Moisture Conditions and Furniture Shrinkage

A discussion of the way changing humid or dry conditions can affect furniture, and how to deal with the risk. April 24, 2014

Question (WOODWEB Member) :
Does anyone have a policy to deal with shrinkage in their furniture after a customer places it into service?

Forum Responses
(Furniture Making Forum)
From contributor M:
I state clearly that my warranty is void if the woodwork is damaged as a result of exposure to excess humidity or moisture. I define this as greater than 55%, or below 25%, which is still generous in my claimant. I also state that the more stable the environment is the better, and that changes in dimension are to be expected. In my care statement I warn against storage in garages, basements, attics etc. I also mention that fine furniture should not be placed over, or directly adjacent to a heat register, or other heat source.

From contributor Y:
Contributor M, do you mean greater than 55%RH? Where I am at the average RH is 60%. I have highs and lows but most kiln dried material settles out at 11% in the shop and bottoms out at 9% in service. I kiln to 7-8% but you just have to sit on it, or it moves while the parts are gaining moisture. Most of the work I do ends up in homes running AC. Average Rh inside is 40-45%. I need to build to those parameters to insure my work gives good service. Not too hard as the last place I lived the summers were 90% RH and the winters 25%. I must say though recently I had a piece in a show that was placed under a high wall mounted AC with 30%RH air bowing right on my work. I had to go back and trim a door that was tweaked from the change in climate. This gave me an opportunity to check the RH in the room which was not too bad, 40%. At some point fine furniture is kin to fine instruments made of wood and they like to hang out at a moderate constant RH. I was not happy that the sliding door took on a slight wind, but a little judicial trim and it was happy again. It's a live and learn. If your clients are going to use AC then you need to consider this. If they live in AZ and you live in FL then you need to realize that you have to build for it or not take the job. I agree with Mike that extremes beyond reason are not my responsibility.

From contributor M:
Yes I do mean greater than 55% Rh and that is specific to my own climate. I am aware this can be vastly different depending on the region in question. Nobody here has climate control in the summer so itís almost inevitable that there are going to be a couple of days each summer that really push or exceed the limits. Then our winters are bone dry. Because of this there are certain ways I will not build stuff. I have a couple of pieces in my own home that "test the rules" and the conditions are certainly not ideal. There are some issues related to seasonal movement that are quite evident. Nothing major, but flaws that could be perceived as defects by a person who cares.

I generally dehumidify my shop to about 45% when conditions get above 48% RH in the summer. This year I am going to get an air conditioner. I can't work when I am dripping sweat all over finished surfaces. If the humidity in my clientís home is greater than my shop by more than 5% I try to match it a bit closer. I have to constantly educate my customers on this. I get asked to build stuff that would self-destruct, and then they second guess you when you say you canít do it their way. It would be nice if all of our customers had archival quality climate control in their homes, but that is just not realistic. I am glad to cover warranty issues that are clearly my fault, for example problems from using wood with a high MC, or from improper construction. However I go to great lengths to prevent this. Beyond that I can't be responsible for what nature does to itself, provided I did everything I needed to do.

From contributor Y:
Not owning a moisture meter and not worrying about it, not paying attention to regional differences in RH is a recipe for disaster. Wood moves a lot, even if kiln dried. New England'ers (and craftsman in other area where the RH has major seasonal swings) get to see it every year from season to season. You build for it. Your reputation is on the line with every piece you make. It's your name going on it. If you own a big production facility and youíre selling $100 whole sale coffee tables then itís not a big deal, but if youíre making upper-end work with joinery then you need to sweat the small stuff. Woodworkers have been working with moisture movement for hundreds if not thousands of years, hence the development of floating panels and many hybridized forms of basic mortise and tenon work. Though they had to worry less about it back then because of the lack of interior heating and cooling like modern dwellings. Even in areas like Hawaii here, one side of the Island is wet and the other dry. The dry hot side uses AC which creates another big drop in the RH in the interior of homes. I work in a shop that is 60% most days but build for homes that are about 40%. Not hard to do, you just need to know where your material is when you mill it and mind your designs to accommodate.

From Contributor K:
I would like to make a few comments and observations.

As we all know, wood will shrink or expand as it will. All we woodworkers can do is to try to mitigate or buffer (or hide) this fact. It all starts with the design, compounded by the process.

My shop was totally un-conditioned, except for spot heating and cooling. As a test, I glued up an 18" wide piece of oak. Over a two year period, its minimum and maximum widths changed by 3/8". So, from a design perspective, if I were to insert an 18" wide board into an 18" wide mortise, there would be a substantial amount of stress points created very quickly, and the piece soon would be destroyed or damaged. As a remedy, one would create two (or more) narrower mortise and tenon joints to lessen those stresses. Another remedy would be to use a different wood. A piece of mesquite under the same conditions moved an almost indiscernible amount.

A properly applied film finish will slow down the wood movement. As relative humidity goes up or down, so too will the wood's moisture content, albeit at a much slower rate because of the additional vapor barrier of the finish, thus minimizing the actual change in the wood's dimensions. By the way, wax is not a finish.

A properly made raised panel door actually allows for wood movement, but the end result can eventually be unsatisfactory because the staining/ finishing process was incorrectly accomplished. Many shops assemble the door, then stain and finish. When that panel shrinks, the unstained edges show.

Molding and trim were invented to hide gaps. It's all about reducing or accommodating the inevitable stresses. Making a well-thought-out bookcase in Arizona and shipping it to New England should be less of a problem than, say, a parquet tabletop. With the bookcase, all of the movement is in a parallel direction and a nominally equal amount, so everything moves as a unit. With the table, the movement is in all different directions, thus offering no stress relief.

How a customer takes care of the furniture is a factor of course. If the owner is constantly moving the furniture piece into the garage for storage and then back into the house, that piece is going to suffer.

An informational packet, discussing the customer's responsibilities in caring for the furniture, goes a long way in preventing future issues.

If a piece of furniture shows wood movement problems in less than 10 years, it is either from poor design, poor workmanship, or customer/ elements abuse. 10-20 years, you may want to help the customer if you can. Over 20 years, hey, that's wood! (These numbers are highly subjective.)

If the problem is of your making, own up to it. The cost of materials and labor usually is far less than the cost of a tarnished reputation.

For woodworkers and many "civilians", solid wood furniture imparts a sense of beauty that few other items match. Making solid wood furniture imparts a sense of satisfaction that only a few other activities match.

From Gene Wengert, forum technical advisor:
Wood only swells if its moisture increases. Wood only shrinks if its moisture decreases. So, the question is "What is the moisture at the time of manufacturing and what is the moisture going to be when the finished piece is put into use?" In most of the US and Canada, the interior humidity in the wintertime is around 30% RH which is 6% EMC, or which means the wood item will be trying to achieve 6% MC in the wintertime. (If we go much drier than 30% RH, we get static electricity, paper does not feed well in copy machines and printers, lips cracks, etc. Dry homes will use a humidifier. Do not believe any fancy looking devices that are supposed to measure RH; they look good but are not accurate at all.)

In the summertime, the average inside a home or office is 50% RH or 9% EMC. (If a home or office gets much more humid, we have paper feed problems, mold in bathrooms, dampness here and there, etc. Air conditioners generally remove moisture and the low RH helps us feel cooler even when temperatures are a bit high.)

Some manufacturers are told two humidify their shops in the wintertime to get 40% RH or higher. This does help prevent lumber problems during manufacturing if the lumber is not dried to a low enough MC, but it also postpones shrinkage problems until the homeowner gets the item. Run the shop at the same RH (or a bit drier) that the homeowner has.

A homeowner has the right to expect that in the normal dryness of his/her home or office in the wintertime that the furniture will not shrink excessively after it was made. One thing that will help you prove that your items are properly made is if you measure their MC with a moisture meter (made in the USA) that costs at least $250, and of course your readings were all around 7% MC and not wetter. On the other hand, if you ship to Denver, the RH can be very dry at any time. If you ship to the Florida coast, the reverse.

As indicated, garages can be 12% EMC; basements 15% EMC, and unheated home under construction, 16% or so. Servicing these markets seems to require that the furniture or cabinet is put in a plastic bag, which (if no holes) will hold the moisture constant for years. Note that outside conditions in almost the entire USA average 65% RH or 12% EMC, summer and winter. Our bodies do not judge humidity very well. When it is 45% RH and 95 F, we think it is hot and humid, but the wood only sees the 8% EMC. Temperature is not an issue. Coastal areas are more humid and higher elevations in the West are drier.

I have several clients that have an old 18-wheeler trailer that is without holes. They store any incoming wood in it and keep the RH under control by heating the inside until they get 30-35% RH (about 25 F above the morning's low temperature) all year round. One client went from over 100 complaints in a year to just three. Another reduces warping issues in their product from 23% to 6%. For about $30, you can get a reasonably good and accurate (as accurate as we need it) digital RH sensor. With this (or several) you will know what is actually happening in storage, manufacturing, shipping, customer's home, etc.

From contributor J:
We stated clearly on our policy that the warranty is void if the furniture is utilized against the guidelines. If the damage is rectifiable then after proper consideration necessary steps will be taken as per the requirements of the customer.

From Gene Wengert, forum technical advisor:
To contributor J: To make such a warranty effective, you must measure the MC of every piece of furniture and document (make a record of) those readings, you must ship the furniture in a manner that will keep the MC constant, and you must show (without question) that the purchaser has conditions that are not proper. It is indeed difficult and expensive to do this. In one court case involving such an issue, when the purchaser questioned the person who was supposedly measuring the MC of every piece, that person was unable to indicate what type of battery was used in the moisture meter, where the spare batteries were kept, and where spare needles were kept. Obviously, he was not doing what was claimed.

From Gene Wengert, forum technical advisor:
Paint, varnish, oils and other finishes slow down the moisture change (some do not have much slowing effect, and others are quite effective). Wax is perhaps the most effective coating. None are effective in stopping moisture change.

From contributor Y:
Gene, do you think wax is as effective as lacquer in the same film thickness?

From Gene Wengert, forum technical advisor:
My guess is that wax is more effective. I do not have numbers to show that, but I do know that a wax coated block of wood can be submerged for a month without any moisture gain.