Humidity and Warping in the Shop

The key is to get the wood at the right moisture content and keep it there. October 26, 2012

Has anyone encountered problems in the winter time regarding too dry of an environment in the woodshop due to heating? Has anyone ever used a humidifier for this? If so, will it work well in a 500 square foot woodshop?

Forum Responses
(Business and Management Forum)
From contributor K:
We had a lot of trouble in the school shop last winter with humidity. We had some boards crack, but the worst was with warping. It was pretty much certain that if you left a stack of panels over the weekend without clamping them to each other the top one was going to be cupped when returning. We rip boards down and alternate end grain to help prevent this, but the fact is they were drying out on the tops and causing them to cup.

We helped the problem by buying a humidifier that put about eight gallons of water in to the air per day which was able to bring our humidity back up into the mid 40's. To fix the warped boards we put down a bunch of wet paper towels and put a couple stickers on them for spacers and then put the board with the cup side down on the spacers. It worked pretty well. The humidifier was an Essick Air ED11 Series. It is pretty small but if you stay on top of it, it does pretty well in what was a 3200 square foot shop.

From Gene Wengert, forum technical advisor:
Air does get drier (the relative humidity drops) when the air is heated. This is true for your shop and also for a home or office. In the wintertime, most homes in the USA and Canada average around 30% RH or 6% EMC (except in coastal areas of high humidity). In a home, moisture comes from plants, cooking, bathroom showers, humidifiers, etc. Offices without humidifiers can get quite a bit drier.

The key to preventing problems is to use wood that is at the same MC when you use it as it will attain in use. In general that would be 6.5% MC. When the wood is wetter than that it will dry and shrink, crack, or warp. In fact shrinkage and warping size changes that occur in five minutes, five hours, five weeks, etc. are always caused by MC change, not temperature or some other factor.

Note that if we have wet wood (over 7% MC), we can humidify the shop to 45% RH or higher (8.5% EMC) and solve the problem in manufacturing, but when this piece gets to the home or office, it will dry further to 6% MC in the home or office and that can be a big problem. So, humidifying the shop much above 30% RH is merely postponing the problem.

The real cure is to get wood at the correct MC (no wetter than 7.0% MC in the wintertime) and store it in a dry location. You need to check the MC with a standard moisture meter (no less than $200). Then humidify the shop to 30% RH at all times. Check humidity with a $30 digital instrument from Radio Shack.

The size of the humidifier depends on the size of the shop and the size of the dust system. Also - the hotter the shop, the more moisture that will be needed. In some cases where the entire shop cannot be humidity controlled, a small plastic-walled room can be used to keep or store wood and components that are in process. Control the room to 30-35% RH and move work in and out of the room as needed. This solves the dust collector issue too, so the humidifier can be quite small.