# Bio-based building insulation

Why stillness of air is the whole mechanism, how four routes — carded hemp web, pressed straw, steam-expanded cork, grown mycelium — reach the same porous network, and where water, fire and settling break it.

Insulation does not insulate; the trapped air does. Every bio-insulation product is a different engineering route to the same objective: an open pore network that holds air perfectly still.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/bio-isolation/
Updated: 2026-09-02



A batt of insulation is mostly empty space — around 90 percent or more by volume. Nothing about that is accidental: the insulating substance in the wall is not the fibre, it is the immobilised air between the fibres. Each differs mainly in how it manufactures stillness, and at what price that stillness is kept.

## The material does not insulate; the trapped air does

Heat crosses a wall three ways. **Conduction** through the solid skeleton is proportional to how much solid there is: fibre is deliberately sparse, thin and randomly oriented so any path through solids keeps meeting dead ends. **Radiation** contributes little at these densities and temperatures. And **convection** — bulk movement of warm air carrying heat with it — is suppressed purely geometrically: once pore dimensions shrink to the millimetre scale, air cannot circulate. Still air has a thermal conductivity of roughly **0.026 W/(m·K)**; an ordinary fibre mat turns in **0.038–0.040**, meaning nearly all of the residual conductivity is air itself plus the unavoidable contact points. That gap between 0.026 and 0.04 is the entire engineering problem.

## Four routes to the same open network

The commercial formats differ in how they build the skeleton:

- **Hemp and wood-fibre mats** are carded into thick nonwoven webs; a minority of sheath-core binder fibres are melted at points just above their softening temperature, gluing crossings without filling pores.
- **Straw board** relies on the feedstock's own lignin: hot pressing softens it so it welds the stalk pieces together without any synthetic adhesive — the same formaldehyde-free logic as [wood impregnation](../bio-impregnation-for-wood/) replacing soluble chemistry with inherent chemistry.
- **Expanded cork** starts from nature's version of foam: a closed honeycomb of air-filled cells with suberin walls. Superheated steam softens the suberin, granules expand and bond themselves while every internal cell stays sealed and air-filled.
- **Mycelium composite** grows its binder: fungal hyphae colonise a bulky agricultural substrate over several days and tie its particles together before the organism is heat-inactivated. Density is set by inoculation pattern and incubation time rather than by a press.

Four mechanically opposite routes, one identical outcome: an uninterrupted volume of air held by the thinnest possible skeleton.

## What the biology adds beyond the conductivity figure

Plant-derived mats have two properties mineral wool lacks. Their **heat capacity per unit mass is roughly double**, and combined with the greater thickness normally installed this lengthens the *decrement delay* — the hours a temperature wave takes to cross the assembly — which flattens indoor peaks in summer far more than the R-value alone predicts. And they are **hygroscopic**: the wall absorbs and releases moisture vapour, buffering indoor humidity. This only works within a vapour-open construction; sealing such an insulation behind an impermeable layer converts a virtue into a liability.

Two failure modes sit outside the material itself. **Water is fatal in principle**: liquid or long-condensed water fills the very pores that carry the insulation, since water conducts about 25 times better than air, and sustained wetness initiates biological decay. Durability therefore depends on the assembly keeping the material dry, exactly as for [hempcrete](../hempcrete-hemp-construction-materials/). **Compression and settling thin the air layer**: squashed fibre means fewer centimetres of trapped air, so performance degrades quietly where panels were installed compressed.

Finally the honest asymmetry: unlike stone or glass, these materials are fuel. Untreated fibre insulation is flammable, so borate treatment — promoting char and suppressing smouldering — is part of the product rather than an option. The carbon-stored-in-the-wall argument likewise holds only while the material remains sound in service; it is deferred emission, not destruction.