Why Knowing Your Climate Zone is Important

Figure N1101.7 (R301.1) Climate Zones-2021 International Residential Code (IRC)

This climate zone map is published by the American Society of Heating and Air-Conditioning Engineers (ASHRAE) and is included in the International Residential Code, chapter 11, Energy Efficiency.  ASHRAE’s purpose is to create standards of how an engineer or HVAC professional calculates and designs heating, air conditioning and ventilations systems to match the insulation, air sealing, and moisture profile of a building.  When designing these often-complicated systems, where the structure is located becomes key, this is the reason the map was created.

The map is broken down by individual counties in each state by their historic climate data.  Based on that data, each county is placed in a numbered climate zone, the numbers start at 0, which is extremely hot, (there are no locations within the United States that have a climate zone 0 designation, yet), and move to climate zone 8, subarctic/arctic (extremely cold).  There are also subcategories to each climate zone indicating whether the location is moist, dry or marine, this results in 19 different climate zones.  Texas, for example, has several different climate zones both in humid and dry categories ranging from climate zone 1 to climate zone 4.

Each climate zone gets its designation based on historic weather patterns gathered from nearly 5,000 weather sites across the US.  These sites monitor several different metrics, such as temperature, wind speed and direction, precipitation, humidity, solar radiation along with other weather-related data.  The most pertinent information used in developing the climate zone map is temperature and precipitation.

Based on Table N1101.7.2 of the 2021 International Building Code (IRC)

The thermal criteria column above is based on heating and cooling degree days which is the annual accumulation of heating or cooling demands.  You’ll need a couple definitions to be able to interpret the table.

The definition of a cooling degree day (CCD) is the average mean daily temperature above a designated temperature.  For the purpose of the climate zone map, the CDD temperature designation is 50°F.  (Not to be confused with interior design temperatures for heating and cooling load calculations.)  As an example, let’s say we have a daily high temperature of 90°F and a low temperature of 40°F.  To calculate the average mean temperature, we simply add the high and low temps together and divide by 2.

(90°F + 40°F) / 2 = 65°F

If the result is above the benchmark temperature of 50°F, we subtract the benchmark from the average mean temperature to calculate the number of cooling degree days, for the previous example, the calculation would be:

65°F – 50°F = 15 CCD

Similar to cooling degree days, a heating degree day (HDD) is the average mean daily temperature below a designated temperature.  65°F is the benchmark temperature used for the HDD calculation.  As an example, if the average mean daily temperature is 0°F, there would be 65 heating degree days for that day in that location.

65°F – 0°F = 65 HDD.

To calculate the total cooling degree days for a given location over the course of a year, you simply add all the daily CDD values together for the year.  The same process is used to calculate the total yearly HDD.  Using climate zone 5 as an example.  To fall into that climate zone, the cooling degree days need to be greater than 6,300 and heating degree days need to be more than 5,400 but less than or equal to 7,200.

The other part of the climate zone designation has to do with rainfall amounts for a given area.  Moist is considered any area that receives more than 20 inches of rainfall annually.  Dry is, you guessed it, less than 20 inches of rainfall for a given location.  The marine zone is a little more complicated.  According to The Department of Energy, “A marine climate is generally defined as a region that meets all of the following criteria:

A mean temperature of coldest month between 27°F and 65°F

A warmest month mean of less than 72°F

At least 4 months with a mean temperature more than 50°F

A dry season in summer.  The month with the heaviest precipitation in the cold season has at least three times as much precipitation as the month with the least precipitation in the rest of the year.  The cold season is October through March.”

Trying to calculate all these metrics manually for the purpose of knowing your climate zone would take a lot of effort, simply reading the map, or looking up the state and county information in the 2021 IRC, Table N1101.7, Climate Zones, Moisture Regimes and Warm Humid Designations by State, County and Territory is much easier.

So, what do we get out of knowing our climate zone?  The Energy Efficiency section (chapter 11) of the International Residential Code is where insulation requirements, air sealing requirements and systems covering HVAC strategies are covered in the codes.  Many of the quantifying details are based on the climate zone of the home.  If you live in climate zones 7 or 8 for example, insulation and air sealing will be more robust than homes built in climates zones 1 or 2.

It’s hard to believe that our current climate zone map based on historic climate data has only been around for the past 20 years.  2004 was the year it was first published as a supplement to the International Energy Conservation Code (IECC).  The map illustration above is from the most current addition of the IRC at the time of this writing, 2021.  Some areas may be using this map for code requirements, others may still be using the original version from 2004.  We’ve seen some big improvements in how we handle energy efficiency in our buildings, the current climate zone map has greatly simplified that process.

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