Wednesday, January 30, 2019

The price of burning damp wood

Moisture content of most woods as kiln dried, dried at 70% relative humidity and 90% relative humidity

Freshly cut soft maple, aspen and willow are about 50% water, by weight.

How bad is that? That means you need to add about equal amounts kiln dried wood to get the green wood to burn...and all the heat goes up the chimney as water vapor.

Things go around the bend at about 30% moisture. You need to burn twice as much wood, by weight, to net the same amount of heat as air dried wood.

Bonus chart


  1. I air dry my firewood for a year. I 'think' I get it down around 15-20%.

  2. Now that's looking familiar! As I recall, I also had a chart that showed curing time to ~10-15%. It was in Alaska, and winters were dryyyyyyyyyyyyyy.

    We lived in a log cabin, and had (often) a 120 or 130 degree temperature differential between outdoors and indoors. I also calculated the heat flux. We burned only wood - heating oil could not warm the house - not nearly enough burner.

    I was the Bubba about wood when we lived there . . .

  3. I can tell you that I have extreme prejudices on the trees I use for firewood. A black oak snag (also northern red, and other red oak group trees) will be dry in the top and damp in the lower half. The open pores of red oak wood will let it dry quickly once it is split and stacked. A live tree will have to season for a year, but a dead one is ready within weeks. If you are caught short, build a fire on one side of your firebox and warm up black oak on the other side of the firebox and it will dry before you roll it into the fire. White oak, with its water-tight pores needs to sit in the barn two years before it will burn well. It wants to sit there and skunk along instead of giving heat. Ash is good with a short drying session after splitting, and I have several that will die from emerald ash borer in the next couple years. Black locust will dry in a couple months if you split it and stack it out of the rain. Hickory will not keep because it is candy to powder post beetles. Hickory snags can be burned after splitting and a quick seasoning. Live hickories need to be planned so they have time to dry for most of a year, but don't hold them to the second year. Bradford pears need to be burned, and six months split, out of the rain will make them do pretty well. Keep your saw sharp! (Elm is a pain in the butt. Makes a lot of ash and not much heat. You need two pieces of dry oak to burn one piece of elm.)

    1. Ours was a real birch to cut. As in the only firewood we had was birch. Which was awesome.


Readers who are willing to comment make this a better blog. Civil dialog is a valuable thing.