What is a ‘good insulation’?

Question - what is a good insulation

A good insulation is measured by its ability to restrict heat transfer. This is expressed by a number of different ‘values’.

Lambda values

‘Lambda’ [lam-duh] is the 11th letter of the Greek alphabet (λ). However we’re not here to talk semantics.

Also known as ‘thermal conductivity’ or portrayed as ‘λ–value’, a material’s lambda value shows how well it can conduct heat. It’s measured in units of W/m·K (Watts per metre Kelvin).

As mentioned in last week’s post, a good insulation will have a low a lambda value as possible to reduce heat loss. It’s worth noting that lambda values can vary widely depending on the type of insulation, so be sure to do some research into which of your insulation options achieves the lowest.

Lambda values are general measurements. If you want to be more specific and know how a certain thickness of insulation affects heat transfer, you need to know the R-value.


Also known as ‘thermal resistance’, R-values tell you how well an insulation resists heat transfer at a specific thickness. It is calculated by dividing a material’s thickness (in metres) by its lambda value, and is measured in units of m²·K/W (metres squared Kelvin over Watts).

So you can see how the lower the lambda value, the thinner the insulation needed and in turn the higher the R-value achieved.

The table below gives an example of how you need different thicknesses of different insulation materials to achieve the same R-value of 2.857 m²·K/W:

Type of insulation Thickness Lambda value (W/m·K)
OPTIM-R 20 mm 0.007
Kooltherm K100 range 55 mm 0.018
Kooltherm 60 mm 0.020
PIR 65 mm 0.022
Rock mineral fibre 100 mm 0.034

Thicknesses rounded to the nearest 5 mm.


Yes, these can be known by a different name too – this time it’s ‘thermal transmittance’.

A U-value is the sum of the R-values of all the layers that make up an entire building element (for example, a roof, a wall or a floor), including adjustments for any fixings or air-gaps.

U-values are measured in units of W/m²·K (Watts per metre squared Kelvin) and demonstrate the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element. So you can see how a low lambda insulation, which achieves a high R-value, can help you reach those U-values required to meet Building Regulations.

When brought together, these building elements make up the whole building envelope – the barrier between inside and outside. For insulation to reduce heat loss and help you achieve those low U-values, the building envelope must have as few thermal bridges as possible. We’ll look at this in more detail in next week’s post. In the meantime, take a look at last week’s post, which explains just how insulation works.


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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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