Wood is an amazing material to work with but you should carefully consider what kind of wood you want to use for every project. Because especially if your project is going to be sitting outside the choice of wood is very important. Things like temperature and humidity can cause different kinds of wood to contract or expand or even weaken the durability of the wood.
This is why it is so important to know how different kinds of wood react to humidity and temperature. Because then you can choose the perfect kind of wood for your project and thus ensure the longevity of all of your woodworking projects.
Temperature does affect wood in several ways. It can cause wood to contract or expand but only indirrectly. Humidity is the actual reason for the contracting and expanding of the wood. The more water there is inside the wood, the more it expands and vice versa.
So while temperature does play a big role in the expansion and contraction of the wood it is not actually directly the cause.
Higher and lower temperatures just indirectly change the moisture content of the wood resulting in the shrinkage or expansion of the wood.
The effective moisture content of the wood can be calculated by taking the weight of the water present within the wood and dividing it by the weight of the wood. Then times that by 100 and you have the percentage of moisture inside the wood relative to its weight.
Moisture content = (weight of the water inside the wood / wight of the wood itself) * 100
The higher the moisture content of the wood is the more the wood will expand and the lower it is the more it will contract.
The relative humidity surrounding the wood has a higher impact on the moisture content than the temperature. The temperature only indirectly influences the moisture content of the wood.
Different types of wood will also expand and contract differently. So for outdoors, for example, I would recommend a type of wood that doesn´t move as much but is still resistant enough to the elements to ensure its longevity.
Wood Expansion and Contraction Chart
There are a lot of different aspects, that can influence how much exactly a piece of wood can contract or expand, such as the way it has been cut, the exact wood species, and even the thickness of the wood.
My chart, however, will only consider the kind of wood. Most other charts will tell you exactly by how much the wood can expand and contract but my chart will only tell you an average of how much each type of wood can expand and contract.
If you want more detailed information on how to calculate and predict wood movement, then check this article from “thiscaepentry.com” out.
|Hardwoods||Factor (Multiply the Volume by |
This Number to Get the Potential
Contraction and Expansion)
|Alder, Red||.0015 – .0025|
|Ash, White||.0017 – .0027|
|Beech, American||.0019 – .0043|
|Birch, Yellow||.0026 – .0034|
|Cherry, Black||.0013 – .0025|
|Hickory, True||.0026 – .0041|
|Maple, Red||.0014 – .0029|
|Maple, Silver||.0010 – .0025|
|Maple, Sugar||.0017 – .0035|
|Oak, Red||.0016 – .0037|
|Oak, White||.0018 – .0037|
|Walnut, Black||.0019 – .0027|
|Poplar, Yellow||.0016 – .0029|
|Bald Cypress||.0013 – .0022|
|Cedar, Eastern Red||.0011 – .0016|
|Douglas-Fir, Coast||.0017 – .0027|
|Hemlock, Western||.0014 – .0027|
|Pine, Eastern White||.0007 – .0021|
|Pine, Lodge Pole||.0015 – .0023|
|Pine, Slash||.0019 – .0027|
|Pine, Sugar||.0001 – .0020|
|Pine, Western White||.0014 – .0026|
|Pine, Yellow||.0018 – .0026|
|Redwood||.0010 – .0023|
|Spruce, Sitka||.0015 – .0026|
|Spruce, White||.0013 – .0027|
As you can see, some types of wood move quite a lot while others are a lot more stable.
So choose the kind of wood carefully before starting your project. Especially if your project is going to sit outside.
It is also a good idea to let the wood acclimate to their surroundings for a couple of days before you start working with it. This way you can avoid nasty surprises later on.
Effects of Temperature on Wood
I already told you a little bit about the effects of temperature on wood and why moisture is the main reason for wood to expand and contract and not temperature.
But in this section, I want to go more into depth about why temperature does affect the wood in the first place and why exactly humidity is so important.
How Does Humidity Effect Wood
Moisture does affect wood significantly. Wood is a hygroscopic material. This means that wood is constantly reacting to environmental climate changes. It absorbs and releases moisture relative to its surrounding humidity resulting in either expanding or contracting of the wood.
What does that mean exactly?
Well, wood is always striving to balance its own humidity levels with the humidity levels of its surroundings. Wood wants to be at EMC levels.
EMC stands for Equilibrium Moisture Content and is the point at which wood is at the same moisture level as its environment.
If the humidity level of the environment is higher than the humidity level of the wood then the wood will absorb water and therefore expand.
And if the humidity level of the environment is lower than in the wood then the wood will release some of its moisture resulting in the contraction of the wood.
So the effect of humidity on wood is actually the reason for the contraction and expansion of wood and not the temperature.
How Does Cold Temperature Affect Wood
I get a lot of questions about how wood is being affected by colder temperatures and why. One example of wood expanding in cold, that a lot of people seem to experience, is a wooden door not closing properly anymore when the outside temperature starts to get cooler. So I decided to answer this question and explain the reason behind wood expanding and shrinking in colder temperatures.
Wood can expand in colder temperatures and it can also contract in colder temperatures. Wood doesn´t directly contract or expand because of the surrounding temperatures but because of its internal humidity level. The higher the humidity level of the wood the more it expands and vice versa.
So, as weird as it may sound, wood can contract as well as expand in the cold.
This is because the humidity levels of the wood itself are responsible for the movement of the wood. And they can change regardless of the temperature that is affecting the wood.
So does temperature even matter?
Yes, the temperature does still matter because the temperature is responsible for changing the surrounding humidity most of the time.
We will take the wooden door that won´t work properly when it gets cold outside, as an example.
Let´s say that the door leads outside and is made out of wood. When it starts to get colder outside, let´s just say that winter is around the corner, then the humidity levels outside will drop.
This will lead the wood to disperse some of its humidity resulting in a lower humidity level inside the wood. This, in turn, will lead to the wood contracting.
But in that exact example, the door could also expand even though it is still cold outside.
Let´s say there is some snow lying outside and you just got home and you leave your wet coat hanging right next to the wooden door. You turn up the heater because you are cold, and now the wet coat will start to dry slowly.
This will raise the humidity levels indoors and the wooden door will start to absorb some of the humidity, remember wood wants to strike a balance in the humidity levels of itself and its surroundings.
This additional humidity, that the wood absorbed, will cause the wood to expand slightly.
You don´t even need a heat source, the wood could also become wet because some snow has set right in front of it. This can also lead to the wood absorbing some of the moisture of the snow resulting in the expanding of the wood as well.
As you can see from the three examples above, even though it’s always cold the wooden door can react very differently depending on the surrounding humidity.
How Does Cold Weather Affect Wood Furniture
A common question I get is how cold weather or temperatures can affect wooden furniture. And the answer is quite simple.
Wood furniture can be affected by cold weather or temperatures but most wooden furniture is built in a way, that already considers that. Wood furniture can expand and contract slightly if the surrounding humidity or temperature changes. But well build furniture won´t take damage from that movement so they can be stored in colder locations.
So basically wooden furniture will react much the same way that any other wood does when the surrounding temperature and humidity change.
But mos well build furniture will be built in a way, that does anticipate the movement of the wood, so even when the wood contracts or expands your furniture will most likely be fine.
So you can store your furniture in the garage or in your cellar without any problems.
You can leave it there in cold weather and they will survive. As long as they don´t get wet.
How Does Warm Temperature Affect Wood
I already explained how wood is being affected by cold temperatures so we are only taking a quick look at how warm temperatures affect wood.
Warm temperature affects wood only indirectly. Warmer temperatures will change the humidity levels of the environment and wood will either start to dry, when the surrounding humidity levels are low, or it will absorb moisture when the surrounding humidity levels are high. Dry wood will contract while wet wood will expand.
As you probably realized, the answer is much the same as with cold temperatures.
Wood can start to shrink in warm temperatures but it can also expand in warm temperatures.
It all depends on the humidity levels and how much moisture the wood is in direct contact with.
The temperature only indirectly influences the contraction and expansion of the wood.
How to Keep Wood From Shrinking or Expanding
Wood movement is annoying I completely get that. You always have to account for it, especially when you need to be very precise, and there are so many different factors, that can influence the contraction and expansion of the wood. So naturally, we are all searching for a way to keep wood from moving.
You can only keep wood from shrinking or expanding by completely sealing the wood airtight. This is only possible with multiple layers of some sort of liquid epoxy, like urethane. So it is generally better to account for the movement of the wood instead of keeping it from moving entirely.
So, unfortunately, there is no surefire way to keep wood from moving.
You can limit the amount of the movement by carefully choosing the right type of wood, by choosing wood, that is cut in a certain way, and by letting the wood acclimate to the room or area, where it is going to be later on. But that´s about it.
Does Sealing Wood Prevent Expansion and Contraction?
There is one way to keep wood from moving but only theoretically and I would not recommend doing it. The main reason for the contraction and expansion of wood is that wood is hygroscopic.
Meaning that wood always tries to balance its own humidity levels with the surrounding humidity levels. So if you seal the wooden surface completely airtight, then the wood will no longer be able to change its own humidity levels. Which means it won´t be able to move anymore.
Sealing wood does prevent expansion and contraction if it is sealed airtight. The only way to seal wood airtight is with some sort of epoxy resin, like urethane. If the wood is sealed airtight, then it can no longer change it´s own humidity levels. Wich means it can no longer expand or contract.
But, even though it is possible to hinder wood from moving at all by sealing it airtight I would still not recommend doing this.
The main reason is, that you would need to apply a lot of layers of urethane in order to get it really airtight. And I also think that this solution is not a permanent one.
So I would recommend just account for the movement of the wood when you are building or you can also choose a type of wood that doesn´t contract or expand much.