Walk into any British stove shop or fuel merchant, and you will see the same phrase stamped on every bag: "Moisture content below 20%". Most people know dry wood burns better than wet wood. But what does sub-20% moisture actually mean for combustion physics, and why is it especially transformative for alder logs?

The Physics of Heat Theft: Burning Water

Freshly felled timber contains up to 50% to 60% water by weight. When you place a freshly cut log onto a fire, half of what you threw into the stove is liquid water. Water does not burn. Before the wood fibres can reach ignition temperature, that moisture must be boiled away.

The energy required to turn liquid water into steam is immense. It takes 2,260 kilojoules of energy to vaporise just one single litre of water. When you burn unseasoned firewood, your stove spends all its energy boiling off sap and moisture instead of radiating heat into your living room. The firebox temperature stays low, water vapour cools the chimney walls, and thick, tarry smoke is released into the neighbourhood.

The Magic Threshold of 20%

Combustion scientists have long demonstrated that 20% moisture content is the critical tipping point for domestic firewood. Below 20%, the energy required to evaporate residual internal moisture drops to a tiny fraction of the wood's total caloric value. The fire is able to sustain high combustion temperatures without self-cooling.

When alder is kiln-dried down below this threshold—often reaching 12% to 16% in commercial kilns—its combustion behavior changes completely. Our comprehensive guide on Kiln Dried Alder: Low Moisture Secrets for an Effortless Blaze highlights how proper drying unlocks the full energy potential of this lightweight hardwood.

Why Sub-20% Moisture Supercharges Alder

Sub-20% moisture affects alder even more dramatically than dense timbers like oak or hornbeam. Because alder has an open, diffuse-porous structure, water evaporates freely from its vessels during the drying process, leaving bone-dry cellular tubes.

When you light dry alder:

• Instant surface ignition: With no lingering moisture to hold back temperatures, the surface fibres ignite within seconds of meeting a flame.
• Total volatile combustion: The wood's natural gases escape and ignite cleanly, creating tall flames and leaving behind almost no unburnt volatile residue.
• Zero sizzle and spit: You will not hear the dull hissing of boiling sap bubbling out of the log ends. The burn is quiet, steady, and crisp.

Testing Your Own Fuel at the Hearth

Never take a supplier's word for it without checking for yourself. A calibrated digital moisture meter is a fantastic tool for any stove owner. Always split a log in half before testing; measuring the outside bark will give a false reading, as the outer surface dries first while the inner core may remain wet.

If you do not have an electronic meter on hand, there is also an ingenious physical method you can try right at your hearthside. Check out The Bubble Test: How to Check Your Alder Firewood Is Truly Dry to see how blowing through the open capillaries of an alder log proves its internal dryness.

Maximum Warmth, Zero Waste

Burning sub-20% alder logs means you are paying for usable thermal energy, not water weight. Your fire lights effortlessly, your stove runs at peak efficiency, and your living room warms up fast. Keep your moisture low, and your fire will look after you all winter long.