Keeping Cosy with Thermowood Garden Buildings

Stack of Thermowood with large Telehandler visible in background

When winter really starts to bite, there’s nothing better than stepping into a garden building that feels warm, snug, and instantly welcoming. Staying cosy in your space can boost your motivation when working remotely, spark creativity while crafting a handmade Christmas, or simply give you a peaceful escape from the festive chaos.

Lock in the warmth with Thermowood

Thermowood has excellent thermal insulating properties. This chemical-free, heat-treated timber locks in warmth while withstanding the cold, damp winter weather. Used as a cladding material for garden buildings, Thermowood is not only durable and beautiful but also comfortably warm even during the chilliest months.

So, here’s everything you need to know about Thermowood:

What is Thermowood?

If you’re searching for a natural alternative to traditional treated timber, Thermowood is well worth knowing about. Originally developed in Finland (where cold winters are very much a way of life) Thermowood is made using a specialist heat and steam process that enhances the natural properties of wood. No chemicals. No preservatives. Just carefully controlled heat that transforms the timber from the inside out. The result is a material that looks beautiful, lasts longer, and performs brilliantly outdoors, especially when it comes to keeping warmth in.

Thermowood vs pressure-treated timber: what’s the difference?

Traditional pressure-treated timber relies on chemical preservatives being forced deep into the wood to protect it from rot, moisture and insects. While effective, this approach can affect both appearance and environmental impact.

Thermowood is made using only heat and steam. This changes the wood’s cellular structure, reducing moisture absorption and improving stability while keeping its natural look and feel.

The end result is a long-lasting, low-maintenance timber that’s ideal for external cladding and far kinder to the environment.

How is Thermowood made?

Our Thermowood supplier is a proud member of the Finnish Thermowood Association and a family-owned pioneer in this specialist process. Using slow-grown Nordic timber (known for its straight grain and tight growth rings) the wood undergoes a carefully controlled three-stage treatment.

The process of thermal modifying timber to create Thermowood – image shared with the kind permission of HJT-Holz

Drying the timber

The process begins in the kiln, where temperatures are raised quickly to around 100°C, then gradually increased to 130°C. This stage removes almost all moisture from the wood.

Heat treatment

The temperature is increased to 212°C (Thermo-D grade), where the wood is held for two to three hours. This intense heat alters the timber at a cellular level, locking in durability and stability.

Cooling and conditioning

Finally, the timber is slowly cooled using water spray systems until it reaches around 80–90°C. Moisture is carefully reintroduced, bringing the final moisture content to around 4% — roughly 40% lower than untreated softwood.

This low moisture content is key to Thermowood’s excellent insulation and resistance to cold, damp conditions.

Why Thermowood is perfect for Malvern Garden Buildings

When it comes to cladding garden buildings that are used all year round from summer evenings to winter mornings Thermowood really comes into its own.

Exceptional stability and durability

Thermowood is far more stable than untreated softwood. It’s less likely to warp, swell or shrink, even as temperatures drop and weather conditions change.

Naturally resistant to rot and insects

The heat treatment makes Thermowood highly resistant to fungal decay and the three most common wood-boring insects found in Europe without a single chemical being added.

A warm, natural appearance

Thermowood’s rich golden-brown tones feel especially at home in winter gardens. Left untreated, it will weather naturally to a soft silver-grey, much like cedar or larch, giving your garden building a timeless look.

A stack of Thermowood - image shared with the kind permission of HJT-Holz — A stack of Thermowood – image shared with the kind permission of HJT-Holz

Improved thermal performance

Its thermal conductivity is around 20–25% lower than standard kiln-dried softwood, meaning it helps retain heat more effectively.

That makes a noticeable difference when you’re using your garden building as a home office, studio or cosy retreat during the colder months, especially with the heater on inside.

No chemicals, no leaching, no resin bleed

As no preservatives or resins are used, Thermowood won’t leach chemicals into the environment. The process also removes natural resins, preventing sticky resin bleed on painted or finished surfaces.

How to care for Thermowood

Thermowood is refreshingly low maintenance, but a little care will keep it looking its best for years to come.

Use mild soap and water: A gentle detergent or specialist wood cleaner is all you need.
Brush lightly: Use a soft brush or sponge, working in the direction of the grain.
Rinse thoroughly: Remove any soap residue with clean water.
Let it dry naturally: Always allow the surface to dry fully before applying any finish.

Why Choose Thermowood?

Thermowood combines modern timber technology with natural beauty and it’s particularly well-suited to winter living. With improved insulation, outstanding durability and zero chemicals, it’s the ideal cladding choice for Malvern Garden Buildings.