Indoor Humidity and Wood Moisture Content

      A discussion with data, about typical indoor humidity levels and the corresponding effect on wood moisture content. October 13, 2010

Humidity is an important part of wood product design. Does anyone know what the average indoor humidity is for homes in the US? What effect does AC play if the home has a lot of indoor/outdoor traffic? How often do homes reach 70% humidity and above?

Forum Responses
(Architectural Woodworking Forum)
From contributor F:
It depends a lot on your location. Michigan is severely different than Arizona, etc.

From the original questioner:
I guess I am looking for a resource or any studies from any areas, or personal experience.

From contributor F:
Try a Google search. You might be able to find a chart or something. I’m just north of Toronto. In the summer with the AC running, our house sits around 45 to 55% when it’s around 75% outside. As I type this it is 35%. It’s about 32 degrees Fahrenheit and snowing right now. In February, our driest time of the year, it is between 25 and 35%. So I guess our house swings from 25% to 55% with both of those being extremes.

From contributor S:
The basic text that belongs in every professional woodworker's library is Bruce Hoadley's Understanding Wood. This explains the science and facts behind humidity and wood and EMC. Hoadley will answer most of your questions. However, even with accurate baseline data, a responsible fabricator will build for the odd situation when humidity levels are much lower or higher than normal. When the in-laws come to visit and the A/C goes down and the house goes to 90% RH, should the frame and panel work come apart?

From contributor Y:
My guess is that there is no such information available. If you can find it, I wouldn't trust it. My home in Texas is different from my next door neighbor's home if they run the A/C differently than I do. As far as wood movement is concerned, then you have to consider how the wood is sealed, as well as how the wood was cut and dried at the mill.

From contributor U:
Maybe this will help?

Click here for higher quality, full size image

From Carl Hagstrom, Systems Administrator at WOODWEB
“Does anyone know what the average indoor humidity are for homes in the USA?”
Below is a chart that provides the mean humidity levels throughout the United States, for four periods during the year.

“What effect does AC play if the home has a lot of indoor/outdoor traffic?”
To the best of my knowledge, air conditioning dehumidifies along with reducing the temperature. Given that the chart below was based on data from the early 1900's, I think it's safe to assume that air conditioning was not a factor. So, it would seem that buildings with air conditioning would have humidity levels lower then shown on the chart.

“How often do homes reach 70% humidity and above?”
I would think humidity levels of 70% are extremely rare (unless there's an indoor pool/hot-tub or something similar). In my own personal (and limited) experience, when humidity levels approach 60%, other issues start to come in to play - mold, for instance.

Your question points indirectly to the issue of Equilibrium Moisture Content (EMC), which is used to predict the moisture content (MC) of wood when it acclimates to a given humidity level. The values below indicate the EMC of wood at various humidity levels, and when used with the chart, the change in MC for a given piece of wood can be accurately predicted, and this change in MC can also be used to accurately predict the dimensional change in the width of the piece of wood.


If you search the Knowledge Base using the term "EMC humidity" (without the quotes), you'll find quite a bit of information on this topic. The paragraph below also offers some information regarding humidity levels. This was taken from an article written by myself and Gene Wengert. If we're lucky, Gene will weigh in with his comments as well. I don't know of anybody else with as deep a knowledge of this topic then Gene.

Homes in most of the U.S. that lack humidity control typically experience interior levels of humidity from 25% RH to 65% RH. This range of humidity will cause a 6% change in the MC of the wood. This change in MC will cause a 12" wide maple board to change 1/4". Exterior humidity levels range from 60% RH to 70% RH (lower in the SW, and higher near large bodies of water), and cause a 2% change in the MC of the wood. When material is installed that was delivered at an unacceptable MC, or the humidity range in the structure exceeds typical values, the amount of wood movement increases and can cause problems even in well design trim details. It's worth noting that panel material (plywood, MDF, composite materials) move at about 1/10th the rate of solid wood.

Click here for higher quality, full size image

From the original questioner:
Thank you for all the responses. You are right, I am trying to find the parameters that I need to design to for fine custom millwork that will ship all over the US.

As for summer, AC plays havoc on the data because AC can extract moisture but is also lowers the ability for air to hold moisture. So air right before it leaves the duct would have around 100% humidity. I cannot get my head wrapped around how this plays into each other.

From contributor O:
In the Midwest we have summer days with 100% humidity. AC or dehumidifiers, if you have either will reduce that. In winter the humidity drops and if you do not have a humidifier the percentage can drop to next to nothing.

It plays absolute havoc with musical instruments, to the point that I refuse warranty coverage on mine unless the owner can keep it in the 40 to 70% area. A three foot wide 1/8th inch soundboard glued to the perimeter of a case can have sufficient change in width across the grain to buckle it in the summer and produce cracks in the winter.

From contributor S:
In the late 80's I attended a few tech seminars sponsored by Purdue University that featured Weinig and Weyerhauser engineers. One of the issues that came up was moisture content for folks shipping all over the US.

A major frame and panel door manufacturer was having problems with parts fitting. They made parts in two different locations, and assembled in a third. They also had problems with the finished product. After two days or wrangling with this problem, using good data from the manufacturer on plant RH and MC of the wood, the consensus of the engineers and attendees was that the only solution was to decentralize the manufacturing into regional cells. One could not make parts in New Mexico, ship and assemble in Iowa, then ship the final product to Minnesota without having problems (over 5% failure).

In the conversation, almost everyone told tales of problems with solid wood problems when fabricated in one area and used in another. The problems always related to the EMC differences, and were always with solid wood.

This is what you are seeing with the wide statistical variation. This is also, by the way, a major reason for any movement away from solid woods to man-made panels and such. By the way, the door manufacturer determined that regional production was not going to happen, and they fell from being one of the top producers of such doors to closing the doors. The reason was unreliable quality, with joints opening, stiles warping, etc.

From Gene Wengert, forum technical advisor:
It is fairly easy to calculate the interior RH and EMC if you know the outside RH and temperature. In fact, the US Forest Service has made such calculations (years ago) and published them. For an office without humidification, the values are fairly accurate for the wintertime, but with summer humidification, they are way off. Likewise, for a home that has humidity added from plants, bathroom showers, and cooking, plus even a furnace humidifier in the colder climates, the values are way off in the wintertime. If the home has central air, they also are off. Anyone that has been in Denver, CO will also recognize that the AWI map is not accurate. In short, there is not a lot of reliable data published that reflects the interior environment in our homes, especially with today's tight construction.

It is well to note that we (the general American public) will be unsatisfied when the RH drops much below 35 to 30% RH in the wintertime in our homes, due to the static electricity, dry nasal passages, etc. So, that sets the bottom limit for many homes. Also, the US Forest Service maps assume heating to 72 F, which today is not always done and which dries out the air. So, the dry values are just not that common. In the summertime, we tend to see mold growth (like in the bathroom) and dampness (like in the bedding) when the humidity gets much above 65% RH. Likewise, the cooling coils on an air conditioning act like a dehumidifier. So, summertime average humidity is seldom above 50% RH inside a home and in many locations are drier.

That having been said, and using the fact that shrinkage is more of a problem that swelling, we will target the lower MC range in millwork, cabinets, furniture and so on. You will find that 6.5% MC will be suitable for most of the USA. In the old days, some said 6 to 8 % MC and others said 6 to 9% MC. This still persists and it is this old value (when used in today's modern structures) that keeps my consulting business running - perhaps 75% of the manufacturing calls I get involve lumber and wood products that are at the wrong MC for their environment. Bottom line is that it is probably impossible moisture-wise to make one product that will perform perfectly in any location in the USA, Canada or the world.

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