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EWI v IWI

A Comparison Of External Wall Insulation v Internal Wall Insulation

Looking at the benefits of the Structherm systems and disadvantages of IWI system.

External Wall Insulation Advantages

  • Less disruptive as all work is external to the property and does not require any access to the inside of the property.
  • Only minor modifications are required to services such as satellite dishes and extension of boiler flues.
  • Insulation will cover to whole of the external wall and will guarantee uniform thermal performance without any cold bridges.
  • Insulation not required to be installed within the first floor as the external wall insulation covers the edge of the floor automatically.
  • No reduction in the available living space.
  • No restriction on U value as the EWI system can be provided with insulation up to 300mm thick if required. Typically 60-100mm applied.
  • The provision of EWI systems will resolve most damp problems automatically as the new cladding will protect the wall from further wetting. Rising damp issues would however need to be addressed before applying the cladding.
  • There will be no risk of interstitial condensation as the external wall insulation elevates the temperature of the wall to a level where condensation will not occur.
  • The existing brick wall will be protected fully from the elements.
  • The cost of EWI is cheaper than IWI.
  • The render coatings can be carefully selected to sympathetically design an aesthetically pleasing design that is compatible with the existing houses that are not insulated.
 



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Internal Wall Insulation Disadvantages

  • Disruptive to occupants as work requires clear access to the inside of the external walls and noise dust and debris will be generated.
  • Electrical re-wiring / socket box modification required. 
  • Insulation generally cannot cover the entire wall due to obstructions – this can lead to cold spots, condensation and mould growth.
  • Insulation will be required to be installed
    within the first floor to avoid cold bridging where the floor abuts the outside wall and separating walls. This will involve lifting floor boards or removal of ceilings hence this is
    sometimes omitted. If this insulation is omitted then heat loss continues to occur.
  • The available living space within the house will be reduced – typically IWI systems are 70 – 100mm thick.
  • U values are limited by the available space to install the system.
  • The existing external wall structure must have any existing damp problems cured before applying an IWI system.
  • To prevent risk of interstitial condensation occurring a vapour control layer (polythene must be fixed to the entire surface of the external wall – this is generally not achievable.
  • As the existing brick wall will be left exposed to the elements and it will become far colder due to the provision of the internal insulation it is possible that degradation of the brick face / mortar pointing will be accelerated.
  • The cost of IWI is more expensive than EWI.
  • No improvement to the external appearance.

 

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