Air drying lumber winter vs summer?

[Perry Holbrook]

Well not exactly fine lumber. I have recently completed a new front porch during our latest remodel. Used premium pressure treated, I think they call it pre-conditioned, from a real building supply place, not the big box stuff. Of course we will stain it as soon as it is dry enough. I was expecting it to take at least 6 months, but it looks like it is going faster, which raised this question.

In the winter with low humidity but low temps, does lumber dry quicker as opposed to summer with higher temps but higher humidity.

In a kiln you control both humidity and temp. It of course dries the quickest in both low humidity and high temp. But in an uncontrolled environment, which has the fastest result, humidity or temp.

I have my opinion, but wanted to know yours.

Perry

[Tom M King]

I keep a supply of clear treated boards drying. We pick ones off the top of the stacks when we go to get lumber for almost any project, and put them aside to dry. I've never had even 5/4 boards to dry all the way through in a year. I like two years for 2x stuff. I've tested it a time or two with a moisture meter, and the outside dries a lot faster than towards the center. I can only getting soaking wet stuff here though. I'm sure it gets hot more in Summer here than over there, so no question it dries in my shed faster in Summer. If it was outside it might not, since the humidity is so high, but the shed is almost as good as a kiln when the Summer sun warms up the tin roof, and sides.

[Kevin Jenness]

Around here (northern VT) the relative humidity outdoors isn't that much different winter or summer. Wood on a porch will average around 12-15% year round. It will dry faster in the summer due to higher temperatures.

[Cody Colston]

Evaporation, which is how lumber dries, is accelerated more by heat with higher humidity than by low humidity with little heat. Cold air has a low relative humidity because it simply can't hold much moisture. Also, in cool temps, I takes a LONG time for the moisture to move from the core to the surface of the wood where it can be evaporated. Heat facilitates that moisture movement. BTW, you also need air flow for drying. Heat, relative humidity and air flow are the factors that determine drying rate.

[Kevin Jenness]

" Cold air has a low relative humidity because it simply can't hold much moisture."

That's actually not quite right. A body of cold air will have a lower absolute humidity, that is it will hold a smaller amount of water vapor than warm air, but its relative humidity may be the same as a similar volume of warm air holding a much larger quantity of moisture. The equilibrium moisture content of lumber will be essentially the same at equal RH regardless of temperature, but it will take longer to get to that EMC at a lower temperature. Many people think of winter as bringing low relative humidity- that is generally true indoors because we are pulling cold air of fairly high humidity inside and then warming it up, thus decreasing its relative humidity.

[Harold Burrell]

This is what I do...

In our house, we have a problem with super dry air in the winter. So...instead of investing in some high priced humidifier...I simply bring stacks of lumber into the house to dry.

The lumber then becomes make-shift furniture as well.

You know...dining room table...entertainment center...8' coffee table...



(None of the above is true, by the way. However, I can dream, can't I?)

[Cody Colston]

" Cold air has a low relative humidity because it simply can't hold much moisture."

That's actually not quite right. A body of cold air will have a lower absolute humidity, that is it will hold a smaller amount of water vapor than warm air, but its relative humidity may be the same as a similar volume of warm air holding a much larger quantity of moisture. The equilibrium moisture content of lumber will be essentially the same at equal RH regardless of temperature, but it will take longer to get to that EMC at a lower temperature. Many people think of winter as bringing low relative humidity- that is generally true indoors because we are pulling cold air of fairly high humidity inside and then warming it up, thus decreasing its relative humidity.

You are correct. The "absolute humidity" is lower the colder the air and yes, if you warm air without adding moisture, the RH decreases.