A look at (interior) insulation: when is it actually a good idea?

At Instacrepi, we focus primarily on facade insulation and exterior crepi applications. However, it can happen that exterior insulation is not feasible, for example with monumental facades or terraced houses on the building line. In those cases, interior insulation can be a smart and sometimes even the only solution. In this comprehensive article, you will discover the main advantages and disadvantages of interior insulation, when you should definitely consider it, and how to prevent moisture problems.

1. What exactly is interior insulation?

Definition and method

With interior insulation, you apply insulation material to the inside of your exterior walls, followed by a finish such as plasterboard or wooden cladding. This improves the thermal resistance of the wall without changing the exterior of the building—a major advantage for those with a historical or protected facade.

Commonly used materials:

• Mineral wool (glass or rock wool)
• Wood fiber boards
• PIR/PUR panels
• Insulating lime-hemp system

Finishing:

• Plasterboard
• Wooden cladding
• Plaster layer directly on the insulation material

Reasons to choose interior insulation:

• Aesthetics: preservation of a historical or characteristic facade.
• Practical constraints: no space to insulate outside, or building regulations (e.g., building line).
• Faster execution: no scaffolding required, work can be planned room by room.
• Cost-efficient: in some cases cheaper than full exterior insulation (provided the existing state of the facade allows it).

 

2. Advantages and disadvantages of interior insulation

Advantages

1. No facade changes: ideal for monumental buildings or protected cityscapes, where the exterior facade must not be altered.
2. No permit required: often no planning permission is required because the appearance of the facade does not change.
3. Flexible approach: you can insulate room by room, which is useful for phased renovations.
4. Cost savings: sometimes cheaper than exterior insulation (but this depends on the existing situation and the choice of insulation).

Disadvantages

1. Space loss: you quickly lose 8-10 cm (or more) per insulated wall, which can be noticeable in small rooms.
2. Condensation risk: if installation is not done correctly (think of a vapor barrier foil), mold or moisture can accumulate in the structure.
3. Thermal bridges: floor connections, window frames, and other penetrations (e.g., balconies) remain difficult points.
4. Reduced thermal mass: the wall can store and release less heat, which sometimes leads to faster temperature fluctuations.

 

3. When is interior insulation truly recommended?

1. Monuments and heritage buildings
   • Historical facades often may not be modified or clad. In such cases, interior insulation offers a solution.
2. Terraced houses on the building line
   • In densely built-up streets, there is sometimes no room to insulate the facade from the outside.
3. Partial or temporary renovations
   • Perfect for those who want to tackle the bedroom first, for example, and the rest of the house later.
4. Specific constructions
   • In timber frame or half-timbered constructions, interior insulation can add an extra layer of thermal protection.

Tip from Instacrepi: Is your home eligible for exterior insulation and crepi? Then that is usually the first choice due to better moisture drainage and less space loss inside. However, we are happy to think along with you if exterior insulation is not feasible.

 

4. Preventing condensation and moisture problems

1. Vapor barrier on the inside – Always place a high-quality vapor barrier (e.g., PE foil) between the insulation and the interior finish. This prevents moist indoor air from reaching the cold exterior wall.

2. Vapor-permeable systems – Natural insulation materials such as wood fiber boards or lime-hemp allow the wall to ‘breathe’ better, but must be installed professionally.

3. Minimizing thermal bridges – Pay extra attention to windows, doors, floor connections, roller shutter boxes, and any penetrations. This prevents spots from remaining uninsulated.

4. Good ventilation – A controlled ventilation system (C or D) or sufficient window vents help to remove moisture from inside.

 

5. Materials and systems for interior insulation

• Integrated insulation boards
  PIR or EPS boards with plasterboard or OSB already bonded to them. Easy and quick to install.
• Wood fiber or clay boards
  Very suitable for historical buildings thanks to their vapor-permeable character and moisture-regulating properties.
• Stud walls with mineral wool
  A classic solution with a wooden or metal support structure, filled with glass or rock wool.
• Spray (foam) insulation
  Seamless finish, but pay attention to the correct vapor retarder and the environmental impact of the foam.

 

6. Practical points of attention

Electricity and piping: count on extra work and costs to relocate sockets and pipes.

Impact on interior space: on average, you lose around 11.5 cm per wall (including insulation thickness, air gap, and finish).

Airtightness: pay extra attention to the sealing around windows, doors, and corners. Gaps and seams are disastrous for insulation efficiency and can cause moisture problems.

7. Practical case examples

• Historical mansion
  • Interior insulation with wood fiber and lime plaster: facade remains unchanged, energy loss drops drastically.
• Terraced house on the building line
  • PIR panels applied to the inside, because the house stands directly on the sidewalk.
• Apartment with limited renovation options
  • Interior insulation possible per room; exterior facade may not be modified due to HOA regulations.

8. Cost price and payback period

• Cost price: interior insulation costs on average between €50-€100 per m², depending on the material and complexity.
• Payback period: although the insulation value is often slightly lower than with exterior insulation, it still yields significant energy savings. With grants such as the Flemish ‘Mijn VerbouwPremie’, the investment can become profitable sooner.

Conclusion

Interior insulation is not always the first choice, but it is a sensible option in specific cases—such as with monuments, terraced houses, or room-by-room renovations. Pay attention to preventing condensation and thermal bridges through proper installation and ventilation. Despite the loss of space, with the right materials and careful execution, you can benefit from more living comfort and lower energy costs without changing the facade of your home.

Would you like advice on (interior) insulation or facade insulation? At Instacrepi, we are happy to think along with you. Our specialty is exterior facade insulation and crepi, but we also have the expertise to advise you on the possibilities and limitations of interior insulation.

Feel free to contact us for personal advice or a non-binding quote.