When we think about expansion and contraction in materials, we normally think of temperature changes. But with wood, the primary cause of expansion and contraction is moisture. Wood always contains moisture content. Although it dries over time, the tubular cells, cavities, and fibers will always contain some moisture.
Does wood contract or expand in the cold? The way temperature affects wood is generally indirect. Humidity, however, is the prime reason for changes in the dimensions and shape of wood. The higher the moisture level, the more the expansion and vice-versa. To get the best out of wood, you need to know how and when it expands and contracts.
Background
When we talk about expansion and contraction in materials an important factor is moisture. Although temperature is an important consideration, you need to consider moisture when you deal with wood.
During dry weather, in the presence of high temperatures, initially, there can be some thermal expansion and the wood may swell and warp. But due to atmospheric dryness, the final result would be for the wood to shrink.
The moisture levels are what ultimately cause wood to shrink or expand. It largely depends on the humidity of the environment and the percentage of moisture in the wood.
Another important factor to consider about how wood contracts or expands is the wood species. Different species of wood have different levels of expansion and contraction. There are some species of wood that have a low degree of expansion or contraction which puts them in the category of dimensionally stable types of wood.
How Humidity Affects Wood
Moisture is the primary cause of the expansion and contraction of wood. Wood is a hygroscopic substance. Its inherent capacity to attract and retain water makes it susceptible to changes in the humidity levels of the atmosphere.
Being a porous material, wood tries to reach its internal levels of humidity with those of the atmosphere. We call this phenomenon equilibrium moisture content (EMC).
The wood, in an attempt to achieve its EMC, takes moisture into its fibers or releases it according to the atmospheric humidity. If the humidity level of the wood fibers is high and the humidity in the wood is low, then the wood will absorb moisture until it reaches its saturation point. In low humidity levels of the air, wood gives off moisture and shrinks.
The Effect of Warm Temperature on Wood
The effect of temperature on wood is indirect. If the atmospheric temperature is warmer, the humidity levels may reduce. There is a change in humidity levels with warmer temperatures.
Warm air will hold moisture more readily, so under such conditions the shrunken wood cells also take in moisture easily and the wood expands. So, if the temperature is warm and moist, wood can expand. It will contract just as quickly with a drop in moisture levels in high temperatures.
The Effect of Cold Temperature on Wood
During cold months, wood might expand or contract. Dry cold air holds less moisture. Wood releases some of its moisture to maintain its EMC. On the other hand, wet, cold air contains excess moisture. Wood will take in moisture and it results in the wood expanding it will swell and warp.
If the weather is freezing, the water within the wood fibers expands and pushes them apart as it freezes. So, here also, we see that wood movement depends on the humidity levels rather than the atmosphere’s temperature.
The Effect of Cold Weather on Wood Furniture
Wood furniture experiences dimensional changes with changes in weather just like other surfaces. During cold weather, especially with changes in humidity, wood furniture expands or contracts slightly.
The dimensional change in wood furniture is not very noticeable because most furniture is built to accommodate the nominal change in dimension. Apart from leaving wood furniture out in the rain or snow, it should survive through hot and cold weather for several years.
Effect of Cold Weather on Wooden Floors and Doors
As mentioned above, wood expands or shrinks during the winter months. The effect varies according to specific conditions during the cold weather.
The air tends to carry less moisture in the winter. To reach an EMC conducive to the low humidity levels of the atmosphere, wood gives off moisture into the air. It causes wooden doors and floors to shrink with a drop in moisture content.
But you also have to consider the wetness associated with winter in the form of rain and snow. It especially applies to outdoor wood furniture and wooden deck flooring. If hardwood doors or flooring are exposed to extreme wetness, they will expand.
For example, if you hang a wet coat on a wooden door to dry, and you have a fire going in the house, you will raise the humidity levels within. The wooden door begins to expand as it starts to absorb moisture, and it can become tight in the door frame.
Dealing with Shrinkage and Expansion in Wood
Wood shrinkage and expansion is a natural processes that you cannot control or stop. However, you can take some measures to reduce the adverse effect of how wood changes dimensions.
Seal the Wood
You can use epoxy resin to seal the wood. The more layers of polyurethane epoxy resin you apply to wood, the more airtight it will become. Once the surface of the wood is sealed off, it will not take in moisture as easily.
Plywood as an Alternative
Plywood is much more dimensionally stable than solid, natural wood. You can use plywood instead of solid wood in critical places like doors, windows, and certain parts of furniture that is more susceptible to dimensional changes due to humidity.
Select Suitable Hardwood
Use only high-quality hardwood of the right type for making furniture and other wooden items. Such wood tends to be very dense, so will not lose moisture or take it in that easily with changing weather conditions, making the wood dimensionally stable.
Season Your Wood
It is not a good practice to use newly-cut wood for making furniture. Sometimes wood from the lumberyard may also not be sufficiently dry to use. Leave new wood to acclimatize itself to the existing environment.
You may have to leave wood standing for months, even years before it is ready to use. Check your wood with a moisture meter before use to ensure that it does not contain too much moisture. If there is too much moisture, it may warp, twist, or crack later on.
Disassemble Your Furniture
An uncommon but not unknown practice is to dismantle expensive furniture during different seasons. You can assemble the furniture again when the weather is conducive. Store your furniture parts in an area that has moisture and temperature control.
Store Outdoor Furniture Inside During the Winter
Although outdoor furniture is designed to withstand the elements, it may not fare well if exposed to freezing temperatures during the long winter months. Similarly, if you live in a place that receives heavy rainfall in certain months of the year, in both cases, move your outdoor furniture to indoor locations.
To Seal or Not to Seal?
You can stop wood from moving by applying sealers to its surface. But it may not be a good thing to stop the wood from moving. Wood is naturally hygroscopic, so it adjusts to its own humidity levels.
If you seal wood, then it cannot change its humidity levels and will not expand or contract. But stresses can build up in the wood and it could still result in the wood becoming distorted.
There are two main reasons for not recommending sealing wood as follows:
Many Layers of Urethane
You would have to add multiple layers of urethane-based epoxy to the surface of the wood that you want to seal. It may unnecessarily add to the thickness of the wood.
Not a Permanent Solution
Even if you manage to seal off the surface of the wood from moisture, it is not a permanent arrangement. Once the surface gets scratched and abrased, you will have to keep adding more waterproof layers to it.
Conclusion
There are two ways to deal with the expansion or contraction of wood in cold weather or during weather changes. First, you can choose types of wood that are more dimensionally stable while exposed to changes in weather.
Then, you can also factor in the expansion and contraction of wood. If you provide adequate provision for dimensional changes when you build furniture, it will remain dimensionally stable even with drastic changes in atmospheric humidity and temperature.
Knowing that wood contracts and expands with weather changes and understanding how it happens is important when you handle wood regularly. It will help you understand what to expect from wood when you use it in your woodworking projects.
