Season firewood until it is at or below 20% moisture on a wet basis (water over total mass), which is 25% on a dry basis (water over dry mass), by splitting it green, stacking it off the ground with the top covered, and giving it from 6 months for softwood to 24 months for oak. Confirm it by weighing, because wood that looks dry can still be wetter than that, and burning unseasoned wood in a closed stove gives far more carbon monoxide and chimney tar.
Part of Coppicing & Fuel Management
Drying firewood: why the moisture basis matters, the seasoning times by species, how to check by weight and what unseasoned wood does to a stove and chimney.
Two Bases, Two Numbers
Wet basis is water ÷ total mass; dry basis is water ÷ dry mass. Convert with u = w ÷ (1 − w): 20% wet basis is 25% dry basis; 40% is 67%; 50% is 100%. The parent article’s 40-60% for green wood and “below 20%” for seasoned wood are wet-basis statements (“water by weight”).
Be careful with the linked Moisture Content article. It defines moisture content as a percentage of oven-dry weight (dry basis), yet also says freshly felled wood is “40-60% water by weight” and that 50% means half the wood is water, which is wet basis; its table lists freshly felled wood at 60-100%+ and a “well seasoned” target of 15-25%, which are dry-basis figures (15-25% dry basis is about 13-20% wet basis). Its figures are not wrong in themselves; the article uses both bases without saying which, so check which one a number is.
Also check your moisture meter: many give a dry-basis reading. A reading of 25% on such a meter is about 20% wet basis.
Seasoning Times (Parent Article)
Minimum seasoning: 6 months for softwoods, 12 months for hardwoods, 18-24 months for oak. The parent says ash can be burned after 3-4 months, or even green in an emergency. These depend on climate, split size and stacking, so treat them as starting points.
Worked Example: What Drying Does
Inputs: a 10 kg freshly cut split at 50% wet basis; target 20%; oven-dry net heat 18.5 MJ/kg; 2.44 MJ per kg of water.
- Dry matter = 10 × (1 − 0.5) = 5 kg.
- Mass at 20% = 5 ÷ (1 − 0.2) = 6.25 kg, so 10 − 6.25 = 3.75 kg of water must leave (37.5% of the mass).
- Heat before: 5 × 18.5 − 5 × 2.44 = 92.5 − 12.2 = 80.3 MJ (the water here is 5 kg).
- Heat after: 5 × 18.5 − 1.25 × 2.44 = 92.5 − 3.05 = 89.5 MJ.
- Gain = 89.5 − 80.3 = 9.2 MJ, which equals 3.75 × 2.44: the heat no longer spent boiling off water in your stove.
Method: Seasoning And Checking
Step 1: Split at once. Split wood dries faster (parent).
Step 2: Stack off the ground, top covered, sides open (Storage & Stacking).
Step 3: Weigh a marked split monthly. Find its dry mass once by drying a matching sample to constant weight at about 103 °C. Moisture (wet basis) = 1 − dry mass ÷ current mass.
Step 4: Burn when it reaches the target and two weighings agree.
Common Mistakes
- Trusting appearance. Surface-dry wood is wet inside; split a piece and test the middle.
- Quoting moisture without a basis. Say “20% wet basis”.
- Covering the whole stack. It stays damp.
Safety
Unseasoned wood in a closed stove
Green wood burns cool and smoky: it gives far more carbon monoxide than seasoned wood and deposits tar in the chimney that can catch fire. Do not burn it in a closed or banked stove, never sleep with such a fire burning, and clean the chimney (Chimney Cleaning Schedule, Carbon Monoxide Risks). Dry the test sample in a metal tray under watch.
What’s Next
- Learn next: Storage & Stacking — how to stack
- Combine with: BTU Content Compared to Wood — what moisture does to heat