How to insulate solid walls

solid brick wall

Solid wall insulation is one of the most common subjects we’re asked about here at Kingspan HQ. So, this blog is a very brief overview of an enormous subject…

First, the basic definition: Solid walls are walls that do not have any cavity included in them.

They are most common in buildings constructed before the 1920s, for example typical Victorian terraces. However, solid walls are also very common in post-war pre-fabricated concrete buildings and are increasingly being used on new buildings. The problem with solid walls (and we’re not talking about new build here) is that there is little or no insulation which means generally they perform very poorly in terms of energy efficiency.

Existing solid walls can be insulated to perform efficiently as long as those walls are structurally sound, weather tight and have any problems or defects rectified before insulation is fitted.

There are three ways of insulating existing solid walls:

  1. Insulate on the outside (External Wall Insulation or EWI)
  2. Insulate on the inside (Internal Wall Insulation or IWI)
  3. Hybrid – where you insulate both internally and externally on the same property

Insulating on the outside

This is a common method of insulating as in effect the insulation acts as a tea-cosy for the building. There is also no internal disruption, speed of application, better control of thermal bridging and weather-proofing. It has the added advantage of almost infinite finishes to upgrade the appearance and aesthetics of the building as well as not using up any internal floor space. However with external wall insulation, there may be limits as to how much insulation can be used because of eaves overhangs and the like which is why it is preferential to use the highest performing insulation to make the most of available space. For example you need just 75 mm of Kooltherm K5 External Wall Board to achieve a U-value of 0.18 W/m2K on solid lightweight blockwork compared to 145 mm of EPS or rock fibre.

Insulating on the inside

This method is used where there may be restrictions externally (such as a conservation area or for aesthetic reasons), or simply lack of space (eg where the building fronts directly onto a pavement). There are a couple of different ways to insulate the inside of a solid wall which will depend upon:

  • Building use
  • Type and condition of the wall
  • How flat or true that wall is
  • The presence or amount of services
  • Where you are located – for exposure to wind-driven rain

Our technical services team can help you answer these questions. The methods vary from using insulated plasterboard (insulation pre-bonded to plasterboard such as Kingspan Kooltherm K13, K14, K17 or K18 Insulated Plasterboard) which can either be glued or adhered to the wall or fixed to battens or steel framing. Another option is to build a free-standing timber or metal frame. Each method uses different amounts of internal space, which is again why using a higher performing material will make the best use of any space.

Hybrid Approach

This is where you would use a combination of both IWI and EWI. For example you may only be able to use internal wall insulation on the front aspect of a building because of planning constraints, but external wall insulation is permitted for the side and rear aspects. With the hybrid approach care needs to be taken where both systems overlap to ensure that thermal bridges are limited. Again the technical team can help with that.

More information:

Internal Wall Insulation Systems and Training

Solid Wall Insulation Guarantee Agency (SWIGA)


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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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