Shop Temperature and Wood Machining

Is it okay not to heat the wood shop at night? Could be, but humidity changes are a potential problem. August 26, 2008

One always assumes that machining solid lumbe in your shop is done at room temperature, whether it’s done at 65 degrees or 70. My question deals with the issue of turning the heat on in a shop that’s allowed to cool overnight and then brought up to temperature in the morning. Will the wood warp, twist, cup etc., normally or am I just delaying the inevitable until I start to glue up?

I built a new shop but haven't had time to winterize it. There’s no insulation, and the cost of heating it skyrocketed the first month so I am tempted to let the shop cool at night. I will be keeping the heat on overnight when I do glue-ups, but am just trying to save a little money until next spring.

Forum Responses
(Solid Wood Machining Forum)
From contributor P:
Well, in twenty five years, I have never had an issue with temperature. I worked ten years in Arizona and fifteen in Seattle. In both places, the temp varies quite a bit.

Humidity is your enemy, but it takes time for wood to react to these changes. I currently work in temps that vary between 50 and 65 for most of the year. I like the cool weather for most work. Glue-ups are fine from 55 on up to say 90. Above 90, glue dries too quickly. Below 50, chalking can occur.

Warping would be a drying or stress relief issue. A temp swing of 15 or 20 degrees from day to night would probably not cause a single problem with materials. If you are finishing, 65-75 is ideal. But again, it is the humidity and warm air holds more moisture than the cold. I have had my shop down in the 20's and never seen a problem. Just think about warehouses and material delivery in winter.

From contributor J:
I agree with Contributor P on the humidity. In our shop, once the furnace starts kicking on we would roll the finished products out into the warehouse at night. Now we have a humidifier and everything is stable. Hardwood will warp overnight but I'm not sure where you are or your climate.

From Gene Wengert, forum technical advisor:
I agree with the earlier postings. Temperature is not the issue, but when you heat air, it does dry out. So, a long weekend at cool temperatures likely means about 10% EMC, so the wood will be trying to get to 10% MC. With the heat on and up to 70 or so, the humidity will drop to 30% RH or 6% EMC, which means that wood will try to achieve 6% MC. Although MC changes take time, it is possible that with thin, short or narrow pieces, you could get enough MC change to cause warp. So, it is advisable to keep some heat on at all times and also not get too hot when the shop is occupied (meaning low RHs).

Radio Shack sells an inexpensive RH meter ($25 to $30). Use an electric humidifier or dehumidifier to achieve 38% RH at all times, if possible. This includes lumber storage areas too.

You can also consider using a humidistat to control the heat in a storage area. Wen the RH goes too high, the heat will cut in until the RH drops to the proper value. I have seen this technique used in insulated 18-wheeler trailers where lumber is stored. It works great, even in the summer. Paint the roof of the van black to develop some solar heating.

From the original questioner:
I live in Canada and the temps right now are heading to the freezing level. I am building kitchen cabinet doors out of hard maple so I am being extremely cautious right now. I agree about the humidity levels and am trying to keep them under 10 % levels.

The wood has been in my shop now for a month and the gauge reads 8.9%. What I plan to do is machine to within 1/8th inch of all final dimension and then let sit for a week and the final finish to size and then glue up. I have had success at this before but never on this great of a scale. I figure with so many doors (39) that so problems will occur.

From Gene Wengert, forum technical advisor:
In most homes in the heating zones, the humidity will be about 30% RH (6% EMC) during the wintertime. That is why we like to look for 6.5% to 7.0% MC in the wood. Your value of 8.9% is a bit high indeed. It is better to dry the lumber before use than to dry the doors after they are made.

From contributor B:
Just to add to the above posts, your concern is not temperature, but humidity. However, you also will need to be careful with the temperature of the wood for both gluing and finishing. Many glues, particularly water based will not work properly on very cold wood. Titebond for example, will chalk over and have no gripping power what so ever.

From contributor G:
Why not just insulate your shop? If you don't have time to do it, hire it out. You are already paying for it by high heat bills. Working in a cold shop is just asking for trouble in my opinion.