The Effects of Climate on Specifying Cool Roofs

The climate in which a building is located is a major factor in determining what kind of cool metal roof to use on that building.

In the simplest terms, climate can be discussed with respect to Heating Degree-Days (HDD).  Cooling Degree-Days (CDD) are also used to help describe the climate for a given area, but we can generally describe the climate by only referring to heating degree-days.  Heating degree-days are physically measured and recorded on a daily basis for a given area. 

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 defines HDD65 as “…a notation that indicates that for any one day, when the mean temperature is less than 65^oF, there are as many degree-days as degrees Fahrenheit temperature difference between the mean temperature for the day and 65^oF.  Annual Heating degree-days (HDDs) are the sum of the degree-days over a calendar year.”

To help simplify this concept, imagine a climate where, during a three-week period, the average temperature was 64^oF for 10 days, 60^oF for 5 days and 55^oF for 6 days. Each of the 64^oF days represents 1 HDD (a total of 10 HDD) because the average temperature was 1^oF below 65^oF. Each 60^oF represents 5 HDD (a total of 25 HDD) and each 55^oF day represents 10 HDD (a total of 60 HDD). The grand total for the three weeks is 95 HDD. 

To simplify this even further, imagine a climate where the average temperature is a constant 64^oF for three weeks.  The climate for this area during this time frame can be described as having twenty-one (21) HDD.  However, more data collection would be necessary since HDD is the sum of all of the degree-days in a year.  A list of HDD values is contained in the latest edition of the ASHRAE 90.1 Normative Appendix “D” for most major cities in the U.S. 

By comparing the heating degree-days for buildings in different areas, one can see if the building is located in an area that will require more heating or cooling.  Areas with low-to-no heating degree-days are hot climates, and areas with high heating degree-days are primarily cold climates.

National Specifications

The requirements for Solar Reflectance and Infrared Emittance as specified by several national organizations, states and municipalities are listed in Table 1 below.  

Table 1: Some National, State and Local Regulations already in effect. 
R_i = Initial, R₃=3 year aged.

Effective in 2008, the initial and aged reflectance will default
to the EPA Energy Star requirements.