Cavity Wall Insulation

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We stock a wide range of Cavity Wall Insulation suitable for all of your new build, self-build and extension projects - including Celotex PIR insulation board, and a selection of glass and rock mineral wool insulation from popular brands such as Knauf, Isover and Rockwool.

What is Cavity Wall Insulation?

Cavity Wall insulation is a type of insulation applied within the cavity walls (usually the exterior) of residential and commercial buildings. Many people are familiar with the form of cavity wall insulation that involves blowing small foam pellets into the cavity of existing cavity walls, but modern houses are instead often built using Cavity Slab insulation batts Instead of being applied retroactively, this is added during the process of constructing the wall - literally from the ground up. It is designed to “friction fit” between the cavity walls, in between the steel wall ties, and is an effective thermal insulation.

What Sizes Does Cavity Wall Insulation Come In?

Our glass / stone Mineral Wool Insulation comes in a range of thicknesses that reflect the most common depth of wall cavities:

50mm, 75mm, 85mm, 100mm, 125mm, 150mm - the most popular thicknesses are 100mm, 125mm and 150mm. 

Our rigid Cavity Wall insulation comes in slightly narrower sizes (usually around 10mm narrower) which allows room for a 10mm residual cavity within the wall - helping with ease of installation of rigid boards, and to accommodate any mortar spread. 

How to Install Cavity Wall Insulation

Cavity Wall Insulation is applied within the cavity of masonry cavity walls during their construction. It is built up at the same time as both sides of a cavity wall and added as the courses of masonry and corresponding wall-ties are added. It is typically cut into 455mm or 600mm sheets designed specifically to fit between these ties. Ensure you select a thickness that corresponds to the space of the cavity - full-fill insulation is designed to have full contact with the wall, whilst partial-fill insulation is typically intended to allow an air gap. 

How to Calculate U Value

The U-value (thermal transmittance) of a building component such as a wall, roof, or window, measures its thermal conductivity or how easily heat can pass through it. The lower the U-value, the better the insulation. To calculate the U-value, you typically need to consider several factors, including the materials used, their thicknesses, and any air gaps. A general formula for calculating the U-value:

U = 1 over R


U = U Value

R = The total thermal resistance of the building component, including all layers of materials and air gaps.

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