Building Fabric 02 – Insulation Materials

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Introduction to insulation materials


In order to create a comfortable thermal environment in our buildings we need to maintain a constant temperature within the building. By providing a building fabric that keeps heat within the building for as long as possible we are able to conserve energy by reducing the requirement for heating and therefore also reducing costs. 

[Check out the posts below for a refresher]




Good insulation will improve the thermal performance of the structure, keeping the building warm in winter and cool in summer (providing solar gain and ventilation are designed correctly). 


The best type of insulating material will have a low density, with atoms spaced well apart. Gases have atoms that are widely spaced, and so air is a good form of insulator. This is why air key is a factor in insulating materials, as the materials allow for ‘pockets’ of air that act as insulators, an example being aerated concrete or glass fibre. 


Types of thermal insulators


There are many forms of thermal insulators used in construction, with a wide variety of properties and applications. 


  • Rigid preformed insulators – aerated concrete blocks
  • Flexible materials – fibreglass quilts
  • Loose fill materials – expanded polystyrene granules
  • Materials formed on site – foamed polyurethane
  • Reflective materials – aluminium foil


Properties of thermal insulators


When specifying insulation for buildings, it is important to consider the requirements of the material. Some of the properties to take into account are:


  • Is the insulation suitable for the purpose
  • Does the insulation have sufficient strength or rigidity for the application 
  • Moisture resistance
  • Fire resistance
  • Resistance to pests and moulds
  • Is it compatible with the materials adjacent to it
  • Is the insulation harmful to occupants or the environment


The choice of insulation products is vast and there is not a one size fits all insulation that is suitable for all applications. For example, a sheeps wool insulation performs well in a ventilated wall construction but is not suitable for unventilated cavities. The choice of insulation must go hand in hand with the overall construction method. 

Insulation Materials


There are three general categories that building insulation materials fall under: organic (or plant/animal derived), mineral or petrochemical derived (oil based) materials. 


Generally speaking the best performing insulations in terms of k-values (see this article for a refresher on k-values) are the materials with the highest embodied energy or carbon. However, this can sometimes be offset by the reduced requirement of space heating and less material being required for the same performance of a lesser material. 


The natural plant/animal based materials tend to have good environmental and performance benefits (such as acoustic) and a lower embodied carbon. They do however, tend to have higher k-values and much greater wall thicknesses are required to achieve a suitable overall u-value. The best scenario is to be able to choose a material that has a low k-value and low environmental impact. 


Organic or Natural Insulations

Organic insulations can be described as cellulose insulation, cork insulation, hemp, wood fibre, hemp crete and sheeps wool. 


The advantages of organic insulations:

  • Usually low, zero or negative embodied carbon
  • Often waste or by products
  • They are recyclable or reusable
  • They can be made air tight with good acoustic performance
  • Suitable for breathing or timber framed constructions


The disadvantages of organic insulations:

  • They don’t always have the best k-values with a range of 0.038 to 0.1 W/mK
  • some materials can rot if exposed to high levels of humidity or moisture
  • Not suitable for installation in flood areas at low levels
  • Not generally suitable for wet construction such as masonry cavity walls
  • Not generally suitable for high load areas like foundations, floors, green roofs
  • A breather membrane must be used if it is adjacent to a ventilated cavity


Organic insulation focus: Sheeps Wool

Sheeps wool slabs and tools are used as insulation between rafters, joists and timber studs in a breathing or ventilated construction. 

The benefits include:

  • Sheeps wool insulation is a waste produce from a renewable source
  • Wool sequesters CO2 during animal growth
  • It is a hydroscopic material which helps to control the temperature throughout the season
  • Absorbs moisture without loss of thermal efficiency 
  • Is possible to reuse if recovered carefully

The disadvantages include:

  • The polyester binder used to create the material is non renewable
  • Contains a boron based flame retardant
  • Imported wool from China is cheaper and becoming widely available but conditions in which the sheep are reared is unknown
  • If wool is imported it adds to the embodied energy
  • Sheep emit significant amounts of methane


[image credit]

Mineral Insulations

Mineral based insulations can be described as rock wool, glass wool, cellular or foamed glass, or aerogel. 

The advantages of mineral insulations include:

  • Average k-values in the range of 0.036 to 0.05 W/mK, with aerogel being 0.013 W/mK
  • Vapour permeable apart from glass foam and aerogel
  • Usually contains recycled or waste products
  • Good fire resistance
  • Recyclable or reusable (except mineral fibre)


The disadvantages of mineral insulations include:

  • Embodied energy is average
  • Can contain formaldehyde
  • Some materials are not hygroscopic and therefore any moisture will reduce insulation performance
  • Aerogel reuse and recycling is difficult due to installation method


Mineral insulation focus: Rock wool

Rock wool is made from quarried rock and recycled steel slag. It is available in a variety of densities which produces varying thermal conductivity. It is versatile in its application and can be used in masonry cavity walls, timber frame walls, roof rafter insulation and lofts and floors. 

Benefits include:

  • Uses up to 23% secondary industrial waste
  • Can be reused if in a good condition
  • Can be recycled
  • Inherently non combustible and is able to resist rot
  • A formaldehyde version has been developed but not yet available in the UK

Disadvantages include:

  • Quarrying results in land degradation
  • Emissions from production include carbon monoxide, formaldehyde and phenol. UK emissions are within legal limits
  • Only formaldehyde brands are available in the UK at present
  • High embodied energy

[image credit:]

Oil based polymer Insulations

Oil based polymer materials or petrochemical derived materials include expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR/PIR) and phenolic foam. 


Advantages of oil based polymer insulations include:

  • Good insulators, with k-values ranging from 0.028-0.04 W/mK, with PIR and phenolic foam able to achieve 0.018-0.024 W/mK
  • Can be recycled and reused
  • Good fire resistance although not always suitable for rainscreen cladding
  • Suitable for damp conditions – ie masonry cavity walls
  • Suitable for high impact areas such as foundations, floors and green roofs


Disadvantages of oil based polymer insulations include:

  • High embodied energy
  • EPS thermal performance may be reduced by moisture
  • Off gassing particularly with phenolic foam
  • Vapour impermeable so not suitable for use in breathable constructions 
  • Closed cell /surface insulation not suitable for timber frame construction as moisture will only be able to escape through the timbers
  • Phenolic insulation can shrink over time so not suited to render finishes
  • Most materials excluding EPS, suffer from reduced performance over time


Oil based polymer focus: Extruded Polystyrene (XPS)

Extruded polystyrene (XPS) is an extruded foam, by mixing polystyrene with a blowing agent and forcing it through a die. It is slightly stronger than EPS and most commonly seen in below ground applications where high loading or impacts are expected.

Advantages include:

  • Recyclable
  • Reusable
  • High compressive strength
  • Water impermeable
  • Generally resistant to both rot and vermin
  • In the UK most blowing agents used in manufacture are zero ozone depletion agents

Disadvantages include:

  • Derived from petrochemicals which leads to resource depletion, pollution risks for the production of oil and plastic. 
  • HBCD which is used as a fire retardant is considered to be hazardous
  • Can release chlorofluorocarbons 
  • Relative high embodied energy

[image credit:]

Table of thermal conductivity of insulation materials

Building Assembly Examples for Thermal Performance

Refer to Thermal Performance article below for refresher on U-values and thermal performance:
The following details and tables demonstrate building assemblies that comply with the required u-values, using different insulation materials. You can see the thickness requirements of the different types of materials and how the performance has an impact on the over all assembly thickness.
U Values for dwellings
[the following examples are based on NHBC guidance]

Wall: Full Fill Masonry

Full Fill Masonry

Full Fill Masonry

Wall: Partial Fill Masonry

02 Partial fill Masonry

02 Partial fill Masonry

Timber Frame Mineral Wool

03 timber frame mineral wool


03 timber frame mineral wool

Timber Frame Rigid PIR

04 timber frame Rigid PIR

04 timber frame Rigid PIR

Solid Ground Floor

Warm Roof with Mineral Wool

Warm Roof with Rigid PIR


Cold Roof



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