Knowing and Understanding Regional Climate Variations
We all know that climate varies significantly by region. Some areas are naturally dry and arid, while others are naturally wet and humid. This difference affects how we live, and also affects how we should install wood floors.
Wood floors can be installed successfully in every region; however, all wood floors cannot be installed in the same manner in all climate zones. This is because average outdoor temperature and humidity varies among regions. This regional variability of the exterior climate will affect indoor conditions.
Interior climate capabilities that can affect interior conditions include HVAC systems, humidification/dehumidification systems, interior and exterior insulation, and methods of construction.
Temperature and moisture (vapor) move from warmer spaces to cooler spaces. This is known as vapor drive. Building interiors are affected by two distinct seasons: heating and cooling.
In the Winter during the interior heating season, and in cold, dry climates, vapor drive is predominately outward. Heating systems raise the temperature of the interior air. Heating the air will increase its ability to hold moisture; therefore, the interior relative humidity decreases.
In the Summer during the interior cooling season, and in hot/humid climates, vapor drive is predominately inward. Cooling systems lower the temperature of the interior air. Cooling the air decreases its ability to hold moisture and the interior relative humidity naturally increases. Fortunately, air conditioning cools the air by removing moisture through condensation.
In North America, for example, there are numerous regions with varying average moisture contents. Because of this, humidification and/or dehumidification systems may be necessary prior to, during, and after installation to maintain an environment appropriate for the wood flooring specified. [image: Moisture map of North America]
The U.S. Department of Agriculture Forest Service suggests the following moisture content average and ranges for interior wood products:
- In most areas of the United States, the average is 8% moisture content, with an average range of 6-10% moisture content.
- In most dry southwestern regions of the United States, the average is 6% moisture content, with an average range of 4-9% moisture content.
- In damp, warm coastal areas of the United States, the average is 11% moisture content, with an average range of 8-13% moisture content.
Actual moisture content conditions in any location may differ significantly from these numbers. Having a general understanding of the climate zone the wood floors are being installed in, and working with the builder and property owner to determine what is necessary for the interior finishes to perform as they are intended, will ensure a successful installation.
Each region also has average moisture content ranges. These are the ranges that are recommended for interior use of wood products from one region to another, and from one season to another within a region. Actual moisture content conditions in any location may differ significantly from these numbers.
These climate regions are based on the climate designations used by the International Energy Conservation Code (IECC) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The map was developed to provide a simplified, consistent approach to defining climate for implementation of various codes. It was based on widely accepted classifications of world climates that have been applied in a variety of different disciplines. The IECC map further defines how moisture impacts the climate zones. Generally, it is classified as moist to the east, signified by the letter A, dry to the west, signified by letter B, and marine along with west coast, signified by letter C.
Each of the eight climate regions is identified based on heating degree days, average temperatures, and precipitation.
The different climate regions include hot-humid, mixed-humid, hot-dry, mixed-dry, marine, cold, very cold, and subarctic/artic.
A hot-humid climate is defined as a region that receives more than 20 inches of annual precipitation, and where one or both of the following occur:
- A 67°F or higher wet bulb temperature for 3,000 or more hours during the warmest 6 consecutive months of the year; and/or
- A 73°F or higher wet bulb temperature for 1,500 or more hours during the warmest 6 consecutive months of the year. In these regions, it is common for the interior space to be air conditioned year-round. This causes water vapor to move from the exterior toward the interior.
The hot-humid climate is represented in zones 1A, 2A, and the lower half of 3A.
A mixed-humid climate is defined as a region that receives more than 20 inches of annual precipitation, has approximately 5,400 or fewer heating degree days, on a 65°F basis, and where the average monthly outdoor temperature drops below 45°F during the winter months. The mixed-humid climate is represented in the middle band of the eastern United States, shown in zone 4A and the upper half of 3A.
A hot-dry climate generally is defined as a region that receives less than 20 inches of annual precipitation and where the monthly average outdoor temperature remains above 45°F throughout the year. The hot-humid climate is represented in southern Arizona and south-eastern California in zone 2B.
A mixed-dry climate is defined as a region that receives less than 20 inches of annual precipitation, has approximately 5,400 or fewer heating degree days, on a 65°F basis, and where the average monthly outdoor temperature drops below 45°F during the winter months. The mixed-dry climate is represented in northern Texas, southeastern Colorado, parts of New Mexico, Arizona, southern Utah, the southern tip of Nevada, and much of California in zones 3B and 4B.
A marine climate generally is defined as a region that meets all of the following criteria:
- A mean temperature of the coldest month between 27°F to 65˚F.
- A warmest month mean temperature of less than 72°F.
- At least 4 months with mean temperatures 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 in the Northern Hemisphere and April through September in the Southern Hemisphere.
The marine climate is represented along the coastline on the entire west coast of the United States, in zones 3C and 4C.
A cold climate generally is defined as a region with approximately 5,400 or more heating degree days, on a 65°F basis, and 9,000 or fewer heating degree days, on a 65°F basis. The cold-humid and cold-dry climates are represented in zones 6A and 6B shown in blue.
A very cold climate generally is defined as a region with approximately 9,000 or more heating degree days, on a 65°F basis, and 12,600 or fewer heating degree days, on a 65°F basis. The very cold climate is represented in zone 7, which are the purple areas.
A subarctic climate generally is defined as a region with approximately 12,600 or more heating degree days, on a 65°F basis. The subarctic climate is represented in zone 8, which are the grey areas.
The North American Climate Zones map shows the eight temperature-oriented climate zones in the continental United States, as defined by the U.S. Department of Energy. The zones are based on a data analysis of the 4,775 U.S. weather sites utilized by the National Oceanic and Atmospheric Administration. These zones are further divided into three moisture regions designated A, B, and C. Thus, the map allows for up to 24 potential climate designations.
The World Climate Zones map shows the same temperature-oriented climate zones for all areas of the world. An awareness of these climate zone listings will help you determine best practices for selection of moisture control systems, installation methods, HVAC requirements, and long-term maintenance of the wood floor being put to use.
All of this information, as well as specific climate zones listed by country, province, state, city, and county is included in the NWFA’s Technical Publication No. C300, Regional Climate Variations. The publication also includes climate zones for every state within the United States, with specific zones identified by city or county. [sample image: Climate Zones – Colorado] The Regional Climate Variations publication is available for free to NWFA members. More information is available at nwfa.org/technical-guidelines.