The safest wood you can use in your garden should be free of toxic treatments and harsh chemical preservatives. That way you can feel certain that no dangerous toxins will leach into your garden soil.
Now, you may already know about the dangers of using certain pressure treated timbers in your garden. But what about kiln-dried wood?
Well, in this post, you’ll learn how kiln dried wood is made — and why this process helps to make wooden boards more stable. You’ll also find out what the difference is between kiln dried wood and treated wood.
And keep reading to discover if a stack of kiln dried wooden boards are safe to use in your backyard.
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First Off, What Is Kiln Dried Wood Anyway?
Kiln dried wood is simply wood that has been dried using a kiln oven.
Typically, after wood is logged and milled, it’s left to season. However, seasoning involves leaving wood to air-dry for months on end.
Related Post: How To Season Wood (7 Tips)
So, to speed up the process, an alternative way to dry out wood involves heating it in a kiln.
Kiln dried wood gets heated for days on end, (up to 36 hours), at nearly 94°C (201.2°F). And once done, that lumber is left virtually bone dry, with a moisture content almost as low as 12 percent.
What Do You Mean By Moisture Content? Moisture content simply refers to the amount of moisture in a piece of wood. When wood is first cut from a tree, it has a moisture content of around 100%. But, after seasoning, that moisture content will have fallen to less than 19%.
And Why Would You Kiln Dry Wood?
The two main reasons are to increase stability, and to remove bugs.
Kiln dried wood will have been heated to temperatures so hot that termites, and other bugs won’t survive it. This is why kiln drying is a much safer way to get rid of insects, (as opposed to using chemical fumigation).
On top of that, once wood has dried, it will have very little moisture left behind in it.
This low moisture content means that wood is less likely to shift and contract due to moisture evaporating. And this in turn makes it more stable, and less likely to warp and twist.
OK. So, Is Kiln Dried Wood Safe?
If it has only been kiln-dried, then yes it is.
Kiln dried wood has simply been put through a high heat process. So, there are no toxic chemicals added to natural kiln dried timber.
But, kiln dried wood can still be (and often is) also pressure treated.
Pressure treated wood has been put through a treatment that involves infusing wood with chemical wood preservatives.
Some of those wood preserving treatments are safe to have in and around a residential property. But some chemical wood preservatives are much more dangerous.
In other words, the kiln drying process alone does not make wood dangerous or toxic. Nevertheless, that does not mean that all kiln dried wooden boards are free of chemicals.
Is Untreated Kiln Dried Wood Good For Outdoor Use?
Unless that Kiln dried wood comes from a rot-resistant wood species, it won’t last outside.
Kiln drying simply speeds up the drying-out of wood, and makes it more stable. But, Kiln drying does not make wood any more rot-resistant (than it already is).
So, if you want to use kiln-dried wood outdoors, it’ll need to be treated with some kind wood preservative. On top of that, it will likely also need a few coats of a waterproofing wood sealer over it as well.
Wait A Minute…What’s The Difference Between Kiln Dried And Treated Wood?
When you kiln dry wood, the process simply aims to dry it out and kill off bugs. That’s pretty much it.
However, when you treat wood, the treatment process aims to preserve timber from wood rot and decay. This is why kiln dried wood often gets put through both processes.
For example, wood that has been stamped KDAT, has been treated and then kiln dried. Which is why it is referred to as ‘K.iln D.ried A.fter T.reating’ wood.
Is Kiln Dried Wood Better Than Pressure Treated Wood?
Neither kiln dried or pressure treated is better (or worse) than the other. That’s because these two different milling processes serve different purposes.
Having said that, Kiln dried lumber is more stable, and less likely to crack and split. This is because the moisture in this wood has been dried out.
As moisture evaporates out of wood, it can cause wood to contract and split. But, if there is very little moisture in wood in the first place, then cracking is less likely to occur.
However, pressure-treated wood will have been treated with rot-resisting wood preservatives. Those preservatives fight off wood rot and decay.
So, for exterior structures, pressure treated wood is better since it is most suited for outdoor conditions.
What Are The Main Disadvantages Of Kiln Drying Wood?
Well, if you decide to personally kiln dry wood yourself, then the biggest disadvantage is time.
If you decide to build your own kiln-oven, you will have to keep monitoring that lumber for days on end. On the other hand, regular air-dried seasoned wood is more of a set-and-forget kind of setup.
And What Happens If Kiln Dried Wood Gets Wet?
That wood will absorb the water. But, kiln dried wood is generally less likely to warp and twist severely as it drys out afterward.
Still, this depends on the wood type. Douglas Fir, for example, is a very stable wood, after it has been seasoned.
But, Hard Maple, will happily bend out of shape due to humidity or damp — even after it’s been seasoned. Which is why Hard Maple needs to be properly sealed up to prevent it from absorbing moisture.
So, Is Kiln Dried Wood Safe For A Garden?
Yes it is. Kiln drying is simply a heat treatment. It does not involve any chemicals.
However, if that kiln-dried wood has subsequently had additional chemicals added to it, then that’s a whole other story.
To Wrap Up, Here Are The 3 Key Takeaways From This Post…
- 1). Kiln dried wood has been put through a heat treatment to dry it out.
- 2). Kiln dried wood is safe to use, provided it hasn’t been additionally treated with toxic chemical wood preservatives.
- 3). Kiln dried wood is generally more stable than wood that has been seasoned over a number of months.
Treated Wood In The Landscape | Home & Garden Information Center
Overview of Wood Preservative Chemicals | EPA.gov