Replies

1. 
   YesMaam27577 | Dec 14, 2006 11:13pm | #1

   I don't know the conversion factors, but......

   Relative humidity in the atmosphere is not equal to moisture content in the wood.

    

    

   Unless you're the lead dog, the view just never changes.

2. 
   SgianDubh | Dec 15, 2006 12:26am | #2

   Sustained RH of 80% will eventually cause wood to reach about 16.5% MC.

   Sustained RH of 40% will eventually cause wood to reach about 7% MC.

   Temperature has some effect on the final MC, as does barometric pressure. Both these factors can cause the wood to vary either side of the numbers I gave you by a percentage point or so, but the primary cause of MC change in wood is atmospheric RH. You ought to allow for a maximum change in wood MC of  about10%.

   Due to the effect of hysteresis a change in atmospheric RH doesn't affect the wood immediately. In fact, wood will absorb moisture when RH is raised faster than it will lose moisture when RH lowers. A barrier between the wood and the atmosphere slows down the rate at which water vapour in the air affects wood. Similarly, a barrier slows down the rate at which moisture leaves wood and migrates to the surrounding drier air. An unbroken paint job is a barrier, not water-proof, but water resistant. It will slow down the movement of water vapour, but can't stop it entirely.

   I don't know what species you are working with, but a useful rule of thumb to calculate tangential wood expansion and contraction goes like this:-

   Changing the MC of wood by between 3% and 4% causes a 1% change in width. In your case therefore, using 3% as your multiplier the 10% MC range identified at the end of the paragraph three above might result in a 3.33% change in wood width, e.g., a plank ten inches wide at 16.5% MC (80% RH) might end up ~9.67" wide at 7% MC (40% RH.)

   If I knew which wood species you were working with and if it's radially sawn or crown cut (tangential) I'd be able to give you a more accurate idea of the likely range of movement. Slainte. 

   Richard Jones Furniture

   
   

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   Edited 12/14/2006 4:35 pm by SgianDubh

1. 
   Truejoint | Dec 15, 2006 06:21pm | #3

   The species I'm using is eastern red cedar (aromatic cedar is what my lumber yard calls it).  I cut a few large trees down about 4 years ago and the wood has been rough cut and has been stickered in my garage (400 board feet) for two years now (the cars were evicted years ago but my wife is ok with it as long as I put it to some use so I'm under the gun to get working).  I cut off the sap wood while milling it, but it was for the most part all plain sawn.  My plan is to use the wood to reconstruct the soffits, facia, and window treatments on my house, but I'm not sure how much movement to design for.  What if a fog envelops the house for an evening?  That would be 100% humidity.

   I guess what I'm asking is does it take a day or a week for wood to absorb moisture and come into equilibrium with its environment?  If it takes even a few days then I wouldn't have to worry about planning for the extreme values and can go with day/night average humidity values which would be much more reasonable.

   I saw that someone wrote into FW magazine and included a link to a movement calculator.  I haven't tried it yet, but it still doesn't answer the question about what length of time you should average humidity values over when determining MC change.

1. 
   DHAM | Dec 15, 2006 11:06pm | #4

   It would take a few weeks for wood to fully equilibrate to a level of relative humidity.  That 10% to 20% spread previously mentioned sounds right, but the wood will not see that range in M% unless the extremes of relative humidity persist unchanged for several weeks, and that just does not happen.  Otherwise, wood would popping off houses everywhere.  I believe that you could temper the extremes and use the wood movement calculator to determine the change between a low of 12% and a high of 18%.  Given that it will be painted and the paint will retard moisture absorption/loss, this is probably more like what the wood is likely to experience in the real world.  That level of wood movement should not be too difficult to engineer into your design.  If you google wood movement calculator, I believe that you will find several.  There is one at WoodWeb that is very easy to use online.

2. 
   SgianDubh | Dec 16, 2006 03:42pm | #6

   As DHam suggested, I too think it's unlikely that your timber would reach the extremes of movement that are possible. Timber doesn't respond immediately to changes in RH, and when it does respond it takes time for it to reach the new EMC. With paint as a barrier and the time factor your Eastern red cedar (Juniperus virginiana) would probably take a few weeks to reach an EMC that suited an atmospheric RH of 80%. The 80% RH would have to be sustained throughout that period.

   This doesn't happen of course. RH changes daily, even hourly. So as DHam suggested you will probably find that your timber will range between about 10-12% MC when the humidity is typically at its lowest and perhaps 18% MC when the humidity is highest. From what you said in your first post I think you said you normally get lower average RH readings in the winter than you do in the summer.

   Eastern red cedar is a quite stable material. The shrinkage factors from 20%MC to 0% MC for tangentially cut Eastern red cedar is 3.4% and for quarter sawn, it's 2.3%. You're  looking at a maximum change of MC of about 6% or 8% which is about a 0.3 or 0.4 of the total movement possible, or about 1.02% to 1.4%. Therefore if you have a 10" wide piece of tangentially sawn wood at 18% MC, it would eventually shrink by about 0.14 inches to roughly 9.86" wide when it reaches 10% MC.

   You won't need to plan for very rapid or instantaneous change in the dimensions of the wood. It just doesn't happen that fast. Slainte.Richard Jones Furniture

3. 
   LeeGrindinger | Dec 16, 2006 07:33am | #5

   Perhaps an EMC chart would be helpful,

   http://furniturecarver.com/emc.html

   Lee