Thermally Modified Wood Windows:
How Baking Works

Choosing a material for window frames always involves technical compromises. Regular wood offers excellent aesthetics and retains heat well, but it reacts strongly to changes in humidity. In the fall, the frames absorb water and expand, while in the winter, radiators dry out the air, causing the wood to shrink back. Constant expansion cycles deform the paintwork, disrupt the geometry of the sashes, and the locking hardware begins to stick and requires regular adjustment.

Physics of the heat treatment process

Thermal modification technology solves the problem of solid wood instability at the structural level — without a single drop of synthetic impregnation. Boards are placed in sealed chambers and heated to 160–220°C in an atmosphere of hot, pressurized steam, which displaces oxygen and prevents charring. The wood is cured under these conditions for several dozen hours.

Over this time, the chemical formula of the material itself changes. Ordinary wood consists of cellulose, lignin, and hemicellulose — natural wood sugars. These sugars attract water molecules and serve as a breeding ground for fungi. When heated strongly, hemicellulose completely disintegrates: the cell walls sinter, lose their natural porosity, and the material’s ability to absorb water decreases fivefold. The wood becomes hydrophobic.

Profile behavior in real-life conditions

The processed profile maintains the frame’s perfect geometry for decades. The dimensional stability of thermally modified wood exceeds that of conventional kiln-dried wood by dozens of times. Customers ordering wooden windows in Kazakhstan, with its harsh continental climate, often choose this material precisely because winter frosts and summer heat no longer cause frame warping. The hardware operates smoothly, without seasonal adjustments.

Reducing the structural weight by approximately 10–15% directly reduces the load on window hinges. Massive sashes with heavy triple-glazed windows sag significantly less over time.

At the same time, thermal insulation properties are improved: the thermal conductivity of thermally modified pine is reduced by almost a quarter compared to conventional timber. Micro-air chambers within the baked cells act as a barrier against the outside cold.

Production specifics and visual changes

Window frames are extremely rarely milled from a single piece. Manufacturers use laminated veneer lumber — three or four lamellas are glued together with alternating grain direction, which reliably relieves internal stress. When working with thermally modified wood, technologists use polyurethane adhesives: conventional water-based PVA adhesives adhere poorly to the hydrophobic surface of baked-on boards, while polyurethane provides a monolithic bond.

Heating also changes the material’s appearance — completely, throughout the entire thickness of the board. Natural lignin caramelizes and darkens at high temperatures. Ordinary light-colored pine or ash acquires a rich shade, ranging from light brown to deep chocolate. Uniform staining throughout the wood means that a small scratch won’t expose the lighter inner layers — it will remain dark and can be easily concealed with a drop of tinted oil.

No heavy paints and easy maintenance

The profile’s dimensional stability allows for a radically different approach to finishing. Conventional wooden frames are heavily coated with acrylic or polyurethane varnish, which creates a rigid film on the surface. Over time, this film cracks due to seasonal micro-movements of the wood, and rainwater penetrates the cracks and erodes the coating from the inside. Thermowood is not subject to micro-movements, so the frame surface can simply be impregnated with natural oils and hard wax.

Oil impregnation doesn’t form a fragile synthetic film, leaving the wood grain exposed. Renewing an old varnish coating on a regular frame is always a messy process: removing the sashes, sanding off the old varnish, and applying a new coating. Restoring the oil protection of a thermal profile is completely different: the surface is cleaned with a damp sponge, then a thin layer of oil is applied, which cures within a few hours. South-facing frames are treated every three years, while shaded ones are treated every five years.

Economics and acoustics

The initial price of thermally modified wood frames is significantly higher than that of standard alternatives: the complex baking process requires high energy consumption and expensive equipment. This price difference is offset in the long term: standard frames require regular professional restoration, and the cost of professional services and paint quickly offsets the initial savings.

Thermal treatment also affects acoustics: profound changes in the density of cell membranes alter the profile’s resonant properties. Combined with thick acoustic glass, thermally modified wood creates a reliable barrier to high-frequency street noise.

Installation Features

Thermally modified wood is somewhat more brittle than regular solid wood; directly driving thick screws into the wood without pre-drilling can cause the timber to split.

Installation crews use sharp drills with a specific sharpening angle and pre-drill all fastening points. Furthermore, the hydrophobic properties of the oil surface hinder the adhesion of some sealants. Traditional, inexpensive silicone peels off quickly from the oil coating, so when working with such frames, polyurethane or hybrid compounds are used, and vapor barrier tapes are rolled with a rigid roller with particular care.

Increased moisture resistance expands the range of applications for wooden glazing: thermal profiles can be easily installed in bathrooms and indoor swimming pools, where traditional wood was previously unsuitable. Hot steam from showers and condensation on the glass do not deform the frame.

Environmentally friendly material

Thermally modified wood is produced using only water and high temperatures — no biocides or preservatives. Regular, inexpensive wood is often impregnated with toxic chemicals, which then evaporate into the indoor air for years. Thermally modified wood is physically protected from mold: there’s simply no breeding ground for fungi. Even construction waste from installation — sawdust and trimmings — can be safely burned in a fireplace.