Written by - Hector Dueck August 1, 2024
A lot of people ask us how wood moves, especially with regards to gaps in the pine tongue and groove boards so we felt this is is a good time to go over this subject in more detail.
The quick answer: Yes wood moves, and yes your tongue and groove boards will adjust with the different seasons in Southern Manitoba. Sometimes the boards will be tight together (without gaps - summer) and sometimes they will open up (with gaps of 1/8" in winter in a heated space for 6" wide board) just like a hardwood floor would do. We will go into a lot of detail below, follow us down the rabbit hole.
Wood is a natural product that is made up of many microscopic fibers that run along the length of the tree trunk (longitudinally). A good way to visualize this is a bundle of straw that has been bound up with bail string. Wood is also hygroscopic; so these fibers will take in moisture when the relative humidity is high and they will expel moisture when the relative humidity is low.
Wood is always trying to reach it's Equilibrium Moisture Content - "moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the relative humidity and temperature of the surrounding environment." - Wagner Meters. This is what causes tables to warp; hardwood flooring to open up and pine boards to open and close.
In colder climates, like Manitoba, it is normal for wood to expand and contract throughout the seasons and there is nothing that can be done to avoid this. However, proper planning insures that you only install pine once or build the table once.
The next question is how much movement should we expect to see?
Cut Types and How They Change Dimensionally (from Popular Woodworking)
The amount of movement (according to: Carl Hagstrom and Gene Wengert - Understanding Moisture Content and Wood Movement) is that for every 4% in Relative Humidity (RH) change you can expect a 1% change in wood movement, when grains run tangentially (flat sawn) or .5% movement when they run radially (quarter sawn). Which is a lot when we look at relative humidity ranges in Winnipeg!
I've included a quick reference chart below so you can see the temperature, RH, and how they affect EMC (Equilibrium Moisture Content - in white numbers). It's a great reference so that when you buy wood you can see how much it will change in different temperatures and humidity.
Photo from: United States Department of Agriculture; Wood Handbook, Wood as an Engineering Material
We have established that wood moves, how wood moves, and how much it moves; however, everything gets more complicated when you live in a place like Winnipeg, Manitoba, because the temperature and relative humidity change so drastically throughout the seasons.
Let's look at what Relative Humidity is so we can better prepare for our wood installations and plan accordingly for our wood movement in our climate.
"Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air. because as temperature increases so does the airs ability to hold water but the RH actually goes down when the temp increases. " - National Weather Service
The question we need to ask is, if your EMC (Equilibrium Moisture Content) will go up as relative humidity goes up then why does wood shrink in winter, even though it's the highest relative humidity throughout the year? We get a sneak peak at the answer above but lets continue down this rabbit hole because that's what we do at H&M Lumber.
We need to make an adjustment to the RH values that you see on your weather app to what you will experience in your cottage or 4 season sunroom if you plan to heat your space.
I have compiled some data from Canada Weather Stats for Winnipeg and have included what your EMC will be for each month in the chart below:
I found this calculator really helped to get my inside humidity levels when heating in winter. Remember that when you warm the air it has a higher capacity to hold water but in winter there isn't a lot of water in the atmosphere so your humidity drops when you heat a space which is why you have a lower EMC inside your home even though the relative humidity is high outside.
I think a good way to break down the data and make it practical is to average the 3 highest RH months and the 3 lowest RH months. Then we can start calculating for the wood movement and plan our jobs accordingly.
In a home without a humidity control system our average high EMC would be 12% (July 24, Aug 23, Sept 23) and the low would be (Dec, Jan, Feb) 4%.
For a home with a humidity control system set to 30% (which Furnasman recommends for Winnipeg), our high EMC would be 12% and low would be 6%.
This is where where we come full circle and get to the point of this article.
Wood Moves; Moisture Content of Wood
When you buy wood in Manitoba you need to consider the movement of the wood after you complete your piece of furniture or install your pine ceilings in your cottage. What will happen to the wood as it shrinks and expands? If you buy pine that is too wet (often not kiln dried) you end up with large gaps in between your boards, even in summer. Conversely, if you buy pine that is too dry you end up with buckling - where the boards have no room to go so they look raised like a bump.
You need to be careful to not just buy the cheapest wood because the last thing you want to do is tear everything down and reinstall it properly.
Acclimating your wood is a really good idea so that, at the very least, the temperature of the wood matches the space and the EMC adjusts. however, in the dead of winter you may be better off installing the wood immediately. If you're not sure we have high quality moisture meters that we can use to check the woods moisture.
We source pine that is dried to 15% so that our customers can rest assured that it won't buckle, even when it is installed in an uncontrolled but protected (not getting wet) 3 season sunroom or gazebo. Also, if you install it so that the boards are tight in your cabin the boards will shrink ever so slightly (1/32" if flat sawn) and in winter time it will shrink (1/8"-5/32" depending if you run a humidifier). These calculations were made on this awesome calculator. Calculations are for a 1x6 board
When working with hardwood you will want to run a dehumidifier in your shop in summer or get to work right after purchasing from us, as all of our hardwood has been professionally kiln dried. We don't buy diy wood so that we ensure that the wood has been professionally dried to the correct moisture levels.
Comments