Q: Is there a rule of thumb for how much of a gap we should have between the boards as we make packs for the kiln? We've been operating under the assumption that a total of 6.5 inches of gap on a 72 inch wide layer was desirable.
 
 
 

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A: For narrow piles (under 8-feet in width), there's a very small, often immeasurable, benefit in leaving a space between the individual pieces of lumber. So, it's not done. In wider loads, there can be an improved drying rate in the interior of the pile during initial air drying.
 
Of course, if you air dry for a long time, then this benefit isn't seen at the end of the air drying. In the kiln with fans that blow the air through the load, the spaces only reduce the total kiln capacity. (In your case, this reduction is about 10 percent). The spacings don't help drying at all and in some cases may even result in poor air flow distribution in modern kilns. So, again, it's not done today.
 
You may wonder why this spacing between individual pieces was ever done. The answer is that 50 years ago before kilns had internal fans of the size we have today and had heating coils underneath the lumber, a lot of the air flow in the kiln was the result of the hot air rising through the packs. The spacing provided little chimneys for this to happen. Also, with large packs of lumber that were more than 8-feet wide, these chimneys encouraged internal circulation in air drying.

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