Digit":2x4mv3fw said:
Seems a bit pointless when you remember that some of the southern states can have very high humidity to contend with doesn't it? Roy.
Not really Roy. Consider the following modified and shortened text lifted from a manuscript I'm working on. It is not a simplistic response and omits discussion of essential factors. The full text goes into significantly more detail, but I think it would be too much for a forum response, and even this might be pushing it. Slainte.
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Target Moisture Content for Kiln Dried Wood
Commercial lumber yards and kiln operators in North America that dry lumber
* for furniture makers, flooring manufacturers, shopfitters, et cetera aim their product at their biggest customers. The target moisture content for this material is 7% MC. As with steamed walnut discussed in the previous section the main customers are the large furniture manufacturers.
*Lumber translates into British English as timber or wood.
Drying to a little below the MC range furniture experiences in service anticipates some moisture gain after the wood leaves the kiln during stacking into packs, banding, end painting, transport, storage and construction. There is also the issue of tension and compression forces on the likelihood of failure in glue lines discussed elsewhere.
Wood used as furniture in residences, offices, hotels, conference centres, etc, generally hovers somewhere between about 7.5% MC and 10.5% MC in service. This is the middle of the range and it’s likely that many items of furniture reach a lower moisture content and others stray up towards 13- 15% MC. It does depend on where the furniture is located and other factors such as long term storage when home owners move house.
From the Kiln to the User
In many cases the business that dries the wood also sells it to both wholesale and retail customers. Some timber yards operate kilns, air dry wood and buy in kiln and air dried timbers from all over the world. There are also large kilning operations with no retail customers that only sell to their large wholesale customers; and yards that don’t dry any wood at all but resell the stocks they purchase to trade and retail customers.
Whatever form the drying operation takes once the wood has reached its target moisture content the owner of the kiln or yard is keen to keep their product as close as possible to its dried moisture content. For instance, it isn’t good business practice for a large commercial kiln operation in North America to allow the timber (lumber) they’ve expensively and carefully dried to 7%MC rise to 10% or 12%MC between their kiln and their customer. A rise to 8% or 8.5% MC is probably acceptable to most of their clients, but much more than that and there would be complaints and returns.
Drying operations package their product to protect it and hold its moisture content close to its dried condition whatever conditions it experiences after leaving their premises. The dried wood might go to a hot and arid environment where RH numbers are low throughout the year, and the timber sent there would remain close to the kiln dried MC. In theory no special packaging and protection is required to hold the moisture content low. Wood kiln dried to 7% MC and transported and stored unprotected outside in Great Britain soon starts to take on moisture in response to the RH that prevails throughout the year in this country, ie, between approximately 60% RH during summer and 80% RH during winter. The kiln dryers don’t know where their product will end up so they parcel up all their wood much the same whatever the species and try to protect it as much as possible from the vagaries of weather, transport and storage, and final delivery to their customers.
There are strategies that help hold the moisture content of a drying operation’s seasoned wood near to the moisture content it came out of the kiln or off the air dried stack. Where and how the wood is stored, packed and transported is critical. For example, after kilning and a period allowed for the wood to cool, dead stacking protects all but the ends of the planks and those faces and edges exposed to the air. Storing or transporting dead stacked 1” (25 mm) thick boards in a tightly closed buildings or containers reduces moisture regain to about 0.2% or so per month: it would take approximately five months for the wood in a tightly packed container to gain 1% MC. Another strategy is tightly wrapping the wood in firmly sealed sturdy plastic wraps to exclude outside air and its variances in humidity. This is common practice with manufactured wooden floors shipped to floor installers, agents and retailers serving the retail trade.
Close stacking a pile of wood under a roofed shed open on all sides results in moisture regain in the first few weeks of between 1% and 2% MC, depending on the atmospheric relative humidity. For instance if atmospheric RH is high at 80 to 90% the “average moisture content of a pile can increase at the rate of about 2% per month during the first 45 days” (Simpson in Dietenberger, et al, 1999, Section 12-14). Simpson goes on to state that, “An absorption rate of about 1% per month can then be sustained throughout a humid season”. What this means then is that timber kiln dried to 9% MC in the UK and stored in an open shed in mid to late September could reach 13% MC by the end of January. This is at the very top of the moisture content range I’d be willing to build furniture for use in a habitable building. With timber this wet I prefer to bring it into a warm and dry workshop and sticker it up for at least a couple of weeks, preferably a month or six weeks to acclimatise and bring the moisture content back down to 10% MC or less.
In an earlier section I discussed my experience of buying and using wood in Houston, Texas where the wood I purchased at the local merchants was frequently 10% MC at the time of purchase, even though it was earlier kiln dried to 7%. Similar considerations are required wherever you live in the world. Nowadays when I buy timber locally from my British suppliers the sheds used to store and display the wood for selection are seldom insulated and heated buildings. The doors are usually open all day during business hours and closed at night. As in Houston I find wood imported from North America that was kilned to 7% MC is often, at the time of purchase, 9-10% MC. The large sheds used to stock the wood offer some protection from moisture regain, but they can’t prevent it.
References
Dietenberger, MA, et al. (1999)
Wood Handbook: Wood as an Engineering Material, United States Department of Agriculture, Forest Products Laboratory, USA.
