Internal wall insulation – why lambda values matter

Internal wall insulation - why lambda matters

This blog is all about slimming down but, don’t worry, we’re not going to berate you if you’ve broken your new year diet plan. Instead, we’re looking at how to slim down the insulation layer in internal wall insulation (IWI) applications, without having to accept higher levels of heat loss.

What are the considerations?

Lambda value

The primary purpose of the insulation layer is to prevent heat loss through conduction. The lower the lambda value (or thermal conductivity) of the insulation layer, the more effective it is at preventing this type of heat transfer. This allows a slimmer thickness of insulation to be used to achieve a target U-value.

Facing emissivity

Insulation product facings can also help to prevent heat transference through radiation. The emissivity (shininess) of an object’s surface is one of the key factors governing radiation emission. As you might expect, foil facings have low emissivity, helping to keep radiated heat inside a property and further reduce the insulation thickness required to meet a target U-value.

How can this affect insulation thicknesses?

To investigate what impact these factors can have in IWI applications, we’ve modelled three different rigid insulation materials in a typical solid brickwork construction.* The results are shown in the table below:

Insulant Thickness (mm)


(Glass Tissue Facer, 0.022 W/m.K)


(Glass Tissue Facer, 0.020 W/m.K)


(Low Emissivity Foil Facer, 0.018 W/m.K)

45 0.34 0.33 0.29
50 0.31 0.30 0.27
60 0.27 0.26 0.23
70 0.24 0.23 0.21
80 0.22 0.21 0.19

As the table shows, by installing the phenolic insulation board with the lowest lambda value and foil facing, it is possible to achieve the desired U-value with a 10 mm slimmer thickness of insulation compared with the other options. This can feel like a considerable saving, particularly in smaller properties where every millimetre counts.

Ensuring correct detailing

Detailing is paramount in any insulation application as any gaps in the insulation layer can easily undermine the building’s thermal performance. As many older properties feature unusual design elements, it’s particularly important to plan and carry out these IWI applications with care. This issue was a central focus in the recently published Bonfield Report (also known as Each Home Counts) which has recommended the introduction of a quality mark for all companies working within the energy efficiency field. Clear detailing advice is provided within the literature for Kingspan Kooltherm K118 Insulated Plasterboard.

In the field

Each IWI application comes with its own challenges. However, by selecting the most suitable insulation products and paying close attention to detailing, it should be possible to greatly improve energy efficiency for years to come. In the next blog in this series, we’ll take an in-depth look at IWI applications on two properties and what the monitoring data shows about their actual performance.

* 3 mm skim-coated insulated plasterboard (12.5 mm plasterboard) on adhesive dabs, 102.5 mm brickwork, 50 mm clear cavity, 102.5 mm brickwork. Insulant thickness only shown.

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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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