Q. What is the best species for wood exposed to the elements on a house, shutters, window sash, etc.? Is one type of pine better? How come old sash without paint lasts longer than modern sash with paint?
A. These are tough questions because of many variables, so I will cover six key points that you can use to find the best answer for your needs.
First, appreciate that exterior window sash, made of wood, and other exterior wood items covered with a coat of paint are oftentimes especially vulnerable to rot, compared to unpainted material. Why? Because if there is a hole, crack or gap in the paint film, water can easily get past the paint film and into the wood underneath.
But then, the water is trapped inside and cannot get out (or evaporate) very easily through the tiny hole or crack. As all rot occurs only when wood is wet, conditions under the paint are ideal for the rot fungi. This is especially true at the ends of the pieces where a stile and rail join together, as a small gap or break is common. So, unpainted wood can indeed last longer than painted in many cases.
Second, except for the heartwood (red colored in pine), there is not much natural decay resistance in pine, hemlock and spruce. However, resin (the sticky sap) does retard decay, which makes some species and some pieces more resistant.
There are some species of wood that do indeed have natural decay resistance, but cedars and redwood often do not have the required strength; white oak, locust, Osage orange and many tropical woods that are naturally decay resistant are very dense (heavy) and hard to work (machine and fasten), and may even move a lot when their moisture changes.
Third, anything we can do to wood to keep the liquid water out of the wood (“dry rot” is a misnomer) will be extremely helpful. Water repellants are therefore very desirable. In fact, years ago, a water repellant penetrating finish with a wood preservative (penta-chlorophenol) was used on a lot of outside wood successfully.
Health hazards eliminated this product. Some newer finishes, especially if they soak into the wood, are now available and could be used on the finished product and provide reasonable protection. (I saw some wood treated with three coats of Sikkens Cetol Natural #078 finish and the color after several years of exposure was still wonderful; no greying.
Fourth, when manufacturing items, because of the fine sawdust in the air and wood waste disposal, most commercial preservative treatments for lumber are not an option for treating wood prior to manufacturing. Some treatments also will leach out of the wood when exposed to liquid water.
Fifth, I suggest you look at the process of using acetic acid to treat the wood, acetylated wood. One trade name is Accoya. The treated wood is stable and decay resistant. The price may be an issue for some products.
Sixth, the biggest issue is whether the consumer is willing to pay extra for higher quality and longer life compared to the cheap stuff. Further, consumers, in case of a complaint, can never go back to the manufacturer with items made offshore.
But they can come back to you in the U.S., even many years later. Do you want to added life-preserving features to your wood products so you can offer a much longer warrantee? (That reminds me to suggest that you might date-stamp what you make, inconspicuous location, using an ultraviolet ink, in order to confirm the manufacturing date years later.)
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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