Heating Your House with Firewood –
Part 2: Drying
Importance of Drying
In Part 1, we discussed the importance of wood moisture content on energy generated from burning wood (Btu). Freshly cut firewood will have a higher moisture content (MC). Therefore, the wood must be dried (or seasoned) to reduce the moisture content and increase the yield of Btus.
The most common method to dry firewood is to stack the wood outside and allow it to dry with time. Drying times will depend on geographic location, method of splitting and stacking, wood species, and protection from rain and snow. In general, it takes 9 to 12 months to dry firewood from green to 20% MC. This period will be less in a dry climate. As the MC dries, the Btu yield will increase. However, eventually the wood will reach an equilibrium moisture content with the ambient conditions, around 12-15%, and further drying time will not reduce the MC. Small-diameter, shorter pieces will dry faster than larger, longer pieces. Higher density species, such as oak and hickory, will require longer drying times.
The Drying Stack
Protection of the drying stack is often neglected. Allowing wood to regain moisture from rain or snow will slow the drying process. The drying stack should have good air flow to maximize drying. Many people make the mistake of putting their drying stack on the side of a building. This will slow drying because the building will block air flow in at least one direction. If the stack is not elevated 6 inches or more off the ground, the drying process will be slowed for pieces in ground contact and these pieces will also be at risk of decay and insect attack, which will reduce their Btu yield. Also, many insects that attack firewood can move and attack the wood components in the adjacent building. This may not be problematic if the building is a metal shed. However, if the building is wood-framed, this could be an issue if preservative-treated wood was not used.
I prefer an open crib pile with alternate tiers to facilitate better air drying. The best drying will occur in an open pole building with no walls. Since this is seldom possible, a nice tarp or tin cover will work. If you pile your wood on top of each other and all pieces are running the same direction, then try to locate the stack so that the prevailing winds blow through the stack, from end to end, rather than entering the stack from the side. Damp basements are not a good place to dry firewood because of the high humidity. Also, I would avoid hot attics because of the risk of termites coming out of the firewood and getting into the house.
Solar kilns can be used to shorten drying time of firewood. Kiln drying of firewood is done on a commercial basis but is not practical for small operations. For small uses, a solar kiln or greenhouse-type design can decrease drying time.
About the Author:
Todd Shupe is the President of DrToddShupe.com and is a well recognized expert on wood-based housing and wood science. Shupe worked as a professor and lab director at LSU for over 20 years. He is active in several ministries including his Christian blog ToddShupe.com. Todd is the Secretary of the Baton Rouge District of United Methodist Men, Database Coordinator for Gulf South Men, and volunteer for the Walk to Emmaus, Grace Camp, Iron Sharpens Iron, Open Air Ministries, HOPE Ministries food pantry. Todd is currently preparing to be a Men’s Ministry Specialist through the General Commission of United Methodist Men.