wood movement

[rich murray]

how much movement can I expect from a 3/4" glued-up poplar shelf 14" D and 33" L if it is painted with an undercoat and a latex enamel topcoat?

[John Thompson]

Difficult question to answer as many factors would contribute concerning your local.. climate.. general humidity in your home.. etc.. etc..

With the seal you mentioned and assuming you don't live outside, I would suggest a 1/32" micro-meter to measure after you install it. In other words... Very little!

Sarge...

[Bill Wyko]

If the wood was kiln dryed it should be pretty stable around 6% humidity. Shouldn't move much at all. IMO

[rich murray]

Good point, John. Oklahoma has an average humidity of 66.7%. The shelf will live indoors where the humidity probably ranges somewhere between 40% and 60%.

[Howard Acheson]

Go to www.woodbin.com and then click on on-line calculators. Then click on the "Strinkulator". Input the species, and beginning width of the shelf. Then input the range of relative humidity where the item will reside. The output will be the expansion/contraction of the wood.

What finish you apply has no bearing on the total wood movement. All finishes allow water vapor to go through. It just slows it down a little bit when you apply a finish. Likewise, kiln drying has nothing to do with expansion/contraction of the wood. It's purely the change in relative humidity in the environment which the wood finds itself.

Now, let me suggest you not use a latex paint. Latex paint is soft and anything you put on the shelf will tend to stick to the shelf, and/or leave indentations. It's called "blocking". Use a waterborne acrylic paint which is much harder and does not have the blocking problem.

[Craig Che]

Is there a way to compute the average humidity in a house? I am not sure how to figure out what the norm is for us from summer to winter and how to apply it to the inside changes.

[Gary Keedwell]

Is there a way to compute the average humidity in a house? I am not sure how to figure out what the norm is for us from summer to winter and how to apply it to the inside changes.
I know what you mean. last year I decided to buy one of those clocks that tell the humidity, temperature and time. I hung it just a few feet from my wood stash.

Gary

[rich murray]

Thanks for the good advice and site, Howard. I have never painted bookshelves before, but now that I think of it, I have seen what you're talking about. Good advice, and I learned a new word! Blocking.

[Howard Acheson]

>> Is there a way to compute the average humidity in a house?

I believe that Radio Shack sells a combination temperature/humidity gauge.