Cavity Wall Insulation

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 such as Rockwool insulation and Knauf Insulation, or rigid boards known as "PIR Insulation" - popular brands are Celotex Insulation and Recticel Insulation. Instead of being applied retroactively, this is added during the process of constructing the wall - literally from the ground up. It is usually designed to “friction fit” between the cavity walls, in between the steel wall ties, and makes an effective thermal insulation for new buildings and extensions.

What Sizes Does Cavity Wall Insulation Come In?

We stock a range of Mineral Wool Insulation such as Rockwool Insulation, Knauf Insulation 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 (such as Celotex insulation and Recticel 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

Where:

U = U Value

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

Knauf Cavity Wall Insulation

Two popular choices for insulating Cavity walls are manufactured by Knauf Insulation, and are known as Dritherm 37 and Dritherm 32. The number relates to their thermal conductivity, with the lower number indicating a lower conduction of heat and therefore providing more effective insulation. They are made from glass mineral wool, which is spun into flexible "batts" that are flexible and easy to fit within the cavity.

Isover Cavity Wall Insulation

Isover Cavity Wall insulation is a glass mineral wool insulation which is spun to create a soft, flexible matt which is cut into "slabs". It is available in two varieties - Cavity Wall System 32 and Cavity Wall System 36, which are named according to their thermal conductivity. The lower number indicates a lower conductivity, and therefore a more effective thermal insulation.

Rockwool Insulation

Rockwool Insulation is a mineral wool insulation made from spun stone, which creates a blanket-like slab that traps air to create an effective insulating layer. The most common type of Rockwool insulation used in Cavity Wall Insulation is Rockwool RWA45, which is a thermal and acoustic insulation.

Kingspan Cavity Wall Insulation

Kingspan Cavity Wall K108 Insulation is a rigid insulation board with a foil facing made from Phenolic foam, known as "phenolic insulation board". It is often compared to PIR Insulation such as Celotex as it's use is quite similar. Kingspan Insulation is typically used in Cavity Wall Insulation as a partial-fill insulation, ideal for smaller cavities or projects where there is concern about thermal bridging.

Celotex Cavity Wall insulation

There are two types of Celotex insulation most commonly used in Cavity Wall insulation: Celotex Thermaclass insulation and Celotex CW4000. Both are a rigid PIR Insulation boards with a foil facing, which are sized at a 450mm width to make it easy to install in cavity walls. The main difference between them is that Celotex Cavity Wall CW4000 is designed as a partial-fill insulation, meaning it is designed to sit in the cavity with an air-gap, and it has smooth, squared-off edges. Celotex Thermaclass is a full-fill insulation board designed to fill the entire cavity (with a small 10mm gap to allow for ease of fitting and mortar spread when constructing the wall) with a tongue-and-groove shaped edge for a seamless fit.