Ideal Humidity in a Wood Shop

Indoor humidity levels correspond with wood moisture content. Here's advice on humidity control. October 26, 2012

What is the ideal level of humidity for a woodshop? Just looking for some food for thought here.

Forum Responses
(Dust Collection and Safety Forum)
From contributor C:
35% Relative Humidity ~=7% wood moisture content. Perfect, or at least really close, for indoor applications in most areas. In the winter I achieve this with heat. In the summer I get it with dehumidification.

From the original questioner:
Well that's definitely an eye opener. If my gauge is right, it's 75%. And I've had the dehumidifier running for half the day. Any suggestions to accelerate the process? It is very humid here.

From contributor C:
You'll be in the 12% + MC range then. Good for outdoor applications. For indoor use, you'll want to be lower. If you buy KD wood and store it in humid conditions, it's going to swell up to ~12%. Maybe you can control a part of your shop where the wood can equalize for a week or two before you dimension, glue, join or hold projects between production steps. I believe that Gene Wengert is the source of the following chart.

Moisture content of wood may be controlled by controlling the Relative Humidity where it is dried. Here is the relationship between RH and wood MC...

58-64 RH - 11% MC
52-58 RH - 10%MC
46-52 RH - 9% MC
39 -46 RH - 8% MC
32 -39 RH - 7% MC
25 -32 RH - 6% MC
19-25 RH - 5% MC
Adding heat drives the RH down and speeds drying.

From contributor J:
Having a hygrometer is a good first step - at least you can easily check the RH whenever you like. And now you also have a sense of where it should be. 35% is probably ideal for interior woodwork, but even 50% would be a huge improvement. 75% is way too high.

You can lower the RH by simply increasing the temperature of the shop, but that's not feasible during summer in most places. You don't want to work inside a dry kiln.

You can also lower it using some combination of air conditioning and/or dehumidifiers, so long as you can isolate the shop air from the outdoors. The AC/dehumidifiers have to be able to keep up with the amount of vapor coming in; if your building is leaky, or if it's in a low-lying area and lots of vapor comes up through the cement floor, or if you work with the bay doors open all day, then dehumidification is a losing battle.

From contributor D:
There is a regional component to this discussion. The 35% RH = 7% EMC is fine for a large part of the US. The Southwest will be lower, since the environment is lower humidity overall, despite HVAC efforts. The Northwest will be higher than 35%, with the higher levels of humidity prevalent. Things get more complicated if you produce in Tucson, let's say, and ship to Seattle.

Bruce Hoadley's book Understanding Wood has an excellent section on this. I have been in several shops that supply humidity in frigid Midwest winters, and keep windows closed and dehumidify in the summer.

From the original questioner:
Thanks for the replies. Good stuff. Could anyone tell me what a reasonable humidity is for a Minnesota shop?

From contributor E:
I am in Eastern South Dakota, so I suppose the humidity would be similar to MN. We have an auto moisturizer that is used in the winter and then we use AC and try to keep windows closed in the summer. It varies, but we try to run 35% in the winter, and the AC will keep it at 40%-50% in the summer. I will agree with the other guys - the right humidity is important.

From contributor W:
This question was perfectly timed for me as well. I work in the Midwest (just outside of Chicago) and during the summer the outside humidity is always above 75% - in fact a lot of this summer it's been over 85%. I am looking at shop space in the range of 4000 sf and have no idea as to how I am supposed to control the air quality for something this large and with 16' tall ceilings. Any ideas besides air conditioning that large a space?

From contributor C:
Consider controlling a part of your shop where the wood can equalize for a week or two before you dimension, glue, join or hold projects between production steps. A drywaller's Zip wall might provide enough of a barrier to allow you to use simple equipment to get closer to moderate humidity levels.

From contributor V:
I have found that you can make yourself crazy trying to regulate humidity too much. Storing wood in a sealed and dried place and then pulling it out into another, more humid area and trying to work it can cause even more problems, as the wood will immediately begin to pick up moisture and move.

Wood moves. Always. Accept that and build to deal with it. Remember, most furniture is going to be placed in environments where there may be little to no humidity control in the summer, and it will keep moving long after it has left your shop.

I agree that it's nice to start with material that has been KD down to 6-8% in the first place, but for me what is more important is that the wood receives no humidity shocks in the middle of a project.