Door Gap and Outdoor Humidity

A woodworker gets expert advice on predicting the moisture-related dimensional changes in a wood door, for purposes of calibrating the door gap to allow for the movement. October 11, 2007

I'm installing a double entry door in Wisconsin. Bone dry in the winter and humid as hell in the summer. The two doors are 72" wide, quartersawn fir stave core, and the stiles are 5.5" wide. What is a safe gap between the two that won't bind up come August?

Forum Responses
(Architectural Woodworking Forum)
From contributor P:
Some assumptions first:
1) initial EMC is 6%
2) final/highest *theoretical* EMC is 16% (80+ degrees and 80+% R humidity for extended periods)

According to my math, you need better than 1/4" between them... However, the EMC should never get to 15% because you will be sealing the door with multiple coats of something (right?) - varnish, paint, whatever - so a high-end EMC of 12-13 is more likely, which will make your 1/4" gap acceptable. It will still seem really big in the dead of winter, though. Make sure the weather seals are up to the task of dealing with that range of movement.

From the original questioner:
Thanks. I guess I should explain the circumstances here. The stationary door has been blowing open in a good wind. The gap in between the two is 1/4", as I believe it should be. The problem is the exposure of the latch bolt when fully extended is only 3/8". That is not long enough. I need to call Emtek and find out if I have the wrong latch bolt or something. Oh, and yes, the MC is good and they are finished with four coats of Sikkens.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
The highest RH in Wisconsin on the average for a day or two is around 65% RH, which is 12% EMC. (Check with the weather people at the closest weather station and you will see that I am correct. When it is hot outside, our bodies tell us it is humid also, but that is incorrect. When at 95 F, often it is 50% RH.) In the winter we can achieve 25% RH or 5% EMC. You can use the shrinkage calculator here at WOODWEB to determine the size change. A finish slows the change but does not prevent it.

Shrinkage and swelling calculator

From contributor P:
Gene, are you sure that Wisconsin will never see a short period of 80 degrees/80% RH? If, like you said, it's just our bodies complaining, I'd sure not want to live in the tropics, where 80/80 must happen regularly.

I understand about the Emtek bolt being a tad short - I've felt the same thing, although have not yet had one actually fail to catch in the dead of winter due to shrinkage.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Wisconsin might see 80/80 (= 16% EMC) for a brief period, but the wood, especially with a finish, takes time to respond, so it will not cause the wood to change enough to consider the short term, high RH. Again, check with the weather bureau and they can direct you to loads of data.

From the original questioner:
I don't want to argue with you fellas about the humidity here in Wisconsin, because that's really not my issue. As I said, I feel the 1/4" gap in the winter is where it should be. I did talk to a tech at Emtek yesterday, and he confirmed the throw of the passage bolt at 3/8" is what it is. I told him that was not long enough for a solid wood door with a 1/4" gap, and the solution he offered was to not use their lock! I will definitely take his advice in the future, but I don't think they will take back this set which has been mounted for a while. I may try to retrofit somebody else's bolt in there with a longer throw.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
EMC values for many cities in the USA and throughout the world are given in the link below.

Monthly Average Outdoor EMC Values

From contributor W:
Thanks, Doc, that is excellent reference information.