Originally posted by: Gene Wengert, wooddoc@uwalumni.com
A: Let’s consider four aspects of cold weather on wood manufacturing: machining, gluing, finishing and movement.
1. Cold wood machines the same, from a practical point of view, as does warmer wood…if the humidity and moisture content is the same.
2. Cold wood glues quite differently than warm wood. The real issue is that when the adhesive contacts the cold wood, even if the adhesive is warm, it will quickly cool and become much thicker than normal, so it does not flow well to spread out and fill the microscopic nooks and crannies. Additionally, the drying and/or chemical reactions that are necessary to cure the adhesive slow down or may not even occur. Obviously, the surfaces to be glued and the adhesive should be around 70 degrees. (Note that in the summertime, we might want to alter the glue formulation a bit to avoid too much thinning from the warm temperatures.)
3. Temperature greatly affects the finishing process, including poor curing, fish eyes, and a bunch of other issues. The wood, the air and the finish all need to be at the correct temperature. Finishing is also affected by low humidities in the plant that often accompany cold weather.
4. Cold weather almost always means low humidities in heated areas, including your lumber storage area, your manufacturing plant and customer’s home or office. This in turn means that the wood will be drying out a bit and trying to shrink. If shrinkage is too much, then we can see cracking in the finish or wood and even sometimes a bit of warp. Inserted door panels can shrink and exposed unfinished areas. Door latches may not fit perfectly. Drawers may stick.
The reason all this happens is that when air is heated, the humidity drops. For example, outside air at 100 percent RH (28 percent MC in wood or 28 percent EMC) and 0 F when heated to 20 F will drop down to 40 percent RH (8 percent MC in wood or 8 percent EMC air condition). So, obviously, by the time we heat the air to 65 degrees, the RH is really low.
What to do
Most customers have humidifiers in their homes and also have moisture additions from bathroom showers, cooking and plants. Offices may not have such sources of moisture. Many wood manufacturing facilities have humidification equipment to avoid the extremely low RHs that we see in cold weather. Because a customer’s home is likely at a low of 30 percent RH (maybe a bit lower on the coldest days, but these brief excursions are not an issue), we should consider keeping our manufacturing plant at 30 to 35 percent RH when the temperatures are frigid. Lower RHs are likely to cause manufacturing issues; higher RHs will mean more drying and shrinkage when the wood reaches the customer’s home.
How about when finished goods are shipped and warehoused? Shipping is seldom a problem, as there is no heat, so the RH is higher—50 to 65 percent MC (9 to 12 percent EMC) perhaps average outside. Similarly for a cold warehouse. The finish on the product will slow the moisture movement, plus in cold weather, the moisture moves very slowly. The temperature itself is not an issue, as wood does not shrink or swell due to temperature changes alone. However, for a heated warehouse, we have the same potentially low RH issues that were already mentioned.
One final comment: Wood can absorb a small moisture change without much difficulty when the change occurs slowly, such as we see from summer to winter to summer. The problem with defect development in finished goods will almost always occur when there is a rapid change…moderate humidity to a low humidity condition, or when higher MC wood is exposed to a low RH or EMC condition. Adjusting manufacturing humidities and wood MC levels for cold wintertime weather is prudent to avoid warp, open glue joints, cracking and so on.
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.