Bathroom fan: the tissue test… a false sense of security

Many people — and even some inspectors — place a tissue on the bathroom fan grille. 👉 Does the tissue stick? 👉 Quick conclusion: the fan is working .

❌ ERROR.

❓ What this test really demonstrates

✔️ The engine is running

✔️ There is a ceiling suction

⚠️ But this test does NOT confirm that the air is being vented outside.

🚨 The real danger (which we see far too often in expert assessments)

In many homes, the air is humid:

• is discarded in the attic

• is blocked in a crushed or disconnected pipe

• has no functional external outlet

Result :

💧 excessive humidity

❄️ Winter condensation

🦠 hidden mold

🪵 structural degradation

🏠 major problems in the building envelope

And yet… the paper still sticks to the grid.

🧠 To remember (simply)

👉 A fan can operate mechanically

👉 but be ineffective and non-compliant

It's like starting a car in neutral and thinking it's moving.

✅ What you ABSOLUTELY MUST check

✔️ Presence of an outside air vent

✔️ Air actually expelled outside

✔️ Continuous, insulated, and unobstructed conduit

✔️ Functional non-return valve

📸 Without this verification, no reliable conclusion is possible .

🎯 Key message

👉 The tissue test is insufficient.

👉 The outside air outlet must always be checked.

In buildings, poorly drained moisture always ends up causing damage .