Skip to Content

Kiln-dried wood vs pressure treated: what is better?

Kiln-drying means the removal of water or moisture content (MC) from the cells of wood, while pressure treatment means the removal of air from the cells of the wood and the addition of fire-retardants and waterborne preservatives such as a co-biocide, micronized copper azole, etc deep into the cellular structure of the wood providing long-term protection against fungal decay, rot, insects and termite attack. The preservatives used during the pressure treatment of wood are non-toxic and are certified by the Scientific Certification Systems (SCS) as Environmentally Preferable Products (EPP).

Kiln-dried wood vs pressure treated: what is better?

Kiln-drying and pressure treating wood are two different processes. Kiln drying lowers the moisture level of the wood, where pressure treating wood is used to add a preservative. Most pressure treated wood is first kiln dried as the lower moisture level makes absorbing the preservative easier.

Kiln-drying does not change the natural color of your wood, in contrast to it, the pressure treatment of your wood changes its natural color to a darker green color. Kiln-drying reduces the natural weight of your wood to a great extent so it becomes easy for shipping, in contrast to it, the pressure treatment of your wood adds considerable weight to the natural weight of your wood.

Kiln-dried wood is crackproof but pressure-treated wood is not guaranteed against warping, shrinkage, or splitting. Pressure-treated wood can dry in about 1 month, whereas fresh wood provides an environment where fungi and bacteria produce rot and insects infest. Kiln-dried wood is lighter, easier to work with, and less likely to split. You can easily buy timber that has been kiln-dried but it will cost you a premium price.

If you have purchased green or wet timber, you should kiln-dry it out as best you can before working with it by stacking it on spacers and storing it in a shaded, dry, and well-ventilated area so the air can circulate evenly all around it. You can check the moisture content of wood by using a moisture meter at your timberyard or cross-cutting a board and examining the interior wood. Wood shrinks in all dimensions as it kiln-dries, but most problems are caused in how the boards handle the shrinkage in width and thickness.

How long does pressure-treated wood last?

Pressure-treated wood is the best material for outdoor construction such as benches, decks, pergolas, fences, and gates, as it has a long, useful life span that will beat all kinds of weather conditions and is much less costly than its alternative wooden construction material. Pressure-treated wood can last more than forty years altogether. The pressure treatment process involves placing a load of wood in a huge cylindrical chamber called a retort or vacuum pressure vessel.

It has a door on its one end that can be sealed airtight, then some waterborne chemicals are forced into the timber under high pressure. There are a handful of species of wood commonly used for the construction of structural framing, and their properties affect how far they can span as joists. Most wood used for the construction of the deck is considered fresh and wet because it has a moisture content of over thirty percent water weight.

Greenwood will shrink as it dries out and is less strong and stable, which can cause it to check and warp.

What preservatives are used pressure treatment of wood?

For many many decades, the pressure treated timber industry faced a deservedly bad reputation for preservatives that included environmentally toxic chromate copper arsenic (CCA). In 2003 the pressure treated timber industry voluntarily suspended the use of chromated copper arsenic CCA for residential use, although it continues to be used for commercial applications.

Moldex 45 Mildewcide is an isothiazolinone chemical used for the temporary control of mold and mildew on pressure-treated wood building products. Moldex 45 is 100% environmentally friendly and does not persist in the environment.

Today’s pressure-treated wood preservatives still include some form of copper, which is not toxic and inhibits the growth of mold and mildew and repels insects. Some manufacturers avoid the telltale incisions used to help preservative chemicals penetrate the wood. Instead, they employ new high-pressure techniques that drive the preservatives deep into the heart of the timber.

Below I will discuss 6 different preservatives that are used for the pressure treatment of wood, they have the following features: low environmental toxicity, easy to incorporate, low dosage requirement, cost-effectiveness, good supply, and online availability, and above all global production capabilities.


ACQ (alkaline copper quaternary) is used for pressure treatment of raw, organic timber. It is a water-based wood preservative that prevents decay from fungi and insects (fungicide and insecticide). It also has relatively low risks, based on its components of copper oxide and quaternary ammonium compounds.

Water-based preservatives like ACQ leave a dry, paintable surface. ACQ (alkaline copper quaternary) is officially registered for use on building and utility poles, and other wooden structures.

● Borates

Disodium octaborate tetrahydrate (DOT) is used for pressure treatment of raw, organic timber. It is specially formulated for use as a water-based wood preservative and is registered by EPA as well as government agencies throughout Asia, North America, and Europe. Typical applications include furnishings and joists.

● Copper Azole

Copper azole is used for pressure treatment of raw, organic timber. It is a water-based wood preservative that prevents fungal decay and insect attack; it is a fungicide and insecticide. It is widely used throughout the USA and Canada.

Water-based preservatives like copper azole leave wood with a clean, paintable surface after they dry out completely. Copper azole is officially registered for the treatment of millwork, and other wooden products that are used in above-ground, ground contact, freshwater, and saltwater splash (aquatic) decking applications.

● Copper Naphthenate

Copper naphthenate is used for pressure treatment of raw, organic timber. It was first officially registered in 1951 and is used to brush, dip, spray, and pressure treat wood that will be used in ground contact, water contact, and above the ground such as utility poles and landscape timbers. Copper naphthenate is effective in protecting wood against insect damage.

● Copper- HDO

Copper – HDO is also used for pressure treatment of raw, organic timber. It was first officially registered in 2005 and is used today for the pressure treatment of wood that can be used as decking and posts. It is restricted from use in aquatic or marine areas, construction of beehives, or any other application associated with the packaging of food or feed.

● Polymeric Betaine

Polymeric betaine is a preservative used for pressure treatment of raw, organic timber. It was first officially registered as an active ingredient in the United States in 2006. It is a borate ester that, when applied to the pressure treatment of wood, breaks down to DDAC and boric acid. Polymeric betaine today commonly is applied by pressure treatment to forest products.

Adding water repellants to the preservatives to help the timber fend off moisture is very useful. No matter what pressure treated timber you decide to use, you must always wear a dust mask and eye protection when cutting pressure-treated wood, and wear gloves when handling the material.

How to pressure treat wood?

Step 1: Vacuum pressure tank

Move the wood into a large steel cylinder; called a vacuum pressure vessel or tank.

Step 2: Seal the pressure tank

Once the wood is loaded into the cylinder, you must close all the doors and seal the cylinder tightly.

Step 3: Pump the air out

Us an industrial vacuum pump to remove the air from the cylinder, and that includes pulling air out of the wood too.

Step 4: Preservative solution

The cylinder must then be flooded with the preservative solution.

Step 5: Apply pressure

The pressure must be applied to the solution in order to force the preservatives deep into the wood cells.

Step 6: Use correct time and pressure

Cycle times and pressure settings must be adjusted based on the retention levels needed and the species of the wood being treated.

Step 7: Drain the tank

Once the cycle has been completed, the cylinder must be fully drained, then the industrial vacuum pump must be used to remove excess solution from the wood which is returned to the work solution storage tank.

Step 8: Remove excess preservatives

A final vacuum must be run within the cylinder to extract the excess preservatives which is returned to the work solution storage tank.

Step 9: Wait 1 to 2 days

Now open the doors and remove the wood. Wait for 24 to 48 hours.

Frequently asked questions (FAQs) about kiln-dried wood and pressure-treated wood:

1. How do I dispose off kiln-dried wood and pressure-treated wood?

Both kiln-dried wood and pressure-treated wood may be disposed of in landfills or burned in commercial or industrial incinerators or boilers. But you must carefully follow the federal, state, and local regulations.

2. How can I remove stamp from kiln-dried wood and pressure-treated wood?

Sawmills mark their kiln-dried wood and pressure-treated wood with an ink stamp that indicates the species, sawmill, moisture content, and other information about the kiln-dried wood and pressure-treated wood. You can easily remove the stain of the stamp by using a palm sander.

3. What is meant by KDAT?

KDAT means kiln dried after treatment. Following the pressure treatment of the wood, it is placed in a kiln, where its excess moisture content is removed and the wood is returned to its original moisture content.  The kiln offers a strictly controlled environment and the timber is dried evenly, this process helps minimize the natural tendency of wood to crack.