Wood EMC in your heated shop

Hey, my boards are cupping! Is your shop to DRY?


As we are all in the winter heating season now, we forget what that does to wood moved into our very dry shops.

If you don't know what the humidity is in your shop, you should get a gauge and check. I bet like most shops I visit, there is no added moisture and its very dry, in the 20-25% RH range.

You bring boards into your shop that are 7-9% MC, and they start cupping right away. Why, because if the RH in your shop is 22% at say 65 deg, the EMC which is (equilibrium moisture content) is around 4.9 %. So now that wood is going to start dropping 3-4 % very fast.

Boards can cup if one side is exposed to the very dry shop air, and the other side isn't, after laying them on a bench, or stacking them up against the wall. One side of the board is drying and shrinking and the other side isn't drying at all, or as fast. Wood also cups toward the outside of the log, so against the curve of the growth rings, which can make it worse. I tested this and it starts in hours. You can move the cupped boards so air can circulate on both sides, which can sometimes flatten the board back the way it was.

Remember that wood is always gaining or losing moisture. It never stops.

So you make that big table but don't have the base done yet? Clamp the top to a few thick straight boards to help hold the top flat. Don't lay that fresh glued up table top on a bench, or lean it up against anything exposing only one side, or you can increase the cupping of the top.

Big slabs are a real problem. You should seal the ends with what every your going to use to finish seal the top. Do this right away, or the ends can start checking in a few days. Wide slabs see a lot more stress that narrow slabs and can check real fast.

So what should you do? I have seen very few woodworkers add moisture to keep the RH up in their shop so adding moisture is a must to keep wood stable, until its fully sealed and the finish is applied to both sides.

Get a humidifier in your shop. You may add 3 gallons of water a day to keep your shop at 35% RH depending on it's size. If your shop is 65 deg and 35% RH, the EMC of any wood will be 7%. Way better than 4.9.

Couple things to remember:

If you keep your shop, or wood storage area 20 deg above the outside temp, the EMC will stay near 7%. Problem is in the winter out shops can be 30-60 deg above the outside temp so WAY to dry.

40% humidity is perfect to keep your wood supplies near 7%.

50-70% RH and your wood will clime to 10-12% EMC

When the RH gets up 80-90%, the woods EMC is up near 16-20%, which is the same as wood that is air dried.
Kiln dried wood stored anywhere the air is not conditioned will gain moisture.


EMC wood calculator