Internal Wall Insulation: When It Works, When It Doesn’t (Benefits, Risks & Alternatives)

Internal wall insulation may seem like a simple way to improve the energy efficiency of a home. In reality, it carries significant risks, especially related to damp, mould and cold bridging. This complete guide explains when internal wall insulation is a smart choice, when it should be avoided, which materials are suitable and which alternatives often perform better.

1. What Is Internal Wall Insulation?

Internal wall insulation involves insulating the inside of an exterior wall. This is typically done by:

• installing insulation boards directly on the wall

• building a stud wall filled with insulation

• applying vapour‑control layers

The goal is to reduce heat loss, but the construction becomes more vulnerable to moisture issues.

2. When Is Internal Wall Insulation Suitable?

Internal insulation is only advisable in specific situations:

1. Protected or listed façades

When the exterior cannot be altered.

2. Apartments where external insulation is impossible

For example due to building regulations or lack of access.

3. Rooms with low moisture production

Such as bedrooms, offices or guest rooms.

4. Façades that are completely dry and in perfect condition

No cracks, no damp, no cold bridges.

5. Solid walls without a cavity

Common in older buildings with thick brick walls.

In these cases, internal insulation can work well if installed correctly.

3. When Is Internal Wall Insulation NOT Suitable?

This is where things often go wrong. Internal insulation should be avoided in:

1. Homes with existing damp problems

Condensation, mould, rising damp, penetrating damp or cavity wall issues.

2. Façades that are not watertight

Porous bricks, damaged joints, cracks.

3. Buildings with cold bridges

Such as concrete floor edges, lintels or balconies.

4. High‑humidity rooms

Bathrooms, kitchens, laundry rooms.

5. Cavity walls that have not been inspected

A dirty or damp cavity + internal insulation = guaranteed mould.

6. Situations where external insulation is possible

External insulation is almost always the superior solution.

In these cases, internal insulation can cause serious structural damage.

4. The Major Risks of Internal Wall Insulation

Internal insulation is technically complex. The main risks include:

1. Interstitial condensation

The wall becomes cold, causing moisture to condense behind the insulation.

2. Mould growth

Due to trapped moisture.

3. Cold bridging

Uninsulated elements become extremely cold → condensation and mould.

4. Reduced drying capacity

The wall can no longer release moisture.

5. Timber decay and structural damage

Especially at timber beam ends or window frames embedded in the wall.

6. Heat loss through air leaks

A single gap in the vapour barrier can compromise the entire system.

5. How to Install Internal Wall Insulation Correctly

A proper installation includes:

1. Vapour‑control layer

Prevents warm, moist indoor air from entering the structure.

2. Airtight finishing

All joints, edges and penetrations must be sealed perfectly.

3. Insulation with the correct vapour resistance

Suitable materials include:

• PIR boards

• Phenolic (resol) boards

• Mineral wool (only with a flawless vapour barrier)

• Multipor mineral insulation blocks

4. Thermal break at cold bridges

At floors, ceilings, window reveals and corners.

5. Improved ventilation

To reduce indoor moisture production.

6. Thorough façade inspection beforehand

The exterior wall must be dry, clean and structurally sound.

6. Suitable Materials for Internal Wall Insulation

1. PIR boards

• high insulation value

• thin build‑up

• good vapour resistance

2. Phenolic (resol) boards

• extremely high insulation value

• very thin solution

3. Mineral wool in a stud wall

• excellent acoustic performance

• fire‑resistant

• requires a perfect vapour barrier

4. Multipor

• vapour‑open

• suitable for solid masonry

• lower risk of condensation

7. Alternatives to Internal Wall Insulation

In many cases, alternatives are safer, more durable and more effective.

1. External wall insulation

• best thermal performance

• no condensation risk

• protects the façade

2. Cavity wall insulation

• fast

• cost‑effective

• low risk

• ideal for homes with existing cavities

3. Façade renovation + impregnation

• prevents penetrating damp

• slightly improves thermal performance

4. Floor and roof insulation

Often far more effective than internal wall insulation.

8. Summary

Internal wall insulation can be a good solution, but only in specific situations. It carries risks such as condensation, mould and cold bridging. It is suitable when the façade is dry, there are no damp issues and external insulation is not an option. In many cases, alternatives like external insulation or cavity wall insulation are safer and more effective.