Acetylated Treated Lumber What's Up with That?

Here's an interesting discussion about the durability, fastener compatibility, and other properties of acetyl treated lumber, such as "Accoya," with some useful links to more. January 2, 2014

I just started working with a wood called Accoya. Comes from Denmark after being in autoclaves that treat it in some way. Has a 50 year above the waterline and 20 year below waterline warranty (whatever that means) and is touted as being more stable than teak. It is actually raddiata pine when they start the treatment system. Anyone else tried it? It might work well for sash, but being flatsawn I'm not sure how well it would work for solid door stiles, etc. I'm paying about $5.75 B/F which is another added benefit.

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
5.75b/f and you think that is a benefit. I've bought radiata plywood and it is no good.

From contributor L:
For $5.75 it better be good stuff! We don't do any boat work, but some exterior moldings, so the heat treat wood might be okay. Other kinds of wood are also available heat treated. As for radiata plywood, we use some when forced to use plywood. Seems okay, but pretty soft.

From the original questioner:
In California, Honduran mahogany is around $9.00 B/F and teak is around $26.00 B/F. This material is supposedly comparable. I do not know if this is heat treated or infused with something. That is why I am asking for feedback from anyone that has used it.

From contributor B:
Sounds more like it falls into the category of pressure treated. Probably more stable but a chemically treated pine product. Just a whole lot more expensive.

From contributor A:
Do you remember those days when you could rely on eastern white pine to last forever with a bad paint job? My carriage house has 150 year old quarter sawn eastern white pine clapboards, which weren't properly painted for at least 50 years. No rot whatsoever on a 2000 sqft building in Connecticut.

Now we have to cook it, treat it, and it's only supposed to last 50 years (with 3 coats of perfect paint no doubt). I would still stick with the Spanish cedar, WR cedar, redwood and sapele for exterior millwork.

I'm not sure why one would use radiata pine below the water line. It has mediocre structural properties. White oak has served the marine industry for a century below the water line and it's half the price and 3 times the strength (a guess).

From contributor B:
I'm also in CT and had to correct a lumber salesman some years ago when he said Eastern white pine is not an exterior wood. I've had original pine clapboards on many pre-1850 houses I've owned and worked on over the years and it's always in great shape, even when left relatively unprotected. Of course that was all old growth lumber.

From contributor P:
I've been reading about this stuff for a while now - they sent me a small sample and it seems to work nicely. No one near me in northern California carries it yet, but I'm anxious to try it for high end exterior millwork, instead of redwood, cedar or (dare I say it) Azek.

From contributor D:
I have talked to two professional millwork shops that do projects with Accoya. One has converted to it for all their exterior work. They are located in Charleston, SC - lots of woodwork and lots of rot. They claim it is a lot like working with eastern pine. Stable and solid, and glues well and paints well.

While it is pressure treated, it is nothing like the green or brown SYP #3 that is underfoot as decking and such. The samples I have are all nice clean, bright pine, hard and dry. Faint smell of vinegar.

In the data sheets found at (pg 8, Wood Information Guide), they claim the dimensional stability is enhanced. Looking at the numbers, I would say it is damn near eliminated! This would be a great asset in frame and panel work, where panels would move only a negligible or undetectable amount. This characteristic has me really intrigued. There are also some considerations with glues and fasteners, but none appear to be big drawbacks.

My distributor is Sierra Forest Products in Chicago. They will send out samples. If the wood is half what they claim, it is a bargain at 5.75 a foot. It may help save the old growth and still be a great choice for long lived exterior work.

From the original questioner:
Thank you. Maybe you could ask your friends that are using Accoya if they have access to vertical grain Accoya and if not, if they feel comfortable using flat grain for sash stiles and rails?

From contributor C:
I've never worked with it, but I've two containers arriving next month of radiata pine for two projects, spec'd by architects. One has 2,000 profiles, the largest being 7.5 x 12.5 x 120 cm, of four laminations, ready for finishing, that will end up as a storefront in Cancun, Mexico. The second one has un-dimensioned lumber in different widths and thicknesses up to 4.5m long. This will be used for a door and window project. Both will have a WB finish.

The only thing I know with any certainty is that Accoya is very acidic and you must use SS fasteners as it'll eat anything else (or so I've been told from UK).

From contributor D:
Look at the PDFs on the website for Accoya. The radial and tangential movement percentages are given, and compared to other woods. If they are accurate, there is almost no movement! This would make for very stable stiles and sash parts, and no need for vertical grain (beyond visual). It appears that the process also slows water/moisture absorption, so that is also very desirable.

From the original questioner:
Thanks. Since I have seen your great work I know you are a journeyman sash and door builder, so I know you know this, but my concern for V/G is about straightness more than stability or movement. V/G annular ring orientation will be more likely to hold a stile straighter (always been my understanding), so I would be less likely to use flat grain for the same purpose. Only with species like gen mahogany or teak would my concern be belayed about flat grain. A sash stile 2-1/2" wide X 1-3/4" thick and 72" long needs to stay straight and I would never use flat grain fir for that purpose. It sounds like Accoya might make this concern unnecessary?

From contributor D:

Not to take this off on a sidebar, but when I think about flat vs quartered on a 2-1/2 x 1-3/4 stile, it is almost a moot question. That is, if you are showing VG or quartered on the 2-1/2" faces, then you are showing a lot of flat grain on the two edges. That is, it is impossible/impractical to have VG on all four sides, though I guess what we call rift could be possible.

When I have a part go bad, I can't see a correlation to grain orientation. I chalk it up to growing stress, or maybe drying stress. Or the alignment of the planets.

I get my rough lengths first, then rough widths, and then like to let it sit for 24 hours or so before facing and edging. It is very rare that a part moves on me once they are faced, edged, then S4S'd.

From the original questioner:
Acetylated wood has been studied for many decades. It has been under commercialization for over 25 years, but cost limited its adoption.

Basically, a chemical is added (acetic acid) to the wood and a chemical reaction takes place that changes the wood so that the wood is more decay resistant, stable with moisture changes, yet easy to work with.

Although expensive, it does merit attention, especially when its long life is considered (like for windows). Because it lasts so long, we will have to cut fewer trees in the next century (unless we remodel a house in 25 - 75 years).

From contributor I:
We have been using Accoya for paint-grade exterior doors and sash for about 1 year now. We're in Canada so climate is a factor. It is very easy to work (like a slightly harder pine) and extremely stable. I have no idea whether it will hold up for 50 years, but short-term it is far superior to the fast-growth pine. Architects seem to love the specs!

From contributor P:
Professor Wengert, could you comment on the need for SS fasteners with Accoya, and the durability of cypress compared to redwood or WR cedar for exterior millwork? Do I need to find out what variety of cypress I'm buying?

From Professor Gene Wengert, forum technical advisor:
We can look in an older text book and find out what the decay and insect resistance of a species was. Unfortunately, we have found that second growth (the trees we are harvesting now) do not have the same resistance as the wood from century old trees.

I assume you appreciate that it is only the heartwood that has natural decay or insect resistance. So, when I look at the Wood Handbook (page 3-18), they give decay resistance in three groups:
1. Resistant or very resistant
2. Moderately resistant
3. Slightly or non-resistant

All three species you asked about are in #1 group.

Regarding fasteners, stainless steel is suggested due to corrosion that occurs in moisture environments. If the environment is not moist, then you probably would not be using Accoya. The info I have for Accoya indicates that the risk of corrosion is similar to the risk of oak in a damp or wet environment.

From contributor B:
Stainless steel is of course the optimal choice. Is there a reason not to use galvanized fasteners though?

From Professor Gene Wengert, forum technical advisor:
Maybe. Galvanized isn't perfect after you hit it with a hammer, so to be safe, they want SS. Their product has a 50 plus year life (above ground), and it would be sad to have to replace it because the fastener failed at 30 years.

From contributor O:
As specialist hardware suppliers to the timber door and window industry here in the UK, Coastal gets asked about Accoya a lot, particularly in relation to the acidity of the material and the effects it has on our hardware. Our whitepaper 'Wood is Corrosive' may be of interest.

From Professor Gene Wengert, forum technical advisor:
Interesting paper (although the idea that the acidic vapors cannot get out of wood during kiln drying is certainly an interesting concept, as is the idea that a lock set can stop vapor movement from dry wood), but when I saw that they were selling a coating to protect the metal in contact with wood, I became more questioning.

I do not see how such a coating would remain on the metal screws and nails when inserted into the wood, yet their pictures show such fasteners and their corrosion. The wood that such fasteners are in shows extensive weathering as well, not what we would see in a normal door or window indeed. We do have corrosion problems with treated wood in order to prevent corrosion and there are recommended fasteners. I would think that they would work for acetic acid treated wood as well.

The picture of a large metal band (perhaps part of a hinge) that is fully corroded cannot be attributed to a chemical wood treatment - it appears to be due to frequent wetting and the expected corrosion of metal when exposed to rain. Certainly not a confined space as the article talks about.

I wonder why there are not pictures of a lockset that is corroded, as the paper discusses that issue. I am not a chemist, but is the corrosion of metal from sea salt in wood the same and as severe as the corrosion from acetic acid treated wood? I look forward to hearing more from your company about these comments and questions.

From contributor O:
Thank you for your comments.
With regards your comments in respect of acetic vapours not being able to escape once kiln dried, please see page 2 of

You make a good point about a few of the images that are used within the paper. A number of these were admittedly stock images. We will revise accordingly.

There are pictures shown of the lockset corroded, with and without the protection. It shows the rear view of a faceplate of a multipoint lockset.
With regard to the corrosion of metal from sea salt in wood being the same and as severe as the corrosion from acetylated wood, again, the pdf above may help explain. I quote: "The natural chloride content of wood is low (0.1 to 5% of the ash, which is 0.2 to 4% of the dry weight of the wood), but cases have occurred of severe corrosion of metals in contact with woods that were not particularly acid, but which were found to contain appreciable quantities of salt, up to 0.8% by weight. Wood can absorb salt; the likely sources of adventitious salt are twofold:
(a) salt spray and mist near the coast;
(b) the floating of logs in seawater

From Professor Gene Wengert, forum technical advisor:
Thanks for the response and the reference. Any idea who wrote the reference? I can find no author or company name on it.

I note that with this article, much of the discussion is for boxes made with wet oak. We certainly do know that wet wood, especially oak, used for shipping containers is a big issue. So, pallets and crates are often dried. We do not want our motorcycle to be rusted or our sugar to taste like oak. They are also heat treated to sanitize for export.

With respect to the statement:
Kiln drying accelerates the production of free acetic acid in wood, but most of the acid does not have time to escape. Kiln dried wood is more acid and more immediately corrosive than air dried wood, though it contains less combined acid that can be set free in later years."

I do not believe it is correct from a practical viewpoint, especially with softwoods. I wonder what "free" acetic acid is. I wonder why there is no mention of tannic acid.

In any case, I am curious why we do not see such corrosion with all our windows and doors normally (made with KD wood), except in extreme exposure conditions.

Specifically, has your company ever tested Accoya wood for corrosion of fasteners? If so, is liquid wetting required for corrosion or can we get dry corrosion under normal humidity conditions in the U.S. or will it only happen in a coastal climate?

From contributor O:
Thanks, Gene. Further testing is currently underway and I look forward to posting the results back in the next month or so, all going well. I am not sure of the actual author, however this is a download from the National Physical Laboratory in London.