If temperature and humidity aren’t properly controlled, or if a product hasn’t been designed, constructed, and stored correctly, it can lead to a range of problems in the woodwork, including warping, swelling, and shrinkage. To maintain the quality and integrity of your products, you should refer to the AWI temperature and humidity standards. AWI 200.
How Does Temperature Affect Wood?
Changes in temperature do not affect the size of wood – but only if the relative humidity (RH) is constant. This is because wood and products made from wood fiber are greatly affected by changes in moisture content (MC), which is linked to RH. Changing the temperature of the air causes the RH to either increase or decrease, which in turn has corresponding effects on the MC.
For example, where a dehumidifier or humidifier are absent, increasing the temperature of the air by around 25°F will cause the RH to drop significantly. As the relative humidity drops, so does the moisture content inside the wood, causing it to shrink in size. In fact, for most species, if the MC changes by just 4%, there will be a ½ – 1% size change in the wood with the most significant change occurring in the width
However, if the RH is kept constant, changes in temperature won’t affect the wood because they will hold no bearing over the MC. The reason moisture plays such an important role here is because wood is a hygroscopic material, meaning it readily absorbs and releases moisture and seeks to come to equilibrium with the moisture content in the surrounding environment Therefore, changes in temperature affecting relative humidity, or the moisture content of the air, may result in wood movement, either shrinking or swelling, which can result in the wood or wood component warping, or splitting, and possible joint failure. Applying finishes to wood will slow down the transfer of moisture. But, it will not stop the wood from moving with changes in RH.
How Does Heat Affect Wood?
When you take the importance of moisture content and relative humidity into account, it’s clear to see how exposure to heat can negatively affect woodwork. Hotter temperatures mean a lower RH level, which translates to lower moisture content. But there’s more going on here than you might think.
Heat is a form of energy. When atoms inside a piece of wood are exposed to energy, they move faster, causing lots of internal reactions to occur. In wood, this process leads to thermal expansion. Not only can this cause the wood to swell, it can also result in warping and even shrinkage.
That said, wood can handle exposure to heat much better than other materials, like steel or aluminum, which become weaker as temperatures increase. But that doesn’t mean you should just cross your fingers and hope that your piece of woodwork can hold its own in the face of heat exposure. Instead, there are several things you can do to prevent the negative effects that heat can cause:
- Choose the right species: Different species of wood react differently to heat. If your woodwork is going to be kept in a place that is subjected to a lot of heat, humidity, or temperature change, take the time to choose the species that are best suited for the conditions. Mahogany and oak, for example, have high heat resistance.
- Store timber under ideal conditions: When storing wood, ensure that its exposure to environmental stresses is minimized as much as possible. Proper storage of wood is crucial at every stage of the woodworking process.
- Use treatments: You can improve the natural resilience of your wood by using timber treatments that are designed to reduce the movement of moisture and withstand high temperatures. Chemical treatments can also be used to change the internal structure of the timber and improve its durability and dimensional stability.
- Apply the right kind of finish: The proper choice, and application, of wood finishes, can vastly improve the heat resistance of your final product.
- Use kiln-dried wood: Kiln drying to a moisture content equivalent to the environment for which the wood is intended is an extremely effective way of reducing the effects of changes in temperature and humidity on the final product..
What Are the AWI Temperature and Humidity Standards?
The effects of temperature and humidity can be mitigated if you take the necessary precautions before, during, and after installation. The AWI 200 – Care & Storage Standard provides all woodworking professionals, design professionals, general contractors, and owners with guidance on how to protect architectural woodwork and related interior finishes from the negative effects of temperature and humidity.
By complying with this Standard, you will achieve a high degree of aesthetic quality, structural integrity, and performance longevity of the product. This is just another step in maintaining quality assurance for all parties.
Here are the key temperature and humidity requirements as specified in the AWI 200 – Care & Storage Standard:
- Dimensional Change: Minor fluctuations in humidity and dimensional change in wood will always be inevitable – but if the product has been constructed properly, the effects of these will be insignificant. To ensure this, the Standard specifies that:
- You must use wood that has been kiln-dried to a suitable moisture content
- Maintain appropriate relative humidity levels at all times
- Responsibilities: Dimensional change in woodwork resulting from improper design is the responsibility of the design professional. Dimensional change as a result of improper relative humidity exposure during job site storage and installation is the responsibility of the general contractor. Meanwhile, the owner is responsible for dimensional change caused by humidity extremes that occurred after occupancy.
If your project is due a QCP inspection, and lack of humidity control is a concern you have addressed with the general contractor (and you’ve already been provided with a written directive from the GC to continue), we recommend you request your inspection to take place as soon as possible following installation.
This will act as proof that the woodwork complied with specifications at the time of installation, whereas if you wait too long, the effects of humidity could compromise the piece, and your project may fail the inspection altogether.