Measuring wood’s moisture content

by sadia_badhon | July 30, 2020 10:36 am

By Jason Spangler

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Wood is an exceptionally versatile and attractive building material that has been used in construction for thousands of years. Wood’s advantages in construction are many. It is extremely strong in relation to its weight, is widely available, and is economically competitive with other types of building materials. Additionally, wood is durable, reusable, and carries the lowest carbon footprint of any comparable building material.

Wood can be used for nearly any facet of a building project, including floors, walls, ceilings, panels, cabinets, doors, window frames, and structural support. It can be fabricated into a variety of different shapes and sizes to fit practically any construction need.

For wood to satisfactorily fulfill its intended purpose in a building project, one must keep in mind the product is susceptible to the influence of moisture in the environment. Wood is a hygroscopic material[2], meaning it has the ability to absorb or lose moisture depending on the relative humidity (RH) and temperature conditions of the surrounding environment. Simply, wood expands when absorbing moisture from the environment, and shrinks upon losing moisture.

Due to its swelling and shrinking tendencies, wood can incur significant moisture-related damage. Problems due to moisture can rear their ugly heads in different ways. Warping (i.e. any deviation from flatness) is a common issue that can occur when wood takes up moisture unevenly or is allowed to dry too slowly or quickly.

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Warping can take several forms:

• bow is a warp along the length of the face of the wood;
• crook is a warp along the length of the edge of the wood;
• cup is a warp along the width in which the edges of the wood are higher or lower than the centre; and twist is a distortion in which the two ends do not lie on the same plane.

Aside from the undesirable appearance, warping can also lead to costly structural failures. For example, in wood flooring, an issue with cupping may create gaps between the boards. Eventually, if the boards do not revert to their original shape, they could separate from the subfloor, or even splinter or crack.

Moisture may cause other types of issues that impair the quality, appearance, and performance of the wood.

Buckling

An extreme reaction to moisture, buckling is in response to prolonged exposure to excess moisture. Buckling happens when wood flooring gets pulled up from the subfloor, lifting as much as several inches in one or more places. Fortunately, buckling is not a common occurrence, and if caught early, spot repair and replacement may be possible.

Mould and mildew

Mould and mildew may grow on wood surfaces when the surrounding environment is damp or poorly ventilated. Aside from causing unsightly black, blue, or greenish-brown patches, spores from moulds may cause allergies or other serious health concerns. While moulds contribute to discolouration or staining, they are usually not a factor in any structural damage to the wood.

Decay or rot

This is another serious moisture-related issue. It is caused by specific fungi that can grow in moist wood and destroy the wood fibres[4], leading to structural damage. The spores of decaying fungi are always present in the air, and cannot be kept away from wood. However, fungi generally grow in wood only when the material’s moisture content (MC) is greater than 20 per cent. Preventing decay is straightforward, assuming wood is kept below this moisture threshold.

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How wood holds moisture

Wood’s hygroscopic nature stems from its individual cells that are designed to soak up the moisture a living tree needs as it grows and matures. Those cells do not lose their capacity to hold and release moisture once the tree is converted into usable lumber.

The cells hold moisture in two ways:

• free water, in which the cell cavities can hold moisture in both its liquid and vapour states.
• bound water, in which the cell walls, made up of cellulose fibre called microfibrils, hold water that is chemically bonded or essentially ‘bound’ with the cellulose molecules[6].

The amount of water in the wood, or the MC percentage, is the combined total of both free and bound water. However, these two forms of water do not impact wood in the same way. It is the movement of bound water, either into or out of the wood, that has the most significant impact on whether the wood is warping, swelling, shrinking, or cracking.

The cell walls expand or shrink depending on the amount of water they hold. In contrast, cells holding free water do not change their shape with any shift in MC in the cell cavities and open spaces. However, free water usually occurs at moisture levels much higher than is used in construction.

The effect of ambient temperature and RH

Ambient conditions (temperature and RH) largely dictate whether the wood cells take on or release moisture. Air temperatures affect the amount of moisture air can hold. Warmer air can hold more moisture than colder air. RH indicates how much moisture the air is holding as a percentage of what it could hold.

Wood absorbs or releases moisture in response to ambient RH. When the ambient RH is low relative to the wood’s MC, moisture gets drawn out. When RH is high relative to the wood’s moisture level, the material will suck moisture from the environment.

Wood reaches a point of equilibrium moisture content[7] (EMC) when it is neither absorbing nor releasing moisture. Attaining EMC requires relatively stable ambient conditions so the moisture within the wood can eventually match its surroundings. When ambient conditions are constantly changing, so will the MC of the wood.

In construction projects, it is important to work with wood that has been dried to the proper MC and has attained the point of EMC. Inside heated buildings, this will often mean aiming for an MC of about six to eight per cent, although this is merely a rule of thumb that may vary according to different environmental conditions and the geographic location.

How to measure wood MC

Although other methods exist, the simplest and fastest way to get an accurate measurement of wood’s MC is to use a handheld moisture metre. Two types are available:

1. resistance or pin-style; and
2. dielectric or pinless metres.

When using a pin metre, two pins are inserted into the wood and the metre measures the electrical resistance between the pins. Since moisture conducts electricity, higher electrical resistance corresponds with drier wood, and conversely, lower electrical resistance corresponds with wetter wood. One drawback to pin metres is they are intrusive, leaving unsightly pin holes in the wood. Another drawback is they can be somewhat slow and cumbersome to use. This is especially true with hardwoods where inserting pins to the depth necessary for an accurate reading can prove difficult.

In contrast, pinless metres are extremely fast to use. Since they employ a flat plate in contact with the wood surface to detect conductivity using electromagnetic waves, they leave no damaging holes in the wood. They can also be used for a broader range of MC measurements than most pin-style metres. One drawback of pinless metres is that some of them may be unduly affected by surface moisture or the ambient temperature.

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Since pinless metres are fast and easy to use, they provide an excellent way of monitoring wood moisture levels in just about any construction or remedial application. During construction, they can be used for assessing when wood materials are ready for installation, such as in a hardwood flooring application. In an existing building, they can be used to detect small trouble spots and help contain moisture-related problems before they grow totally out of hand. Handheld metres can also be used to determine the extent of existing moisture that might cause damage quickly and easily.

It is important to keep in mind it can be useful to monitor more than just the moisture in the wood. Given that ambient conditions play an important role in how wood is affected by moisture, it can be extremely helpful to monitor ambient temperature and RH at any jobsite or building where wood materials are used. Portable data logging devices make great tools for taking temperature and RH measurements 24/7.

Today’s advanced moisture measurement technology

Newer metre technology now make the monitoring of onsite moisture conditions easier and more accurate than ever. For example, some new lines of pinless moisture metres are engineered to take measurements at specified depths which minimize any effect from surface moisture and ensure more accurate results. Some newer devices are especially powerful, and provide accurate wood moisture measurements as well as asses the ambient temperature and RH.

Another recent advance in pinless metre technology is the ability to do in-field calibration of metres. Most handheld metres, whether pin or pinless, must be sent back to the manufacturer for recalibration, but some metres offer the convenience of onsite calibration to help ensure accuracy with every measurement.

While some of today’s wood moisture metres can measure ambient temperature and RH, they do so only when one is actually onsite using the device and taking measurements.

Obtaining ambient temperature and RH data 24/7

A growing number of people in the construction industry are taking advantage of today’s portable data loggers that capture temperature and RH data around the clock, even when no one is there to collect the data. Some new smart loggers are relatively tiny, unobtrusive devices that can be readily installed and left in any spot where one suspects ambient conditions could be a factor leading to moisture damage, mould or mildew growth, or other similar issues.

These data logging devices are useful in a variety of residential and commercial settings, including hospitals, schools, businesses, and government buildings. They save time and money by reducing the number of site visits needed to monitor ambient conditions. They can also serve as a valuable reminder to customers and clients to maintain consistent, optimal conditions in the building or project area.

In today’s construction world, where there is more knowledge about moisture-related issues and their causes and effects, it makes sense to take advantage of highly advanced wood moisture metres and data logging tools that are capable of giving a fast, easy, and accurate picture of conditions in wood and the surrounding environment. Indeed, it is a very practical way people can get a handle on what is happening so the significant expense and headache of wood moisture problems can be prevented or minimized.

[9]Jason Spangler, Wagner Meters’ flooring division manager, has more than 25 years of experience in sales and sales management across a broad spectrum of industries. He has launched a variety of products to the market, including the original Rapid RH concrete moisture test. Spangler, who received an MBA from West Texas A&M University, has extensive industry involvement, including the National Wood Flooring Association (NWFA), the International Certified Flooring Installers Association (CFI), and is vice-chairman of associations for the Flooring Contractors Association (FCICA). He can be reached at jspangler@wagnermeters.com[10].

Source URL: https://www.constructioncanada.net/measuring-woods-moisture-content/

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