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Insulation? External or internal: Which is best?

Heating a house with solid walls can be quite a challenge. Solid walls lack a cavity, so the most common form of wall insulation isn’t viable, but they can still be insulated with either internal or external insulation.

The effectiveness of an insulating material is represented by a U-value.  This measures how fast heat will pass through an object such as a wall or window when there is temperature difference between the inside and outside surface of 1 degree Celsius. The best insulating materials have lower U-values.

UK building regulations require a certain level of insulation. The building regulations Part L stipulates U-values of 0.18 W/m2K for walls, 0.15W/m2K for roofs and 1.4 W/m2K for windows.

Adding insulation improves the U-value. For example an uninsulated solid brick wall 225mm thick might have a U-value of 2.7W/m2K. If this wall is insulated with 100mm of EPS (Expanded Polystyrene) it will bring the U-value down to 0.27W/m2K, much closer to the requirements of the building regulations for new buildings.

Any issues with penetrating or rising damp need to be fixed first before you insulate.

So what are your options when you a looking to insulate a solid wall and what are the pros and cons of each?


External wall insulation: Pros and Cons


  • It is less disruptive as the work takes place outside the house.
  • External wall insulation improves the ability of the wall to withstand the elements.
  • It keeps the wall dry and warm, preventing penetrating damp from entering the building.
  • There is no risk of interstitial condensation as external wall insulation envelopes the house and so helps elevate the temperature of the wall to a level where condensation will not occur.
  • The transmission of airborne noise through the wall is greatly reduced.
  • It only requires modest modifications to services, such as remounting satellite dishes and extending boiler flues.
  • It can improve the appearance of the building by covering cracks or poor rendering.
  • Durability should be good – 30+ years.
  • External wall insulation can be up to 300mm thick, so excellent U-Values are possible. It is typically 60-100mm thick.


  • It is best installed all at the same time.
  • It may require planning permission.
  • Installation requires good access to the outer walls.
  • It is not recommended if outer walls are not sound.
  • Hard render systems may not allow for moisture and vapour movement.
  • It may prevent the building fabrics from breathing, resulting in the need for another source of ventilation.
  • The increased thickness of the walls may cause junction problems with neighbouring houses in terraced or semi-detached arrangements.
  • It might be vulnerable to vandalism as behind the hard render is softer insulation


Internal wall insulation: Pros and Cons


  • Internal wall insulation does not change the external appearance of the building.
  • It may be an option for insulating the front of the house in a conservation area.
  • It can offer thermal insulation where a cavity cannot be filled.
  • Insulation can be installed on a room by room basis rather than having to be fitted all at once.


  • Internal wall insulation can be disruptive and hard to install well, especially in bathrooms and kitchens with units against the wall.
  • Installation requires skirting boards, door frames and fittings to be removed and reattached. This includes decorative cornices and coving which could be damaged during the process.
  • It may make it hard to fix heavy items to inside walls. This can result in additional costs to get special fixings.
  • Avoiding thermal bridging at window reveals can be a challenge.

There is no doubt that insulation increases the thermal efficiency of a building, and saves on heating costs and carbon emissions. Whether you decide to insulate internally or externally will depend on what is the best fit for your building. Consideration may need to be given to cost, choice of materials and how it looks.