Insulation – External Wall

A General Note About Insulation

It is more effective (and less risky) to insulate evenly all around as far as you can, rather than, for example, concentrating on one element and neglecting the remainder. 

Having said that, If you are planning to carry out insulation works over a period of time it makes sense to fully insulate each element that you tackle with the intention of all elements ultimately meeting the same high standard.  This will avoid the need to return later to make further improvements that will be difficult (or expensive) to do.

So however you plan your improvements, a continuous thermal envelope should be the aim. 

The quality of thermal insulation is denoted by its λ* value. The lower the λ value, the better the insulation.

This page on insulation appears now following a request for information.  If you have a particular interest, do let us know.

External Wall Insulation

There are some distinct advantages including:

Room sizes are unaffected

Internal decorations and services unaffected

All the mess occurs outside so you can carry on as normal inside

Continuous insulation across intermediate floors and walls

The effects of thermal mass of an existing masonry wall are enhanced

Less likelihood of condensation risk

Improved wind resistance / airtightness 

A range of finishes possible

Issues that might need addressing include:

-Scaffolding needed

Depth of soffit affects depth of insulation

Rainwater pipes will need adjusting

Windowsills will need extending

Consider spread of fire especially near a boundary.  BBA* systems are available.

A number of insulation materials with different characteristics are available:

Mineral wool, λ=0.034

Woodfibre, λ=0.040

Cork, λ=0.039

Expanded polystyrene, λ=0.033

Polyisocyanurate, λ=0.022

Phenolic foam, λ=0.022

A variety of finishes are possible:

Proprietary renders are thin, light, elastic, waterproof and vapour permeable

Timber cladding

Synthetic cladding

Brick slips (thin slices of real brick)

-Stone slips

Metal (e.g. zinc, copper)

A number of firms have specialised in EWI and they can be expensive.  But if they are too expensive, you can do it with your builder.  Builders with connections to The Green Register, the AECB and CAT should know what they’re doing.

Further reading: (and go to ‘Best Practice Guide’)

Chew Magna Climate & Nature Emergency Working Group

This is information not advice. We suggest you obtain professional advice before undertaking work.


λ  – the lambda value is a measure of the conductivity of an insulation material.  Lower values indicate better insulation.


AECBAssociation of Environment Conscious Building
BBABritish Board of Agrément 
CATCentre for Alternative Technology
ESTEnergy Saving Trust
TGRThe Green Register 

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