Roof Talk-101  What’s a Heating Degree Day?

Martin Grohman  

Director of Sustainability for GAF

Whenever the subject of cool roofing or energy savings comes up, the next topic is usually Heating or Cooling Degree Days, and there seems to be a lot of confusion about what these mean.  It’s tempting to think that a Heating Degree Day is a day on which you need to turn on the heat, and a Cooling Degree Day is a day on which you need to turn on the air conditioning, but that is not the case.

So let me dive right in:  a heating degree day is a way of summarizing the annual heating (or cooling, in the case of Cooling Degree Days) requirements in a particular climate.  This is a complex sounding concept that is actually quite simple.  On a heating degree day (HDD), the temperature falls below a standard “comfortable” temperature (usually 65° F) so a building or home needs to be heated to maintain the target temperature; and a cooling degree day is one where the temperature is above that target, requiring cooling.  Turning climate data into a heating degree day or cooling degree day is a matter of simple math.  If the average temperature on a given day is 80°F, the building needs to be cooled 15°F to reach the target 65°F.  That day would be counted as 15 cooling degree days (CDD).

Another example: consider a typical New York City winter day with high of 40°F and low of 30°F, for an average temperature of 35°F.  This one day would generate 65 – 35 = 30 Heating Degree Days.  A month of similar days would accumulate 900 Heating Degree Days, which gives you an idea how HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements. In the course of a heating season, for example, the number of HDD for New York City is 5,050 whereas that for Barrow, Alaska is 19,990.  Barrow doesn’t have any more days in the year, it just has lower average temperatures!

Average daily temperatures tend to vary farther to the low side, particularly overnight.  Because of this effect, it is common even for warmer climates to have more HDD than CDD.  Some would argue that in any climate where HDD exceeds CDD, a cool roof does not make sense, but that is not correct.  That’s because building occupants and services generate internal heat, and with conventional equipment, cooling is generally more expensive than heating.

There are a number of online resources where you can find heating and cooling degree data for climate stations around the country.  Or, you can just log into the free GAF Cool Roof Energy Savings Tool (“CREST”), which taps into HDD and CDD data for you automatically and helps you compare roofing assemblies to better determine what is right for your project.

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