Days has been less than half the number of Heating Degree Days over the last five years.

Now that I know my HDD, what home improvement should I make?

Knowing the above can help us in choosing which home improvements to make. We know that keeping heat inside the house in winter is more important than keeping heat outside in the summer.  

Radiant barriers are a good example. Radiant barriers are primarily installed in order to keep the heat out during the hot summer months. They do little or nothing to retain heat in the winter. In a place like Southern Florida, Arizona or Southern California, where keeping summer heat outside is very important, a radiant barrier may be a sensible improvement for many homes. Here in St. Louis, not so much.

Another example is choosing between replacing a furnace with a more efficient model and replacing an air conditioner. Each home is different, but the potential energy savings from upgrading the furnace is greater than the potential from replacing the air conditioner. Adding attic insulation may be more important than either because it keeps heat inside during winter and outside during the summer. Windows that keep heat out in the summer may be less important than air leaks in winter.

A simple comparison of Heating and Cooling Degree Days in St. Louis helps us prioritize our home improvements. So …stay warm.