my initial thought is this is a terrible idea but having had a conversation with a very knowledgeable woodworker yesterday - i might just do it. i wanted to get some more feedback before i go this route.

like most of you, we are shoehorned in our current space and moving seems less likely (availability and expense). as a way to increase the size of our existing space i am considering renting, or buying, a 20' shipping container to store in our fenced, concrete yard. i was advised yesterday by said fellow woodworker that this should work without soaking my kiln dried hardwood to 12%+ mc. he advised getting a sealed unit, requesting a dark color, and making sure i have plenty of sunshine to heat it up and cook the moisture out of the contrainer. furthermore, i have a $1200 dehumidifier i could stick in there and a small fan to help circulate air on chiller and cloudy days.

we're in virginia with pretty good humidity in the summer and pretty high temperatures (average 80s+ for four months).

keeping our mc between 6-9% is critical to our business. the only hole i see in the plan would be cloudy/rainy summer days. sunny days should heat up the container to dry up the moisture in the container, but cloudy days would rely solely on the dehumidifier to keep mc down.

would this work? anybody else doing something similar?

Some years ago, I had a customer that ordered a batch of divided light exterior doors. I asked when he needed them on site, and he gave me a date. We delivered on the date, but found the house far from ready. He smiled and said he had the problem taken care of - he would put them in his storage trailer parked on site. I cautioned him that it needed to be cool and dry, etc. and he acted irritated that I would question his cleverness.

Once the doors (Honduras Mahogany, top quality work) were installed, several had leaks where the 9/16" wide integral sticking had curled away from the glass almost 1/16", leaving a pocket in the seal that water could get into, and then find its way into the inside. His site manager told me the container regularly was well over 100 degrees during the 3 months the doors were stored, and it had caused problems with other things stored in it.

We fought over it for about a year, and I eventually settled by getting half my labor to remove the insulated units and reseal and replace them.

Never again.

I wouldn't! Looks like an uncontrolled kiln.

dave -

1/16" deflection on thin sticking doesn't seem like much. was that the only issue with your doors? what was the geographic location?

how much different would a container be from most shops? our shop isn't conditioned in the summer and is too big for us to run our dehumidifiers. i would guess we are in a similar situation to most shops. only thing i can think of is that it wouldn't get as hot, but i'm thinking heat would suck the moisture out of the air and be a benefit to the wood. i'm also talking about rough lumber that would be surfaced and dimensioned further, and not storing any finished goods in the container.

wouldn't it be better to have wood that is on the dryer end of the suitable mc scale than wood that is on the wetter side?

dr. gene?

The sticking was a heavy molding on passage doors - 9/16" x about 3/4", and in Honduras Mahogany - a wood that is renown for no movement in service. If these doors had been any other species, they never would have made it to installation. The fact that the sticking pulled away from the sealant and the glass means a lot of heat/stress was put into the wood.

We are in central Indiana and have plenty of heat and humidity. If I were forced to use one of these temporary storage things, I'd open it every day, have flow-thru ventilation, and try to paint the roof white.

As Larry said, it is an uncontrolled kiln. You pay your lumber vendor to dry the material (and store it...) properly, so why jeopardize it with too much heat? Look for precedent - does your lumber guy ever do this? Have you ever seen it in another shop?

This could be a recipe for a container full of twisted, ruined lumber. Excessive heat, lack of air circulation etc. seem to me a recipe for disaster.

BH Davis

Reefer cars store product at whatever atmosphere would be considered correct for said product . Power consumption to do so can (will) be restrictive . To remain KD proper storage is essential . How long do you store the wood ? Dr.Wengert probably knows offhand how many days/mths. to EMC . Do you check MC on receiving the wood ? Track it through ? Tightly stacked and wrapped inside a weather tight container sounds pretty good to me . What about auto exhaust to prevent over temps.?

can someone please riddle me the difference between an unconditioned (let's stick with summer, so no a/c), uninsulated shop space (or storage warehouse from my lumber supplier) and a shipping container? airflow and excessive heat are the only things that i can think of, although, our shop temperatures are typically higher than outside temps (thanks to a black roof) and air circulation definitely doesn't reach every corner of the shop. i can imagine the facilities at our lumber supplier are equally as unimpressive.

if temperature reaches 100*+ for a few hours a day, will this dry the wood too quickly and to "too dry" conditions for use?

johnny - our lumber typically reaches us as 9-11% and exits in the approximately same condition. according to commonly accepted mc laws, everything we've ever built should have come back to us as it falls apart when accumulating to indoor mc. however, we haven't received any calls, so from my experience, this is all a much to do about nothing (although, i really want to get the mc down and play by the rules).

good stuff about the reefer containers. i found something similar with regards to guys turning containers into kilns. i also found others storing wood in them with success (like i'm asking about). i'd plan on adding a couple of fans to move the air around and my dehumidifier has a humidistat on it to come on automatically when the moisture in the air reaches a certain level. i could open the doors/vents on sunny days to keep the temperature from peaking but my understand is that excessive heat 100*+ won't necessarily reduce humidity by itself. we have plenty of 70%+ humidity days with temps in the 90-100* range.

When storing wood, we basically have two concerns: 1) Insect damage and 2) change in moisture content (MC).

If the container is kept very clean and free of wood debris, the risk of insect damage is no greeter than storing wood in any location....very low risk. Note that we cannot use insecticides in storage as we do not know exactly when the insect will be active, so we would have to use an insecticide with a long life (long period of activity--technical term is efficacy). This means that when we sell the lumber, or use it ourselves, especially when planing and sanding, we have wood dust that has the insecticide and this is likely to be a human health issue. Further, in North America, if you sell lumber with an insecticide in it or on it, you are subject to a bunch of special regulations. Further, if the insect is inside the wood, it is unlikely that any surface treatment will reach inside the wood and kill the insect. So, damage inside the wood will continue. The oily 100% practical cure for insects (mainly powderpost beetles in hardwoods or old house borers in softwoods) is heat--heat the wood to 133 F or hotter (including core temperatures) for a few minutes.

Change in MC will typically result in size change (including warp) in storage, or because the wood does not match the moisture conditions in a shop or in a home or office, change in size during or after the product is manufactured. This change can mean warp, cracks, and poor gluing...and probably more. So, it is important to keep the storage humidity at the correct level. Because we commonly use lumber in a product used inside the home or office, we will want a MC of the lumber to be around 7% MC. This is the typical average MC in many situations. (Of course, Denver is drier and Miami is wetter, etc.). We find that air at a humidity of 36% RH will give us 7% MC in the lumber. So, for convenience, we call air at 36% RH to be 7% EMC (equilibrium moisture content). (Note that 6% EMC is 30% RH and 8% EMC is 42% RH, so precise RH control is not necessary.) Temperature does not affect the MC of wood, or affect wood's size or shape.)

So, how to control the RH in a storage container? Add heat. A little heat will cause the RH to top. In fact, 100% RH air will be 30% RH when the air is heated 25 F. Now, in most of North America, the outside air is 65% RH or 12% EMC--daily average. So, heating the air about 10F or so above the average outside temperature will give us 35% RH. An inexpensive gage from Radio Shack ($30) will let us know precisely. If we are in a reasonably sunny location, painting the outside of the container a dark color will give us enough heat on the inside. Otherwise, we need to hard a safe method of heating the inside. We need to heat the container 10 F or so hotter in summer and winter.

One last thought: In the wintertime, let the lumber warm up before gluing or machining it in the shop.

dr. gene -

thanks for joining the conversation. everything you said is in line with my understanding of mc, emc, and working wood. i would like to clarify a couple of points because some of these principles seem to easy to understand.

1. so long as i keep the temperature in the container 10-20 degrees above the outside air temperature this should lower the rh and wood mc. does the range exist for varying air rh? so higher humidity requires a larger difference in temperature (from storage to outside air) to achieve an acceptable, 6-8% wood mc. is this at all times? in a perfect world, the storage container would always be 10-20 degrees above air temperatures to ensure lower mc in the wood. is this correct?

2. would my dehumidifier (ebac $1200 unit, very nice) supplement the air temperature difference, in keeping the rh down, on muggy, rainy, summer days?

3. i understand what you are saying with regards to the temperature of the wood in the winter time and we would bring the wood into the shop a few days in advance to bring it up to temperature before working it. anticipating that my shop rh will be higher (mostly in the summer time) than the rh of the container - would the mc likely change significantly/detrimentally while being worked in the shop? for argument's sake, the container rh would be 35% and the shop would be 65%. our projects run the whole spectrum of custom woodworking - casework, doors, tables, etc. but we typically complete projects in 2-3 weeks.

thanks for your time and advice.

#1. For 7% MC lumber, keeping the container at 25 F above the morning low temperature should be perfect, as in the early morning, the RH is very close to 100% RH.

Usually, the weekly forecast or maybe a four day forecast can be used...average the high and low to get the average temperature forecast. Seldom would it be necessary to make daily changes in RH.

If you want to fine tune, then get a humidistat that when the RH is too high you will turn on the heat. Works perfectly indeed.

#2. You can certainly use a DH unit. In a tight container with lumber going in that is at the correct final MC, you could just use a hone-type DH unit that you buy at Home Depot...45 pints a day would be more than adequate.

The whole idea is that this is to store lumber and not to dry it.

#3. You might consider running the storage unit only 2% EMC below the shop and the customer's home's EMC conditions. So, in the summer, 9% EMC would be common for an average. But, if you make 9% MC items, will the customer call you when the heat comes on in the winter and the RH drops? Probably will. So, it is best to shoot for the lower MC. To be really safe, keep the shop goods that are exposed for more than a few days to high RH back into a controlled RH area. But, first, get the $30 RS sensor and measure your actual conditions in the shop.

Now if you do indeed make something at 7% MC in the summertime and then the customer has a more humid home, we both know what is going to happen...swelling. Will this be a defect?

I am called as a consultant and as an expert witness in numerous situation each year involving moisture change. Some are really dumb, like there was a flood that raised the RH abnormally high. Some are with some doors installed just before the drywall people came and then after they left, they put in heaters to encourage rapid drying of drywall compound and paint, etc. Others are just the natural behavior of wood in a normal environment. And others, lots of them, are having wood a slight bit too wet in a very dry winter condition.

Is this avoidable? I have one customer that had about 300 reject calls in the wintertime. After putting in an in-line MC meter to reject the wet pieces, he had only three. An RV manufacturer had rework items of 30% on a bad day; after controlling the MC in storage and the incoming MC, it dropped to 2%. There are several large cabinet makers that check every load of lumber coming to them for the MC; their quality has improved since they eliminated slightly wet pieces.

As you probably know already, the three most important aspect of making interior products from wood are MC, MC and MC.

Questions?

Hello Dr.Wengert , Been awhile since you've been to my yard . Smoke pile ??F-250 with 3 ash logs on the roll bar ? Anyway curious why still 10 * heat in winter .Manitoba does freeze good !

I'm not too sure whether you can get an air conditioner unit installed into the side of the container to help with the heat problem, and perhaps some dehumidifiers - silica gel or something to help absorb any moisture in there. There has to be a way to condition the internal environment so this idea is feasible...

In almost all of North America, the outside humidity, summer and winter, is 65% RH or 12% EMC. Therefore, to get a drier condition, we need to add heat...about 12 degrees F above the average outside temperature, or about 25 F above the morning's low temperature. A container painted black will get about the necessary heat, on the average, from solar.

Note that water in wood under 30 % MC does not freeze, as the water is not present in liquid in the wood, but is absorbed water.