Insulation materials and their thermal properties

Thermal insulation is the reduction of heat transfer (the transfer of thermal energy between objects of differing temperature) between objects in thermal contact. (19)

Key issues


• Reducing the amount of energy used from fossil fuels is the most important factor in promoting sustainability.

• Insulation has the greatest potential for reducing CO2 emissions.

• Energy conserved through insulation use far outweighs the energy used in its manufacture. Only when a building achieves a ‘LowHeat’ standard does insulation’s embodied carbon (see below) become significant.



The most important aspect of an insulation material is its performance – that it consistently provides the designed-for resistance to the passage of heat throughout the lifetime of the building. Though the insulation manufacturer’s published performance expectations will be an essential guide, other factors associated with the ‘real-life’ installation of the material need to be considered as part of the design process:

Ease of installation – the ultimate performance will be determined by how effectively a builder can install a material using conventional skills. For example, insulation slabs need to be installed so that no gaps result either between adjoining slabs, or between the slabs and other construction components that form part of the overall insulation envelope, such as rafters or joists. Any gaps left over will enable the passage of air and result in a reduction in performance.

Shrinkage, compaction, settlement – Some materials are likely to suffer a degree of dimensional instability during their installed life. In many instances this is anticipated and can be overcome through careful design and installation methods. In all other instances, the specifier should seek guidance concerning associated risks from the insulation manufacturer – particularly where materials have not had an established record of installed performance.

Protection against moisture – some insulation materials will suffer a degradation of performance when moist or wet. The designer should, through careful detailing, ensure that vulnerable insulation is protected from moisture. If moisture is a high risk (ingress or over 95% RH), then a suitably resistant material should be specified.

Below we take a look at performances exhibited by a range of common and increasingly common construction insulation materials. 
Insulation materials, particularly where 'green' specification is concerned, divide into so-called 'natural' materials and 'man-made' materials. 
When considering how to specify an insulation material in terms of environmental impact, it is often the case that the 'natural' material is the most beneficial in terms of environmental attributes. However, in some cases, the inherent efficiencies of man-made materials can be included into the environmental equation to provide a wider environmental benefit eg where space for insulation is at a premium such as in retrofit.

What are the performance terms and what do they mean?


Thermal Conductivity / λ (lambda)

Thermal conductivity measures the ease with which heat can travel through a material by conduction. Conduction is the main form of heat transfer through insulation (read more about heat transfer).  It is often termed the λ (lambda) value. The lower the figure, the better the performance.


Thermal Resistance (R)

Thermal Resistance is a figure that connects the Thermal Conductivity of a material to its Width - providing a figure expressed in resistance per unit area (m²K/W) A greater thickness means less heat flow and so does a lower conductivity. Together these parameters form the thermal resistance of the construction. A construction layer with a high Thermal Resistance, is a good insulator; one with a low Thermal Resistance is a bad insulator.
The equation is Thermal Resistance (m²K/W) = Thickness (m) / Conductivity (W/mK)


Specific Heat Capacity

The Specific Heat Capacity of a material is the amount of heat needed to raise the temperature of 1kg of the material by 1K (or by 1oC) . A good insulator has a higher Specific Heat Capacity because it takes time to absorb more heat before it actually heats up (temperature rising) to transfer the heat. High Specific Heat Capacity is a feature of materials providing Thermal Mass or Thermal Buffering (Decrement Delay).



The density refers to the mass (or 'weight') per unit volume of a material and is measured in kg/m3. A high density material maximises the overall weight and is an aspect of 'low' thermal diffusivity and 'high' thermal mass.


Thermal Diffusivity

Thermal Diffusivity measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. For example metals transmit thermal energy rapidly (cold to touch) whereas wood is a slow transmitters. Insulators have low Thermal Diffusivity. Copper = 98.8 mm2/s; Wood =  0.082 mm2/s.  
The equation is: Thermal Diffusivity (mm2/s) = Thermal Conductivity / Density x Specific Heat Capacity


Embodied Carbon (aka Emodied Energy)

Though not an aspect of the thermal performance of an insulation material, Embodied Carbon is a key concept in balancing the global warming gases in producing the material with the that conserved throughout the lifetime of the insulation. Embodied Carbon is usually considered as the amount of gases released from usually fossil fuels and used to produce energy expended between the extraction of raw material, via the manufacturing process to the factory gates. In reality, of course, it goes much further than that including transportation to site, the energy used in installation through to demolition and disposal. The science of embodied carbon is still evolving - consequently, firm and reliable data is difficult to obtain. Look out for EPDs which detail the inputs and outputs of the industrial processes. Read more.....


Vapour Permeability

• Vapour Permeability is the extent to which a material permits the passage of water through it. It is measured by the time rate of vapour transmission through a unit area of flat material of unit thickness induced by a unit vapour pressure difference between two specific surfaces,under specified temperature and humidity conditions.
• Thermal insulation is usually characterised as Vapour Permeable or Non-vapour Permeable. Often referred to, erroneously, as 'Breathing construction', walls and roofs so termed are characterised by their capacity to transfer water vapour from the inside to the outside of the building - so reducing the risk of condensation.


How insulation works

Insulation commonly through a combination of two characteristics:

• The insulation material's natural capacity to inhibit the transmission of heat &
• The use of pockets of trapped gases which are natural insulants. 

Gases possess poor thermal conduction properties compared to liquids and solids, and so makes a good insulation material if they can be trapped. In order to further augment the effectiveness of a gas (such as air) it may be disrupted into small cells which cannot effectively transfer heat by natural convection. Convection involves a larger bulk flow of gas driven by buoyancy and temperature differences, and it does not work well in small cells where there is little density difference to drive it. In foam materials small gas cells or bubbles occur within the structure; in fabric insulation, such as wool, small variable pockets of air occur naturally to form gas cells.


Construction insulation materials

Wood fibre

Industrially produced wood fibre insulation was introduced around twenty years ago after engineers from the timber producing areas of Europe devised new ways of transforming timber waste from thinnings and factories into insulation boarding.    Read more....


Rigid (available in: boards, semi-rigid boards)

Thermal conductivity/ λ (lambda)  W / m . K = 0.038

Thermal resistance at 100mm K⋅m2/W = 2.5

Specific Heat Capacity J / (kg . K)= 2100

Density kg / m3 = 160

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = n/a

Vapour permeable: Yes


Flexible (available in: batts)

Thermal conductivity/ λ (lambda)  W / m . K = 0.038

Thermal resistance at 100mm K⋅m2/W = 2.6

Specific Heat Capacity J / (kg . K)= 2100

Density kg / m3 = 50

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = n/a

Vapour permeable: Yes

(Source: Steico)


Cellulose (blown/sprayed)

Cellulose insulation is a material made from recycled newspaper. The paper is shredded and inorganic salts, such as boric acid, are added for resistance to fire, mould, insects and vermin. The insulation is installed either blown or damp-sprayed depending on application.

Thermal conductivity/ λ (lambda)  W / m . K = 0.035 in lofts; 0.038 - 0.040 in walls.

Thermal resistance at 100mm K⋅m2/W = 2.632

Specific Heat Capacity J / (kg . K)= 2020

Density kg / m3 = 27-65

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = 0.45

Vapour permeable: Yes

(Source: Warmcel and others)


Wool (available in batts; rolls)

Wool insulation is made from sheep wool fibres that are either mechanically held together or bonded using between 5% and 15% recycled polyester adhesive to form insulating batts and rolls. Sheep are no longer farmed primarily for their wool; however, they need to be clipped annually to protect the health of the animal. The wool used to manufacture insulation is the wool discarded as waste by other industries due to its colour or grade.(19)

Thermal conductivity/ λ (lambda)  W / m . K = 0.038

Thermal resistance at 100mm K⋅m2/W = 2.63

Specific Heat Capacity J / (kg . K)= 1800

Density kg / m3 = 23

Thermal diffusivity m2/s  = = n/a

Embodied energy MJ/kg = 6

Vapour permeable: Yes

(Source: Thermafleece)


Hemp (available in: batts; rolls)

Hemp fibres are produced from hemp straw of the hemp plant. Most hemp is imported, but an increasing amount of home-grown crop is becoming available.  Hemp grows up to a height of nearly 4 metres within a period of 100-120 days. Because the plants shade the soil, no chemical protection or toxic additives are required for hemp cultivation. The product is composed of, usually, 85% hemp fibre with the ballance made up of polyester binding and 3-5% soda added for fire proofing.

Thermal conductivity/ λ (lambda)  W / m . K = 0.039 - 0.040

Thermal resistance at 100mm K⋅m2/W = 2.5

Specific Heat Capacity J / (kg . K)= 1800 - 2300

Density kg / m3 = 25 - 38

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = 10

Vapour permeable: Yes

(Source: Thermafleece and Ecological)


Hempcrete (available in: blocks; in-situ)

Hempcrete is a mixture of hemp hurds (shives) and lime (possibly including natural hydraulic lime, sand, pozzolans or cement) used as a material for construction and insulation. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints. Hempcrete walls must be used together with a frame of another material that supports the vertical load in building construction, as hempcrete's density is 15% that of traditional concrete. (19)

Thermal conductivity/ λ (lambda)  W / m.K = 0.06

Thermal resistance at 100mm K⋅m2/W = 1.429

Specific Heat Capacity J / (kg . K)= 1500 - 1700

Density kg / m3 = 275

Thermal diffusivity m2/s  = 1.5 10-7

Embodied energy MJ/kg = n/a

Vapour permeable: Yes

(Source: Lime Technology)


Cellular glass (available in: boards)

Largely manufactured from recycled glass (e.g. windscreens) and mineral base materials such as sand and without the use of binding agents.(21) The ingredients are melted into molten glass, which is cooled and crushed into a fine powder. The powdered glass is poured into molds and heated (below the melting point) in a "sintering" process that causes the particles to adhere to one another. Next, a small amount of finely ground carbon-black is added and the material is heated in a "cellulation" process. Here, the carbon reacts with oxygen, creating carbon dioxide, which creates the insulating bubbles in the (material). CO2 accounts for more than 99% of the gas in the cellular spaces.(20)

Thermal conductivity/ λ (lambda)  W / m . K = 0.041

Thermal resistance at 100mm K⋅m2/W = n/a

Specific Heat Capacity J / (kg . K)= 1000

Density kg / m3 = 115

Thermal diffusivity m2/s  =  4.2 · 10–7

Embodied energy MJ/kg = n/a

Vapour permeable: No

(Source: Foamglas (T4 slab))


Straw (available in : bales, pre-fabricated units)

Straw is an agricultural by-product, the dry stalks of cereal plants, after the grain and chaff have been removed. Straw makes up about half of the yield of cereal crops such asbarley, oats, rice, rye and wheat.

Thermal conductivity/ λ (lambda)  W / m . K = 0.08 (for loadbearing construction)

Thermal resistance at 350mm K⋅m2/W = 4.37 at 350mm

Specific Heat Capacity J / (kg . K)= unavailable

Density kg / m3 =  110 - 130

Thermal diffusivity m2/s  =  unavailable

Embodied energy MJ/kg = 0.91 (source ICE database 2011)

Vapour permeable: Yes

(Source: BRE + FASBA + others )


Glass mineral wool (available in : batts, rolls)

Made from molten glass, usually with 20% to 30% recycled industrial waste and post-consumer content. The material is formed from fibres of glass arranged using a binder into a texture similar to wool. The process traps many small pockets of air between the glass, and these small air pockets result in high thermal insulation properties. The density of the material can be varied through pressure and binder content.

Thermal conductivity/ λ (lambda)  W / m . K = 0.035

Thermal resistance at 100mm K⋅m2/W = 2.85

Specific Heat Capacity J / (kg . K)= 1030

Density kg / m3 =  circa 20

Thermal diffusivity m2/s  =  0.0000016

Embodied energy MJ/kg = 26

Vapour permeable: Yes

(Source: Knauf (Earthwool OmniFit Slab) )


Rock mineral wool (available in: boards, batts, rolls)

Rock (Stone) mineral wool is a furnace product of molten rock at a temperature of about 1600 °C, through which a stream of air or steam is blown. More advanced production techniques are based on spinning molten rock in high-speed spinning heads somewhat like the process used to produce candy floss. The final product is a mass of fine, intertwined fibres with a typical diameter of 2 to 6 micrometers. Mineral wool may contain a binder, often a Ter-polymer, and an oil to reduce dusting.(19)

Thermal conductivity/ λ (lambda)  W / m . K = 0.032–0.044 (18)

Thermal resistance at 100mm K⋅m2/W = 2.70 – 2.85

Specific Heat Capacity J / (kg . K) = n/a

Density kg / m3 = n/a

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = n/a

Vapour permeable: Yes

(Source: Various)


Icynene H2FoamLite / LD-C-50 (available in: wet spray; poured)

H2FoamLite is a proprietary insulation manufactured by Icynene, a company based in Canada. H2FoamLite is a spray-applied open cell, water blown, low density polyurethane foam. The product is prepared from two liquid components, isocyanate (BaseSeal) and resin (H2FoamLite), and is yellowish in colour. (22)

Thermal conductivity/ λ (lambda)  W / m . K = 0.039

Thermal resistance at 100mm K⋅m2/W = n/a

Specific Heat Capacity J / (kg . K) = n/a

Density kg / m3 =  7.5 - 8.3

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = n/a

Vapour permeable: No

(Source: Icynene)


Phenolic foam (available in: boards)

Phenolic foam insulation is made from a resole resin in the presence of an acid catalyst, blowing agents (such as pentane) and surfactants.

Thermal conductivity/ λ (lambda)  W / m . K = 0.020

Thermal resistance at 100mm K⋅m2/W = 5.00

Specific Heat Capacity J / (kg . K) = n/a

Density kg / m3 =  35

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = n/a

Vapour permeable: No

(Source: Kingspan (Kooltherm K3 Floorboard )+ others)


Polyisocyanurate/ Polyurethane foam (PIR/PUR)

Polyurethane (PUR and PU) is a polymer composed of organic units joined by carbamate (urethane) links. Polyurethane can be made in a variety of densities and hardnesses by varying the isocyanate, polyol or additives. 
Polyisocyanurate, also referred to as PIR, is a thermoset plastic typically produced as a foam and used as rigid thermal insulation. Its chemistry is similar to polyurethane (PUR) except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol. Catalysts and additives used in PIR formulations also differ from those used in PUR. Prefabricated PIR sandwich panels are manufactured with corrosion-protected, corrugated steel facings bonded to a core of PIR foam and used extensively as roofing insulation and vertical walls (e.g. for warehousing, factories, office buildings etc.).(19)

Thermal conductivity/ λ (lambda)  W / m . K = 0.023–0.026(18)

Thermal resistance at 100mm K⋅m2/W = 4.50

Specific Heat Capacity J / (kg . K) = n/a

Density kg / m3 =  30 – 40

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = 101 (17)

Vapour permeable: No

(Source: TPM Industrial Insulation & others )


Expanded polystyrene (EPS) (available in: boards, loose fill)

Polystyrene is a synthetic aromatic polymer made from the monomer styrene. Polystyrene can be solid or foamed. Expanded polystyrene (EPS) is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. Polystyrene is one of the most widely used plastics, the scale of its production being several billion kilograms per year. 
Polystyrene foams are produced using blowing agents that form bubbles and expand the foam. In expanded polystyrene, these are usually hydrocarbons such as pentane
Although it is a closed-cell foam, both expanded and extruded polystyrene are not entirely waterproof or vaporproof.
Discarded polystyrene does not biodegrade for hundreds of years and is resistant to photolysis. (19)


Thermal conductivity/ λ (lambda)  W / m . K = 0.034–0.038 (18)

Thermal resistance at 100mm K⋅m2/W = 3.52

Specific Heat Capacity J / (kg . K)= 1300

Density kg / m3 =  15 - 30

Thermal diffusivity m2/s  = 

Embodied energy MJ/kg = 88.60 (16)

Vapour permeable: No

(Source: DOW and others )


Extruded polystyrene (XPS) (available in: boards)

Extruded polystyrene foam (XPS) consists of closed cells, offers improved surface roughness and higher stiffness and reduced thermal conductivity. (19)  It is slightly denser and therefore slightly stronger thatn EPS.
Water vapour diffusion resistance (μ) of XPS is very low -  making it suitable for application in wetter environments.(19)


Thermal conductivity/ λ (lambda)  W / m . K = 0.033–0.035 (18)

Thermal resistance at 100mm K⋅m2/W = 3

Specific Heat Capacity J / (kg . K)= n/a

Density kg / m3 =  20 - 40 

Thermal diffusivity m2/s  = n/a

Embodied energy MJ/kg = 88.6 (16)

Vapour permeable: No

(Source: DOW and others )



 Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely lowdensityand low thermal conductivity. Nicknames include frozen smoke and solid air, or blue smoke owing to its translucent nature and the way light scatters in the material. It feels like fragile expanded polystyrene to the touch. Aerogels can be made from a variety of chemical compounds.
Aerogels are good thermal insulators because they almost nullify two of the three methods of heat transfer (convection, conduction, and radiation). They are good conductive insulators because they are composed almost entirely of gas, and gases are very poor heat conductors. They are good convective inhibitors because air cannot circulate through the lattice. Aerogels are poor radiative insulators because infrared radiation (which transfers heat) passes through them.
Silica aerogel is the most common type of aerogel. The silica solidifies into three-dimensional, intertwined clusters that comprise only 3% of the volume. Conduction through the solid is therefore very low. The remaining 97% of the volume is composed of air in extremely small nanopores. The air has little room to move, inhibiting both convection and gas-phase conduction. (19)

Thermal conductivity/ λ (lambda)  W / m . K = 0.014

Thermal resistance at 50mm K⋅m2/W = 3.8 for 50mm

Specific Heat Capacity J / (kg . K)= 1000

Density kg / m3 =  150

Thermal diffusivity m2/s  = 

Embodied energy MJ/kg = 5.4kgs / CO² per m²

Vapour permeable: No

(Source: Spacetherm & Thermoblok )



16 ICE Database
17 'Polyurethane Rigid Foam', I Boustead, PlasticsEurope (an industry-sponsored report)
18 BRE
19 Wikipedia
20 Building Green
21 Foamglas EPD
22 BBA Cert 08/4598


Read more about Building Physics:

  • Heat transfer: Conduction, Convection & Radiation  MORE
  • U-value for dummies MORE
  • Air tightness MORE
  • Air barrier design MORE
  • Thermal bypass MORE
  • Decrement Delay & Thermal buffering MORE
  • Thermal mass MORE


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