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Insulation basics: What you need to know

Kingspan offers a brief history and practical guidance on insulation materials and the key points contractors must bear in mind.


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Kingspan logo

Over the past half century, there has been a fundamental shift in the way insulation is viewed within construction.

Once a virtual afterthought, it is now a key concern for designers and contractors. This transformation means that specifiers and installers are now faced with a dizzying array of products. It is therefore important to understand how the performance of insulation is measured, and the benefits that innovative new materials can offer.

A brief history

The most basic function of insulation is to resist heat transfer through conduction. The measure of how well insulation conducts heat is its thermal conductivity (also known as lambda value), which is recorded in units of watts over metres Kelvin (W/m·K). The lower the thermal conductivity of an insulation material, the better its thermal performance.

Over time, the materials used to insulate buildings have changed as manufacturers responded to demand for products with lower thermal conductivities, allowing the desired thermal performance for a building element (whether it be a roof, wall or floor) to be achieved with a slimmer build-up.

Cork and straw – Thermal conductivity: 0.040-0.065 W/m·K

Cork and straw are amongst the oldest known insulation materials. There is clear architectural evidence of cork being used as roofing insulation material in 1st century Rome, while thatched roofs were common during the Middle Ages.

Mineral fibre – Thermal conductivity: 0.034-0.038 W/m·K

Spun from molten glass, stone or slag, mineral fibre was developed during the Victorian period. It was initially used for industrial applications but has been mass produced for a wider market since the 1920s. 

Polyurethane (PUR) / Polyisocyanurate (PIR) – Thermal conductivity: 0.022-0.028 W/m·K

PUR and PIR were manufactured as building insulations in the decades following the Second World War. They offered noticeable enhanced levels of thermal performance relative to alternatives of the time. Use of these products increased significantly in response to the oil crises of the 1970s.

Phenolic – Thermal conductivity: 0.018-0.022 W/m·K

Introduced in the 1970s, phenolic insulation provides improved thermal performance when compared with PUR and PIR. Modern phenolic insulation has the lowest thermal conductivity of any commonly used insulation material.

Vacuum insulation panels (VIPs) – Thermal Conductivity: 0.007 W/m·K

VIPs have only recently been introduced as a building insulation material within the UK market. VIPs feature a microporous core which is evacuated, encased and sealed in a gas-tight envelope. This can allow a desired thermal performance to be met with the slimmest possible construction, making VIPs well suited for areas previously considered too hard to insulate due to a lack of depth.


While thermal conductivities provide a clear indication of the performance of an individual material, to see how it impacts an entire building element, a U-value must be calculated.

The first step is to measure the thickness of each component within a building element (in metre units) and divide this value by its thermal conductivity to produce an R-value (expressed as m2 K/W). This indicates the material’s ability to resist heat transfer at a certain thickness – higher R-values are better for insulation materials.

To calculate the U-value, the R-value of all the components within the building element is considered using the following formula: U-value = 1 / (sum of all R-values). The lower the U-value, the better insulated the building element is. So, a wall with a low U-value should prevent heat loss better than a wall with a high U-value.

This formula can be used to work out the U-value for a particular application. However, there are other aspects which should be considered, including thermal bridging factors of fixings or stud work.

To ensure accuracy in these calculations, the British Board of Agré ment (BBA) offers a voluntary competency scheme. Registrants are put through a rigorous assessment process to confirm their technical competency and procedural rigour. For quality assurance, it is always worth checking the calculations provided, whether from suppliers or via a U-value calculator, are approved under the BBA/TIMSA competency scheme.

While U-values are the primary consideration in insulation specification, the standard of detailing should also be closely controlled. As buildings are insulated to higher levels, these areas take on greater importance and poor workmanship can badly undermine the long-term performance of the building envelope. Condensation risk analyses can help to highlight potential issues, allowing them to be effectively addressed during construction.

Condensation risk analysis (CRA)

There are two ways condensation can affect insulation:

Surface condensation

This occurs on the face of a construction. It can lead to mould growth, compromising internal air quality and the appearance of walls. Thermal bridges are one of the primary causes of this type of condensation. Heat is drawn out at these points, leaving the facing colder than its surroundings.

Interstitial condensation 

This occurs between layers of the building element and can cause deterioration or even failure of the components. It is essential that constructions are designed to prevent this, or that an adequate ventilation solution is provided – removing any condensation. 

CRAs are performed in accordance with BS 5250: 2011+A1: 2016 (Code of practice for control of condensation in buildings). The analysis considers various factors including the materials specified, their order within the building element, the overall building use and the local climate (using Met Office data).

The placement and thermal performance of the insulation layer is crucial in maintaining other materials above their dewpoint temperature and so avoiding the formation of condensation. Vapour control layers can also be placed on the warm side of insulation, minimising any water vapour passing from warm to cold sides of the construction and condensing. CRAs are also considered under the BBA/TIMSA competency scheme.

In practice

The Each Home Counts review brought further focus on the gap between the designed and actual performance of properties. By carefully installing innovative insulation products, backed with accurate U-value calculations and CRAs, it should be possible to close this gap and deliver slim, highly efficient constructions.

For example, the latest generation of rigid phenolic insulation boards can achieve thermal conductivities of just 0.018 W/m·K. Compared with other commonly used insulation materials, it can achieve the same level of thermal performance (R-value)[1] with a reduced thickness:

Lambda (W/m.K)InsulationThickness (mm)








Mineral Fibre


One application where this enhanced performance is particularly beneficial is cavity wall constructions. Contractors are reluctant to expand the thickness of these constructions much beyond 300 mm; however, remaining within these dimensions while also achieving a compliant level of thermal performance can be challenging.

At Cwrt Y Bedw, a collection of 82 homes near Swansea (pictured, top), phenolic cavity insulation boards with a composite foil facing were chosen. This allowed the development to secure a compliant level of thermal performance, within a standard cavity depth, using a material the installation team was already well used to.

The need for accurate calculations and top-performing insulation is greatest when space is at a premium.

At the Woodside Fountain Health Centre in Aberdeen, designers wanted to create a spacious roof terrace with a floor insulated to a U-value of 0.15 W/m2. K. At the same time, they needed an even transition from the internal space to the terrace to meet access requirements. U-value calculations showed this wouldn’t be possible with conventional insulation so instead they specified a vacuum insulation panel (VIP) system.

The system comprises 30 mm VIPs with a thermal conductivity of 0.007 W/m. K and PIR infill panels of the same thickness, which were fitted around the perimeter and to allow for penetrations. A layer of rigid extruded polystyrene insulation was installed above the system, followed by a waterproof membrane and the balcony surface. This slim construction ensured level access while meeting the U-value requirements.

Accurate results

The design and performance of insulation materials has progressed significantly over recent decades. However, for the products to achieve their potential, it is essential for specifiers and installers to make use of accurate, independently verified U-value calculations and CRAs.

By checking specifications through these services, and taking due care and attention during the installation, it should be possible to create buildings which deliver excellent long-term energy performance with slim building envelopes.

Answer the following questions – to complete, please use this link to complete a quick registration and answer form.

Q1) What roof insulation did the Romans use?

  • Chalk
  • Cork
  • Stone
  • Wood

Q2) What does a thermal conductivity represent?

  • How well a material conducts heat
  • The thermal performance of a building element
  • The thermal resistance of a material
  • The rate of thermal loss through a material

Q3) Who operates the U-value and CRA competency scheme?

  • BRE
  • DCLG
  • UK-GBC

Q4) Which of these insulants typically provides the best thermal performance?

  • PUR
  • Mineral fibre
  • Phenolic
  • VIPs 

Q5) Condensation which occurs between the layers within a building element is:

  • Interstitial condensation
  • Internalised condensation
  • Surface condensation
  • Interlinial condensation

[1] R-value of 2.857 m2K/W

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