Rendering wood fibre insulation: the Baumit system step by step

A facade insulated with wood fibre can be base-coated and rendered successfully if the boards are dry (up to 13% moisture) and the layers are applied in the right order, with the right pauses. This is confirmed by a Baumit technical opinion of 19.10.2026, prepared by its chief technologist Dr. Eng. Krasimir Delov.

Wood fibre is not EPS and not mineral wool. It is vapour-open, buffers moisture and can absorb a significant amount of water without losing its properties. In the wall this is an advantage, but wet trades demand discipline: render applied over damp boards locks the moisture in and leads to stains, delamination and cracks.

Step 1: Preparing the substrate

A render is only as good as the substrate beneath it. Before the first coat:

• Clean the boards thoroughly of dust and any contamination.
• Repair or re-fix damaged or poorly fastened boards.
• If the installed boards have been exposed to the sun for more than 4 weeks, sand the surface lightly. Sunlight turns the top layer dusty, and this dust must be removed so the base coat has a sound substrate.
• Fill gaps between the boards with strips cut to size from the same boards — never with expanding foam or directly with base coat.
• Fill narrower joints with naturheld FILL — an elastic joint filler and installation adhesive for wood fibre boards that can be rendered over.

This keeps the insulation layer uniform and avoids thermal bridges or lines along which cracks could appear later.

Step 2: Weather protection and moisture check

The maximum permissible board moisture before base-coating is 13%. This is the single most important rule of the whole method.

If the boards will not be base-coated right after installation, protect them from rain and direct sun — for example with a cover or scaffold netting. Wet boards must dry out before work continues.

1. With a wood moisture meter — the accurate method. Measure in several places, especially at the bottom of the facade and around openings.
2. With a plastic film test — a quick preliminary check. Tape a piece of polyethylene (PE) film about 70 × 70 cm airtight along its edges onto the boards and check its inner side after 24 hours.

If there is condensation under the film, the boards are too damp and base-coating must not start.

Step 3: Levelling coat

The first coat is Baumit StarContact White, about 2–3 mm thick. It closes the board surface and creates a flat, sound base for the reinforcement.

• Apply it with a notched trowel, then smooth it.
• Let it dry completely: at least 1 day per mm of thickness, i.e. 2–3 days.

Step 4: Profiles and reinforcement around openings

Before the reinforcing coat, fit the appropriate Baumit profiles:

• corner profiles on external corners;
• drip profiles above openings and along the bottom edge of the facade;
• window and door profiles.

Add extra reinforcement around all openings with strips of Baumit StarTex (glass-fibre mesh). The corners of windows and doors concentrate stresses, and that is where diagonal cracks appear first.

Step 5: Reinforcing coat

The reinforced base coat is again Baumit StarContact White, at least 5 mm thick. It can be applied in two passes — just as with mineral wool insulation.

• Embed Baumit StarTex mesh (145 g/m²) in the upper third of the coat over the whole area.
• Overlap the mesh strips by at least 10 cm.
• Cover the mesh with about 1 mm of mortar — it must not show through.

The total base coat thickness (levelling and reinforcing coats) is about 7–8 mm.

Step 6: Primer and finishing render

Once the base coat has dried — at least 5 days, depending on the weather — prime it with Baumit UniPrimer. After 24 hours apply the finish: Baumit ready-to-use facade render.

In cool and damp weather, wait longer. Use every product according to its technical data sheet and the waiting times stated there.

The system at a glance

Total base coat thickness: about 7–8 mm. Source: Baumit technical opinion of 19.10.2026.

The most common mistakes

• Base-coating boards that have stood in the rain, without checking moisture.
• Filling joints with expanding foam or directly with base coat.
• Laying the mesh directly on the boards or in the lower part of the coat instead of the upper third.
• Skipping the extra reinforcement at window and door corners.
• Shortening the pauses between layers to “save” time.

Conclusion

naturheld wood fibre insulation and the Baumit system work reliably together when the substrate is dry and the waiting times are respected. The difference from EPS is not in the products but in the discipline of execution. More on the wall layers in the article Wood fibre ETICS for timber houses. The boards: naturheld 180, naturheld 140 and naturheld 110.

Who will install it? Your builder. The material is cut with a saw and fitted like mineral wool — we give them the manufacturer’s instructions and help over the phone. No builder yet? Write to us and we will help.

Questions

Sources

• Baumit technical opinion of 19.10.2026, Dr. Eng. Krasimir Delov, chief technologist
• naturheld FILL — joint filler and installation adhesive