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How to insulate soffits

Image of insulation being applied to a soffit

Insulating soffits can be an awkward and time-consuming process, often requiring installers to work at height. For these reasons, it is important to give some thought to the insulation material you are installing as this may support faster, simpler installations. 

In this article, we’ll cover some of the key points to consider when selecting soffit insulation, including:

  • Which insulation products are best suited for soffit applications 
  • What U-value soffits should achieve
  • Best practice tips for installation
Which insulation is best for soffits?

When specifying soffit insulation, it is important to refer to the Building Regulations/Standards as some applications may require products to achieve a European Classification rating of A2-s1,d0 or better. 

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Where this Euroclass rating is required, either under the regulations or as part of the project specification, K-Roc Soffit Slab could be used. The mineral fibre slab has a Euroclass rating of A1. Alternatively, if you are looking for a thinner option, AlphaCore Pad can provide the solution. With a Euroclass of A2-s1,d0 and a thermal conductivity of 0.020 W/mK, AlphaCore Pad can allow project U-values to be met with a reduced thickness of insulation compared with other A2-s1,d0 or A1 rated alternatives. This makes it an effective solution not only for new builds, but also for refurbishment projects where minimising any loss of floor to ceiling height is critical. 

Its microporous silica-based core is water repellent, providing peace of mind in basement applications. The boards can be simply lifted and fitted with appropriate mechanical fixings and can be layered where required to achieve the desired U-value. Take, for example, a typical soffit insulation application beneath a 200 mm concrete deck and fixings with a cross-sectional area of 25.97 mm2 and a thermal conductivity of 50 W/mK. To achieve a U-value of 0.17 W/m2K, a triple layer of AlphaCore can be fitted, comprising two 40 mm boards and one 50 mm board, together with a suitable 6 mm soffit lining board installed with 3.82 metal fixings per m2. This results in a total insulation depth of just 130 mm.

Our Kooltherm K110 Soffit Board and Kooltherm K110 PLUS Soffit Board can provide a thin, lightweight solution for applications where an A2-s1,d0 rating is not required. The insulation component of both products achieves a thermal conductivity of 0.019 W/mK. This can make it possible to achieve a target U-value with a slimmer thickness of insulation than may be possible with lower performing materials, making the boards more practical for manual handling.

The standard board dimension of 2.4 m by 1.2 m is also significantly larger than that of some other commonly used soffit insulation materials. This means a greater area can be covered with each board that is installed. In addition, in some cases it is possible to install the insulation with as few as 11 fixings per board (refer to the Sitework section of the product literature for detailed guidance on fixings and fixing patterns). This can support faster fit-outs. 

Kooltherm K110 PLUS Soffit Board also features a building board facing, providing a more robust finish. 

What U-values should soffits achieve?

Under the Building Regulations and Standards, the U-value requirements for soffits are the same as for ground floors. The required U-Value will depend on the building type, the country it is located in and whether the installation is on a new or existing building.

For new build projects, the Building Regulations/Standards in England, Scotland and Wales all set an area-weighted limiting U-values, which the total floor area (including soffits) must meet on average. In all cases, no area of soffit should achieve a worse U-value than the threshold value shown below. 

An area-weighted Notional Dwelling/Building specification U-value is also provided. This specification is used in energy calculation software using a theoretical building of the same dimensions as the one you plan to build. They set the targets for the building. Whilst not compulsory, they often form the starting point for building specification.

How to insulate soffits table

Learn more about the energy targets in the Building Regulations and Standards

Our online U-value calculator can be used to identify the correct thickness of Kooltherm K110 or K110 Plus to meet your U-value in standard soffit constructions. 

Tips for installing soffit insulation

Below are a few best practice tips when installing either Kooltherm K110 or K110 PLUS soffit boards.

  • Insulation boards should be butted to form a continuous layer with no gaps.
  • Where the insulation boards may be subject to external wind pressure, the requirement for additional fixings should be assessed in accordance with BS EN 1991–1–4: 2005 + A1: 2010.
  • The number of mechanical fixings required to fix Kingspan Kooltherm K110 and Kingspan Kooltherm K110 PLUS will vary with the geographical location of the building, the local topography, the height and width of the soffit, and the soffit construction.
  • The fixings should be evenly distributed over the whole area of the board, and must penetrate at least 40 mm into a solid substrate. Refer to the Sitework section of the product literature for fixing patterns.
  • Fixing should have a minimum head diameter of 25 mm. Depending upon the fixing type, insulation boards can also be fitted by a shot-fired fixing method, which can result in significantly faster installation times.

For specific best practice when fitting Kooltherm K110 or K110 PLUS, refer to the Sitework section of the product literature.

Kooltherm K110 Soffit Board

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