Wood EMC in your heated shop

Hey, my boards are cupping! Shop to DRY?


As we are all in the winter heating season we forget to see what's happening to wood moved into out shops.

If you don't know what the humidity is 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.


So you bring in boards 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 going to start dropping 3-4 % very fast.


The boards can cup if one side is exposed to the shop air and the other side isn't, like laying on a bench, or stacking boards 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 cups toward the outside of the log, so against the curve of the growth rings. I test this and it starts in hours.


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 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 and tell customers to seal the ends with what every your going to use to finish seal the top with right away or you will see end checks appear in a few days. Wide slabs see a lot more stress that narrow slabs and can split real fast.


So what can you do? I have seen very few woodworkers add moisture to keep the RH up.

One customer Kieth actually uses a humidifier in his shop. He adds 3 gallons of water a day to keep his shop at 35% RH. His shop is kept at 65 deg so 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.

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

50-70% RH outside and yout wood starts increasing to 10-12% EMC

When the RH gets up 80-90%, the wood s EMC is up near 16-20 which is air dried. Kiln dried wood stored anywhere the air is not conditioned will gain moisture right back to air dried wood.


EMC wood calculator