Wood flooring is constantly exposed to both long-term (seasonal) and short-term (daily) fluctuations in relative humidity and temperature of the surrounding air. Thus, it is always undergoing at least slight changes in moisture content.
The ideal average moisture content for wood flooring can range from extremes of 4%MC up to 13%MC, depending on many variables, including geographic location and time of year. Additionally, a wide range of conditions can be experienced between individual homes in the same locale.
The individual who specifies the wood floor for a project should ensure that all jobsite conditions are capable of meeting or exceeding the minimum standards and requirements of the flooring being used. This floor selection decision may involve a builder or designer, or a flooring sales associate, or even the homeowner.
Interior climate capabilities of the facility include type and functionality of the HVAC systems, humidification/ dehumidification systems, interior and exterior insulation, and methods of construction.
The methods of construction differ across the world, based on many variables. One of the largest variables involves the average outdoor temperature and humidity, and how it varies from region to region.
Across every region, wood floors can successfully be installed; however, all wood floors cannot be installed in the same manner in all climates.
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 help ensure for a successful installation.
The following 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 IECC climate zone map was developed by the U.S. Department of Energy (DOE) researchers at Pacific Northwest National Laboratory with input from Building America team members, in particular, Joseph Lstiburek of the Building Science Corporation. The IECC 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 U.S. map was defined on an analysis of the 4,775 U.S. weather sites utilized by the National Oceanic and Atmospheric Administration (NOAA). The U.S. Department of Energy divided the United States into eight temperature-oriented climate zones. These zones are further divided into three moisture regions designated A, B, and C. Thus, the IECC map allows for up to 24 potential climate designations.
The climate region definitions are based on heating degree days, average temperatures and precipitation, which are defined as follows:
1) Hot-Humid: This is defined as a region that receives more than 20 in. (50 cm) of annual precipitation and where one or both of the following occur:
i) A 67°F (19.5°C) or higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive months of the year; or
ii) A 73°F (23°C) or higher wet bulb temperature for 1,500 or more hours during the warmest six 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.
2) Mixed-Humid: A mixed-humid climate is defined as a region that receives more than 20 in. (50 cm) of annual precipitation, has approximately 5,400 heating degree days (65°F basis) or fewer, and where the average monthly outdoor temperature drops below 45°F (7°C) during the winter months.
3) Hot-Dry: A hot-dry climate generally is defined as a region that receives less than 20 in. (50 cm) of annual precipitation and where the monthly average outdoor temperature remains above 45°F (7°C) throughout the year.
4) Mixed Dry: A mixed-dry climate is defined as a region that receives less than 20 in. (50 cm) of annual precipitation, has approximately 5,400 heating degree days (65°F basis) or less, and where the average monthly outdoor temperature drops below 45°F (7°C) during the winter months.
5) Marine: A marine climate generally is defined as a region that meets all of the following criteria:
i) A mean temperature of the coldest month between 27°F (-3°C) and 65°F (18°C)
ii) The warmest month mean of less than 72°F (22°C)
iii) At least four months with mean temperatures more than 50°F (10°C)
iv) 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.
6) Cold: A cold climate generally is defined as a region with approximately 5,400 heating degree days (65°F basis) or more and fewer than approximately 9,000 heating degree days (65°F basis).
7) Very Cold: A very cold climate generally is defined as a region with approximately 9,000 heating degree days (65°F basis) or more and fewer than approximately 12,600 heating degree days (65°F basis).
8) Subarctic/Arctic: A subarctic climate generally is defined as a region with approximately 12,600 heating degree days (65° basis) or more.
Building Thermal Envelope
What happens with the exterior climate also will affect the indoor conditions of the space.
The Building Thermal Envelope includes everything within the basement walls, exterior walls, floor, roof, and any other building element that encloses conditioned space.
Conditioned space is defined as an area or room within the building that is intentionally heated or cooled, and humidified or dehumidified, to be maintained at the same expected conditions as the living/interior space either for the comfort of occupants, or for preserving temperature and humidity-sensitive goods (wood floors).
Unconditioned space refers to exterior space, or a space within the shell of a building, that is uncontrolled, and is neither directly nor indirectly heated, cooled, humidified, nor dehumidified.
The exterior conditions (unconditioned spaces) of a building affect how a building is built to keep the interior from being negatively affected by adverse exterior conditions. This involves building practices that control the flow of moisture vapor, most of which are detailed in local building codes.
In order to understand how moisture vapor moves into and through the building, and ultimately can affect the wood flooring system, it is important to know which direction heat, air, and moisture vapor are likely to flow.
Moisture vapor migrates through materials by diffusion. The law of thermodynamics states that temperature moves from a warmer space to a cooler space. This type of moisture transport is called thermally driven diffusion. The moisture vapor moves from the warm side of the building assembly to the cold side of the building assembly. Building interiors are affected by two distinct seasons – heating and cooling.
a) In the summer and hot/humid climates, during the interior cooling season, vapor drive is predominantly 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.
b) In the winter, during the interior heating season, vapor drive is predominantly 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.
Warm air also has higher pressure and is more buoyant than cooler air.
Air moves from regions of high air pressure to regions of the home with lower air pressure. As the temperature warms, air pressure increases (e.g., truck tire pressure in the summer versus the winter).
Vapor diffusion is the movement of moisture (in the vapor state) from an area of higher vapor pressure concentration to an area of lower vapor pressure concentration. The loss of warm air within the home creates a negative pressure inside the home. In lower levels of the building, low-pressure cool air rushes in to replace the rising air in an attempt to maintain pressure balance within the structure. This is known as the stack effect.
Minimizing Moisture Migration through the Structure
In many climate zones, building codes dictate construction methods related to placement and type of insulation and moisture control systems installed below the living space, such as in crawlspaces.
Whether vapor barriers are placed below the slab, over the bare earth in a crawlspace, or on the underside of the floor joists, these products are designed to stop moisture from finding its way into the structure. The type of insulation systems and use of vapor control systems vary throughout many parts of the country.
With new construction, the ultimate objective of the builder is to control the temperature and moisture gradient from a conditioned space to unconditioned space.
We are seeing promising changes to some of the building codes that are requiring vapor control systems to be installed where continuous moisture is a concern.
When installing a wood floor in a remodel situation, the homeowner and flooring professional should both be aware of any existing situation that could have the potential to affect the long-term performance of the wood floor. In many cases, it may be necessary to ask the owner to consider installation of a moisture control system as a part of their overall flooring project.
Interior Humidity and the Average Moisture Content of Wood
Humidity is important because wood products exchange water molecules from the surrounding air based upon the amount of moisture in the air.
To understand why temperature and humidity affect wood, it is important to respect the relationship between temperature and humidity. Heating the air will increase its ability to hold moisture; therefore, the relative humidity decreases, whereas cooling the air will decrease its ability to hold moisture; therefore, the relative humidity increases.
Humidification and/or dehumidification systems are often necessary before, during, and after installation to maintain an environment appropriate for the flooring specified.
Wood, like humans, likes consistency when it comes to the temperature and humidity to which it is exposed. Fortunately, both wood and humans like to live in similar environments.
Wood dried for interior usage generally is stable in a controlled environment maintained between 60-80°F at a relative humidity between 30-50%.
Humans generally are most comfortable in a controlled environment maintained about the same conditions. The following chart provided by ASHRAE shows optimum RH for human health based on a 70°F temperature.
To ensure the facility receiving a wood floor can sustain the requirements necessary for the wood floor, it is imperative that the interior conditions are capable of maintaining those requirements. The values on the map on page 62 provide examples of how average moisture contents for interior use of wood products can vary from one region to another, and from one season to another within a region.
The climate surrounding the wood floor is an important factor in the overall performance of the floor. It is everyone’s prerogative to give the wood floor what it needs to allow it to do what it’s supposed to do. However, not every floor will work in every scenario. Having a general understanding of the capabilities and limitations of the jobsite, and aligning the wood floor selection around those conditions, will go a long way in how well that floor will live out its life in the structure.
Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at firstname.lastname@example.org.