Unusually Low Humidity

[Jan Bianchi]

I have been working on a Hutch in my basement shop off and on for about 2 months now. When I started it, the swing in relative humidity was only 15 degrees varying between 55 and 70 degrees. Since that was also the case upstairs where the Hutch is going to live eventually, I wasn't particularly worried about it. But Seattle has been having very unseasonably cold weather and now the humidity in both the shop and upstairs is between 34 and 36. The shop is heated by the main furnace and maintains the same temperature as the upstairs. I had been assuming that I didn't need to worry about humidity because the piece is going to be subject to the same humidity swings upstairs as it is in the basement. But I didn't expect this much swing. I've only rough cut and stickered most of the pieces so far. All the lumber has been stored in the shop. Should I be worried about this? If so, is there something I should be doing about this?

[John Keeton]

More info would help. What wood are you using, how is it cut and how dry is the wood? What type construction, any wide solid panels and how are they used, etc.? Obviously, you are constructing, and I assume going to finish this piece during the winter at lower humidity. Depending on some of your answers to the prior questions, you may have to allow for some expansion. But, at this point in your question, I am not sure I could offer an answer.

[Joe Chritz]

Depends.

The wood won't expand/contract fully immediately so if the swing is short it isn't a problem at all. If you have wide panels the movement could be a lot if the swings are regular and seasonal. Meaning they stay very low for more than a few days.

Check out a google search for the shrinkulator. It allows you to plug in sizes and moisture contents to see how much shrinkage you should get from changes in humidity.

Joe

[Jan Bianchi]

Thanks. The Hutch I'm making happens to be republished in the latest version of Fine Woodworking on page 93, should you have that. I'm using kiln dried cherry. I didn't check it with a moisture meter but it all seems pretty dry except for the 8/8 pieces I cut for the lower legs which were admittedly a little wet--at least moisture appeared at the end of band saw cuts. Now that you mention it I guess I should have checked it all-- that's what I bought that expensive moisture meter for.:o

I have rough cut all the pieces for the lower part of the Hutch on a band saw, jointed an edge and roughly dimensioned each piece leaving about 1/16th of an inch and an inch of length on all the pieces. There are several panels, the largest is 17 x 22 inches between stiles and rails in the lower part of the Hutch and on the sides of both the top and bottom. The upper top will be one large board, edge glued, but I haven't bought that.

I doubt very much that this humidity will last in Seattle. It is not normally anywhere near this cold. So believe when this cold snap is over the humidity will return to 70 - 80 which is average humidity in Seattle in the winter.

[Jan Bianchi]

That shrinkulator is a handy dandy little tip. The humidity is really almost never this low here, but it looks like we're going to have it for a week or so. Might be a good time to just watch how much it does move. Thanks

[Jan Bianchi]

I just checked the legs--under 6%. Of course I don't know what it was a week ago when the humidity was 70%.

[Joe Chritz]

Interesting, I really need to get one of those meters. My shop humidity hovers around 40% RH since I heat with wood primarily. The house is usually around 60%. That is a pretty good jump but I have never had a problem in any of the projects I have built for the house. Just leave a bit extra wiggle room in the panels.

Remember the wood shrinks/expands differently across the grain. I forget which term is which but on a flat sawn board it will expand more in width than in thickness and almost nothing in length.

Joe