THE IMPORTANCE OF PROPERLY ACCLIMATING EXTERIOR HARDWOODS FOR BEST PERFORMANCE

 

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Wood is a natural organic product and requires some special care and consideration when used outdoors for exterior construction projects. Wood is vulnerable to the elements, particularly when it is first placed outdoors in its new environment. For best results, the longer a wood product is allowed to acclimate on site, the better the long term results for your project.  While high density hardwoods have proven to be the most durable wood species for outdoor use, they too, need to acclimate properly for best looks and results.

 

All woods will expand and contract with their moisture content, seasonally. Depending on the amount of variation from initial moisture content of the wood and how quickly the wood dries to acclimation, will affect the behavior of the wood. For example, if you take a soaking wet piece of green wood and put it in a full exposure to the sun in a low humidity area, you can expect the wood to behave badly; shrink quickly, possibly check or split, possibly twist, etc. All wood species behave differently, depending on their cellular structure. The more stable the wood species, the better they will perform in extreme conditions.

 

Equilibrium Moisture Content (EMC) research provided by the USDA Forest Products Laboratory is a great tool to help predict wood behavior relative to the humidity levels. When a wood species dries naturally to the EMC range within a particular area, it is considered to be 'fully acclimated' or 'at equilibrium' with its new environs. Proper acclimation of wood is imperative for best overall performance and extended lifetime of wood products in an exterior project.

 

No matter which species of wood you choose for your exterior project, care should be taken to protect the wood, particularly during its initial acclimation period. High density hardwood decking and wood siding species perform remarkably well in a wide range of conditions from extremely arid to extremely humid moisture conditions and also extremely hot to extremely cold temperature conditions. It is certainly true that high density hardwoods such as Ipe, Cumaru, Garapa and Machiche can be allowed to weather to a silvery gray patina with little or no maintenance for a long period of time. Nevertheless, it is still highly recommended to coat these durable decking and siding materials with a UV inhibiting sealer, such as Penofin Penetrating Oil or a similar product to allow them to acclimate more slowly and evenly during their initial installation period.

 

In hot, dry climates, wood may tend to dry out very quickly (depending on humidity and time of year).  This can possibly make wood tend to exhibit surface checking at times during the acclimation process. It mostly depends on exposure to the sun, moisture content of the wood and relative humidity at the time of installation. Take care of your wood to help it adjust properly during its initial acclimation period and it will help improve the overall performance of the wood throughout its life cycle.

 

Depending on where your project is, the seasonal EMC (Equilibrium Moisture Content) can be extreme or very mild. In Austin, TX for example, the EMC is very stable and typically ranges from 12.4% to 14.1% seasonally. In Houston, the EMC, while on the humid side, is about as consistent as anywhere in the world, typically staying within the 14% to 14.8% range. High density hardwood decking and wood siding species such as Ipe, Cumaru, Garapa, Machiche and Santa Maria hardwood perform remarkably well in environments such as these.

 

In more temperate climates, like San Francisco, CA where the EMC typically ranges from a low of 13.4% to a high of 15.2%, high density hardwood species such as Ipe, Cumaru, Machiche and Santa Maria hardwood perform extremely well as decking and siding materials. These durable and stable hardwood species are used on a variety of exterior projects and structures from decks, docks, benches, walkways and bridges to boardwalks and more.

 

In Yakima, WA, the EMC typically varies from a low of 8.8% to a high of 16.5%. This wild fluctuation in humidity levels can potentially wreak havoc on exterior wood. Stable, highly durable wood species like Ipe, Garapa and Machiche typically perform very well in extreme humidity ranges like this. For decking applications, we would absolutely recommend installing your decking material with screws in areas where there are extreme fluctuations in seasonal humidity levels.

 

In extremely arid areas, such as Phoenix, AZ, where EMC varies from a low of 5.1% to a high of 9.8%, special care should be taken to allow wood species to acclimate properly. The general rule is to allow wood to acclimate as slowly as possible. A protective coating of a UV inhibiting sealer is imperative for best results in an arid climate with extreme variations such as those found in a desert-like environment. Long lasting durable wood species like kiln dried Ipe wood and Garapa hardwood are known to perform very well in this hostile environment, due to their high density, durability and great stability.

 

In general, kiln dried material works best in hot climates or hot weather that has direct exposure to the sun. Air dried Ipe can work well in most climates, depending on relative humidity, time of year, how much time it has to acclimate, etc. If you put a piece of wood that is approximately 16% moisture content (typical air dried Ipe) directly in a hot, dry southern or western exposure, in an area that has an EMC of, let’s say 11.1%, the wood will want to lose its moisture quickly. If wood loses its moisture too quickly, you may see signs of surface checking or possibly deeper checking or cracking.

 

This type of behavior (expansion and contraction) is typical of all wood species and even more so in less stable wood species. High density hardwoods are typically very stable, but even so, putting a UV inhibiting sealer on any wood species is an excellent idea, particularly during its initial acclimation period. We have found that Penofin Penetrating Sealer works well to help Ipe, Cumaru, Machiche, Garapa or Santa Maria during the initial acclimation period.