Vapour Diffusion in Walls: How Water Vapour Really Moves Through Materials (with Misconceptions & Myths)

Vapour diffusion is one of the most fundamental processes in building physics. It determines how water vapour moves through walls, floors and insulation materials. Yet it is also one of the most misunderstood concepts — especially in discussions about insulation, ventilation and moisture problems.

This guide explains how vapour diffusion actually works, what influences it, and why the idea of “breathable walls” is often misinterpreted.

1. What Is Vapour Diffusion?

Vapour diffusion is the process by which water vapour moves through materials due to a difference in vapour pressure.

Key points:

• it involves water vapour, not liquid water

• it is a slow process

• it occurs in almost all building materials

• it is not the same as air leakage or capillary action

Vapour diffusion = water vapour moving from high vapour pressure to low vapour pressure.

Just as a scent spreads through a room, water vapour spreads through materials.

2. How Does Water Vapour Move Through Materials?

Materials contain pores or micro‑channels through which vapour molecules can move.

The speed of vapour diffusion depends on:

• pore size

• material structure

• temperature

• vapour pressure difference

• moisture content of the material

Vapour pressure difference is the driving force behind vapour diffusion.

High vapour pressure → low vapour pressure Warm air → cold air Humid air → dry air

3. Vapour‑Open vs Vapour‑Tight: What Does It Really Mean?

Many people think “vapour‑open” means a wall must be able to “breathe”. That is a misconception.

Vapour‑open materials

• allow water vapour to pass slowly

• prevent moisture from being trapped

• examples: lime plaster, brick, wood fibre insulation

Vapour‑tight materials

• allow almost no vapour transport

• block diffusion

• examples: aluminium foil, bitumen, plastic paints

Important insight:

Vapour‑open does not mean moisture magically escapes. It only means vapour transport is possible.

4. Vapour Diffusion Is Not the Same as Air Leakage

This is one of the biggest misunderstandings.

Vapour diffusion

• slow

• through the material

• driven by vapour pressure

Air leakage

• fast

• through cracks and gaps

• driven by wind and ventilation

Air leakage transports far more moisture than vapour diffusion ever can.

This is why airtight construction is essential.

5. How Much Moisture Does Vapour Diffusion Actually Move?

Much less than most people assume.

Example:

• 1 m² wall

• 24 hours

• normal vapour pressure difference

→ only a few grams of water vapour per day.

Compare that to:

• a 1 mm air leak → hundreds of grams of moisture per day

Conclusion:

Vapour diffusion is slow and limited. Air leakage is the real moisture highway.

6. When Does Vapour Diffusion Become a Problem?

Vapour diffusion only becomes risky when:

1. Vapour moves from inside to outside and condenses along the way

For example:

• in a cold cavity

• in a cold roof structure

• behind interior wall linings

2. The vapour barrier is missing or incorrectly placed

A common mistake in interior insulation.

3. Vapour‑tight layers are placed on the wrong side

For example:

• vapour‑tight paint on the inside

• vapour barrier on the cold side

4. The construction cannot dry properly

Often caused by cement render or plastic exterior paint.

7. Common Myths About Vapour Diffusion

Myth 1: “Walls need to breathe.”

❌ Incorrect. Walls do not need to breathe — they need to dry.

Myth 2: “Vapour‑open materials prevent moisture problems.”

❌ Not always. If condensation occurs, vapour‑open materials won’t stop it.

Myth 3: “Interior insulation is safe if the material is vapour‑open.”

❌ Wrong. Interior insulation requires a perfect vapour barrier.

Myth 4: “Vapour diffusion causes wet walls.”

❌ Rarely. The real causes are usually:

• air leakage

• thermal bridges

• capillary action

• leaks

8. How to Design a Vapour‑Safe Construction

1. Warm side = vapour barrier

Prevents condensation inside the structure.

2. Cold side = vapour‑open

Allows drying to the outside.

3. No vapour‑tight layers on the cold side

Prevents trapped moisture.

4. Build airtight

Stops massive moisture transport through air movement.

5. Ensure proper ventilation

Prevents high indoor humidity.

9. Summary

Vapour diffusion is a slow process where water vapour moves through materials due to vapour pressure differences. It is often misunderstood, leading to poor decisions in insulation and renovation.

Key takeaways:

• vapour diffusion is slow and limited

• air leakage transports far more moisture

• vapour‑open ≠ breathing

• vapour barrier must always be on the warm side

• vapour‑tight layers on the cold side cause damage

• ventilation and airtightness are crucial