Breathability

The Vapour Side — μ and MBV

Two key parameters describe how a plaster deals with water vapour:

μ (Vapour Permeability): describes how easily water vapour diffuses through the plaster compared with still air. Lower μ means easier vapour flow and faster drying, but if too low, the material can also absorb more vapour and become moisture-active.
MBV (Moisture Buffer Value): measures how much vapour the plaster can temporarily absorb and release as indoor humidity changes. A high MBV value means the plaster helps stabilise indoor humidity and reduce condensation.

The μ-value describes flow — how fast vapour moves through a material.

The MBV describes moisture storage and release — how much vapour a material can buffer when humidity fluctuates.

Together, they describe the “vapour behaviour” of a plaster: how it lets moisture out and how it temporarily stores it to stabilise the environment.

The Buffering Effect — Why MBV Matters

Let's illustrate the buffering effect of lime plasters through a simple example:

An average room measuring 4.5 m × 3 m × 2.5 m has about 33 m² of wall surface and 34 m³ of air.
A typical lime plastered wall with MBV = 2 g·m⁻²·%RH⁻¹ can absorb 2 grams of moisture per m² for each 1% RH change.
For this room (33 m² wall surface) that’s roughly 66 grams of moisture buffered per 1 % RH change.
For a 10% RH change (e.g. after the heating has been turned off), the plaster buffers (absorb) 660 grams of water which otherwise would condense — a significant stabilising effect.

Thus, buffering prevents humidity spikes, which would otherwise lead to condensation on cold surfaces (especially around 15 °C or lower).

A high-buffering plaster absorbs vapour; when humidity drops, it releases it again. This helps keep relative humidity stable, which is crucial for avoiding condensation and mould growth.

Cocciopesto plasters, used since Roman times, are particularly effective at this with MBV values of 3–4 g·m⁻²·%RH⁻¹. They outperform most modern materials and they can buffer about twice as much moisture as regular air limes.

They contain tiny particles of crushed, low-fired brick that can hold and release heat slowly, much better than ordinary lime. This gives it a higher specific heat capacity — about twice that of pure lime — which helps walls stay warmer for longer. When the room is warm, cocciopesto plasters gently absorb heat into the wall; when heating stops, that stored warmth is released back, keeping surfaces warmer and preventing condensation. This stabilises both heat and humidity, creating a healthier indoor climate.

In everyday life, this means cocciopesto-lined rooms feel more comfortable, absorb short humidity peaks after cooking or showering, and avoid cold, damp patches on walls. It balances heat and moisture naturally — a principle known to the Romans centuries ago.

Historically, they were used in Roman baths and humid spaces for exactly this reason: they breathe, buffer and balance moisture naturally.


Material	Vapour Permeability μ-value (Breathability Index)	Moisture Buffer Value (MBV) [g·m⁻²·%RH⁻¹]	Short-Term Water Absorption (A-value) [kg·m⁻²·h⁻⁰·⁵]	Sustained Water Absoprtion (W-value) [kg·m⁻²·h⁻⁰·⁵]
Air Lime (CL90)	8–10	1.5–2.2	2.6–3.6	0.26–0.38
NHL 2	10–18	1.0–1.6	1.2–1.6	0.17–0.25
NHL 3.5	15–22	0.8–1.4	0.9–1.2	0.12–0.19
NHL 5	20–30	0.6–1.0	0.7–0.9	0.10–0.15
Portland Cement	25–60	0.1–0.3	0.1–0.3	0.05–0.10
Gypsum Finish	8–12	0.3–0.7	0.4–0.5	0.06–0.09
Pozzolanic Lime (Roman-type)	12–18	1.8–2.6	0.7–0.9	0.12–017
Cocciopesto-Lime (Roman-type)	7–10	3.0–4.2	0.5–0.7	0.06–0.10
Rinzaffo MGN (Roman-type)	15	1.3–1.8	0.0–0.1	0.01–0.03
Hand-made Brick	8–15	1.0–1.5	1.7–2.2	0.21–0.29
Medium-fired Brick	10–25	0.8–1.2	0.8–1.1	0.11–0.16
Soft Limestone	25–60	0.8–1.4	1.0–1.3	0.13–0.19
Medium Sandstone	20–80	0.8–1.3	0.4–0.5	0.05–0.08
Hard Stone / Granite	100–250	0.2–0.6	0.2–0.3	0.03–0.05

The Liquid Side — A and W values

While μ and MBV govern vapour, the other half of breathability is liquid water behaviour — how plasters react when exposed to real rain or splashing.

A-value (Capillary Water Absorption): describes short-term wetting, or “how fast it gets wet.” High A means faster soaking, typical of lime; low A means slower absorption, typical of denser cement or pozzolanic materials.
W-value (Sustained Rain Resistance): describes long-term liquid penetration during prolonged rainfall — “how well it keeps rain out.”

These two parameters are related but distinct:

A measures short-term behaviour (splashes, showers, surface wetting).
W measures long-term behaviour under sustained rainfall.

The Complete Picture — Vapour + Liquid Behaviour

A truly breathable plaster balances both vapour and liquid behaviour. A low μ with high A may breathe but soak too easily, while low A with high μ may trap damp. The best materials strike a balance that suits their environment and role.


Parameter	Type	Governs	Ideal Range	Real-Life Effect
μ	Vapour	Vapour flow	8–15 (lime range)	Controls drying rate and overall “breathing” speed
MBV	Vapour	Moisture buffering	1–3	Stabilises humidity, reduces condensation
A	Liquid	Short-term water absorption	< 1.0	Limits rapid saturation during rain
W	Liquid	Long-term liquid resistance	5–10 × lower than A	Prevents persistent moisture ingress

Key takeaways:

Breathability is not a single number but a combination of both vapour and liquid metrics, taking into account how a plaster handles both vapour and liquid water.
Chasing the best breathability (lowest μ value) doesn’t always mean better. If a plaster is too breathable, it will also absorb more moisture, which is not ideal.
Vapour buffering capacity (MBV) is a crucial indoor humidity parameter, describing how good natural dehumidifier a plaster is.
Short-term and Sustained Water Absorption (A and W values) define how materials behave under real weather — together they decide if a wall stays dry or slowly saturates.
Cocciopesto and pozzolanic lime plasters naturally combine these advantages — resisting liquid water while staying vapour open.