10/08/2026
Diminishing Returns: Getting the Right Thickness for IWI
How much wood fibre insulation is enough?
When insulating a traditional solid masonry wall internally, the instinct can sometimes be:
“If 40mm is good, 80mm must be much better, and 140mm better still.”
But is that the best approach?
This is where the principle of diminishing returns becomes important.
Adding wood fibre insulation to the inside of a solid wall significantly increases the thermal resistance of the wall.
However, as insulation thickness increases, each additional millimetre generally makes a progressively smaller difference to the overall U-value of the wall.
Why?
Because the insulation is only one part of the overall thermal resistance. The existing masonry, internal and external surface resistances, and other layers in the build-up all contribute too.
So, for example, doubling the thickness of wood fibre does not mean you halve the overall U-value.
And this is particularly relevant when considering Internal Wall Insulation (IWI) in older solid-wall buildings. You can actually "over-insulate", so that not enough heat gets into the walls in winter months, and then they don't dry out, which can lead to mould and decay.
There is a point where adding more insulation may provide relatively modest additional thermal benefit, while taking up valuable internal space.
That space might instead be used to improve:
🔹 Continuity of insulation around junctions and reveals
🔹 Airtightness and detailing
🔹 Thermal bridge performance
🔹 Moisture management
🔹 Internal finishes and service zones
This doesn't mean “less insulation is better.”
It means the right thickness needs to be considered as part of the whole wall system.
Wood fibre is particularly interesting in this context because we aren't simply choosing an insulation material based on thermal conductivity.
A well-designed wood fibre IWI system can also offer:
🌿 A vapour-open, hygroscopic construction
🌿 Good thermal performance
🌿 High specific heat capacity and thermal mass
🌿 Improved acoustic performance
🌿 Renewable, bio-based insulation
🌿 Low embodied carbon compared with many conventional alternatives
And with traditional solid walls, moisture behaviour and detailing matter just as much as insulation thickness.
The objective shouldn't be to squeeze the maximum possible thickness of insulation onto the inside of the wall.
It should be to design a balanced, robust and appropriately detailed system that delivers a "good thermal performance" while respecting the existing building.
When it comes to wood fibre IWI, the question isn't simply “How much insulation can we fit?”
It's “What is the appropriate insulation system build-up for this building?”
naturheld Baumit UK Ampack Acara Concepts Ltd
