Why Moisture Content Matters: The Science Behind Quality Firewood

If you've ever struggled to light a fire, dealt with excessive smoke, or felt like your wood burner just isn't producing enough heat, moisture content is likely the culprit. Understanding the science behind firewood moisture is key to getting the most from your heating system.

What is Moisture Content?

Moisture content refers to the amount of water present in wood, expressed as a percentage of the wood's total weight. Freshly cut wood (known as "green wood") typically has a moisture content of 50-60% or even higher. This means that more than half the weight of the wood is actually water.

For efficient burning, firewood should have a moisture content of 20% or below. This is the threshold where wood burns cleanly and produces maximum heat.

The Physics of Burning Wet Wood

When you burn wood with high moisture content, here's what happens:

Stage 1 – Evaporation: Before the wood can properly ignite, the water must be evaporated. This requires a significant amount of energy – approximately 2,260 kilojoules per litre of water. This energy comes from the fire itself, meaning it's not going into heating your home.

Stage 2 – Incomplete Combustion: Wet wood burns at a lower temperature, which prevents complete combustion. This creates more smoke, particulates, and tar deposits.

Stage 3 – Reduced Heat Output: What little energy remains after evaporation is significantly less than what dry wood would produce. You'll need to burn far more wood to achieve the same heat.

The Real Cost of Wet Wood

Burning wood with 40% moisture content (compared to 20% or below) means you're getting roughly half the heat output. In practical terms:

• You'll burn twice as much wood for the same warmth
• Your chimney will accumulate tar and creosote much faster, increasing fire risk
• You'll produce more air pollution
• Your stove glass will blacken quickly
• You'll spend more time maintaining your system

Measuring Moisture Content

At NW Logs, we use professional moisture meters to ensure every batch of firewood meets our strict standards. These devices measure the electrical resistance of the wood – drier wood has higher resistance.

If you're buying firewood elsewhere, we'd recommend investing in a moisture meter yourself. They're inexpensive and will quickly tell you if you're getting quality wood. Always measure in a freshly split face of the wood, not the exterior, as the outside can be misleading.

The Kiln-Drying Advantage

Natural seasoning can take 18-24 months in ideal conditions. In North Wales, with our high rainfall, it can take even longer – and wood stored outdoors may never reach optimal moisture levels.

Kiln-drying solves this problem by creating a controlled environment where moisture is rapidly removed. Our biomass-powered kilns reduce moisture content to 10-15% in a matter of days, and the wood is immediately protected from reabsorbing moisture.

Legal Requirements

Since 2021, it's been illegal in England to sell firewood with a moisture content above 20% unless it's sold in volumes over 2 cubic metres for seasoning by the buyer. Whilst this regulation doesn't yet apply to Wales, it reflects the importance of dry wood for clean air and efficient heating.

At NW Logs, we exceed these standards as a matter of course. Every batch of kiln-dried firewood is tested and guaranteed to be below 20% moisture content – typically 10-15%.

Signs Your Firewood is Too Wet

• Difficult to light and keep burning
• Produces excessive smoke, even after the fire is established
• Hissing or sizzling sounds as water evaporates
• Soot and tar buildup on stove glass
• Poor heat output
• Heavy, damp smell

The Bottom Line

Moisture content isn't just a technical detail – it's the single most important factor in firewood quality. The difference between burning wood at 35% moisture and 15% moisture is the difference between wasting your money and maximising your heat.

When you buy kiln-dried firewood from NW Logs, you're guaranteed wood that's been scientifically tested and proven to deliver optimal performance. No guesswork, no disappointment – just quality fuel that burns cleanly and efficiently.

Want to experience the difference that proper moisture content makes? Get in touch with NW Logs today for delivery across the UK.