An Introduction to Soffits

By Knauf Insulation UK & Ireland
January 11, 2022

It is vitally important when you are designing and constructing a building to utilise your space efficiently. The area within your project is limited and not only will the occupant like a bigger footprint but for the client, more space can mean more money, so it is important to not waste it on building elements if it can be utilised efficiently. That is why some builders are insulating in a cold space or soffit.

A soffit is the underside of an external or internal construction that connects a cold space to a warm space. An example of this would be an office or apartment above a car park. A lot of architects or designers make the decision to insulate in the cold space, the car park in the example above, in order to save on space within the warm space – the office or apartment for its primary use.

The typical build-ups that you will see in a cold space will either be concrete or a composite steel deck. We offer two different products that are suitable for this application; Rocksilk® Soffit Linerboard Standard and Rocksilk® Soffit Linerboard Extra.

Both products will provide excellent thermal performance and have a Euroclass A1 reaction to fire classification. Using non-combustible fire-resistant products helps maintain structural integrity and limits the spread of fire and smoke from one area to another, providing safe buildings for occupants, and added peace of mind for specifiers.

Rocksilk® Soffit Linerboard Standard comes with a black tissue facing whereas Rocksilk® Soffit Linerboard Extra comes with a 6mm cementious board which provides increased robustness for when adverse weather is a factor. The addition of this board can appear more aesthetically pleasing and can be painted or plastered onto, giving a more finished look for the end user.

We have recently expanded our range of thicknesses, improved the products’ thermal performance to a market-leading 0.034 w/mK and made each slab more robust, meaning from specification through to install, our slabs will meet your needs.

As the insulation is installed, you will need to use the correct fixings for the product. We recommend two fixing methods using either plastic or metal fixings. For more information on using the correct fixings, please download our Rocksilk® Soffit Linerboard Installation Guide.

To give a consistent and finished look, we are also able to provide colour match fixings which blend in with the products and give an aesthetically pleasing finish.

If you’d like more information on how to use our non-combustible Rocksilk® Soffit Linerboard range to provide excellent thermal, fire safety and acoustic performance as well as creating extra space within your builds, visit