10/03/2026
Traditional Walls - Modern Comfort - Breathable Solutions
I get it.... you want to insulate solid stone rubble external walls from the inside, using wood fibre insulation. That’s a great choice for vapour permeable (breathable) construction. Here’s how you can prepare and carry out the work safely and effectively:
1. Assess the Wall Condition
Check for damp issues:
Solid rubble stone walls often deal with damp or penetrating rain. Insulating internally without solving these problems risks trapping moisture. Check the condition of the drains and down pipes, and install French Drains if required.
Pointing and joints: Ensure lime pointing (not cement) in mortar joints, cement traps moisture and damages stone.
Surface stability: Remove cementitious plaster and flaky or loose material.
2. Wall Preparation
Drying out: Allow the wall to dry naturally before insulating.
Levelling: Uneven rubble walls may need a lime plaster "dubbing-out" coat to create an even surface.
Salt contamination: If salts are present, treat prior to installing the insulation.
3. Installing Naturheld Wood Fibre Insulation
Fixing method:
Directly fix rigid wood fibre boards(Naturheld 180 or 140kg) with thermally broken mechanical fixings and lime adhesive(ie;Baumit RK70).
Or
Apply a levelling plaster coat first(Baumit RK38 or NHL Thermo), then fix boards with adhesive/fixings. In my experience, a levelling coat is always required, the thickness of the levelling coat needed, determines which levelling product is most suitable.
Thickness of Wood Fibre Insulation: Typically 40–80mm dependent on space, U-value requirements, and condensation risk.
Stagger joints: Ensure boards are tightly butted with staggered vertical joints.
4. Airtightness and Moisture Control
Vapour open system: Use lime-based adhesive and plasters, never gypsum or cement. We specify Baumit RK38, NHL Thermo,RK70 and KlimaGlatt.
Airtightness layer: Often the plaster finish on the wood fibre board acts as the airtightness layer, but you will need to look at the airtightness detail around the window and door reveals. We specify Ampack-Ampacoll XT60DS Tape.
No plastic membranes: Avoid vapour barriers, as the wall needs to breathe.
5. Finishing
Apply a lime plaster finish directly on the wood fibre boards. We specify Baumit RK70 and Baumit KlimaGlatt.
Water based paints are compatible breathable finishes. No gloss / oil based paints.
6. Key Considerations
Thermal bridging: Pay attention at junctions (floors, ceilings, partitions, reveals). Wood fibre insulation should wrap around edges where possible.
Condensation risk analysis: A hygrothermal assessment (e.g. WUFI calculation) is strongly recommended to avoid interstitial condensation.
Ventilation: After insulating internally, indoor humidity must be controlled with good ventilation.
naturheld Baumit UK Ampack
