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Hot Topic Highlight – Cavity Wall Insulation (CWI)

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What is this week's blog about?


In this week’s blog, we look at cavity wall insulation (CWI) in residential property.

This will be interesting reading for Residential pathway candidates with Inspection and/or Building Pathology as technical competencies.


A history of walls…


To understand the use of CWI, we need to first look at the history of walls!


Before the 1920s, most walls were built as solid walls of brick or stone.


From the 1920s onwards, cavity walls became increasingly used as a way to prevent water ingress and subsequent damp issues in buildings.


Cavity walls generally measure at least 260mm in width. You can also usually tell the difference between a solid and a cavity wall by the brickwork pattern.


In a cavity wall, you are likely to only see the stretcher (long) face of the bricks, whereas in a solid wall you are likely to see both stretcher and header (short) faces. There are exceptions to this rule, however, particularly if a non-traditional form of construction has been used!


The two brick skins on either side of the cavity are tied together with metal wall ties, which are today made of corrosion-resistant stainless steel. We will discuss wall tie corrosion in a future blog article, however.


A history of insulation...

Early cavity walls were fairly narrow (with a cavity gap of say 50mm) and did not have insulation between the inner and outer brick skins.


From the 1970s onwards, CWI became standard and the internal cavity itself became wider (up to say 300mm), with Part L of the Building Regulations setting specific U-values.


The purpose of CWI is to improve thermal performance and stop heat escaping from inside a dwelling.


In new buildings, cavity walls can be insulated with insulation boards or mineral wool batts that sit between the two brick skins. The cavity may be partially or fully filled, depending on the type of insulation used.


In older buildings with unfilled cavity walls (i.e., where there is no insulating material in the cavity), however, it would not be physically possible to retrofit boards or batts into the cavity. One solution would be to externally insulate the walls, although in this article we are going to focus on blown CWI (i.e., within the cavity).

  How can CWI be retrofitted?

Where CWI is retrofitted, the masonry needs to be in good condition and the cavity generally needs to be at least 35mm (although this may be wider depending on the type of insulation specified). A borescope inspection through a hole drilled in the mortar will establish the cavity dimensions and whether it is clear of rubble and debris.


Blown CWI is installed by small 22-25mm holes being drilled into the mortar externally at regular intervals of circa 1m.


Small polystyrene beads are then blown into the cavity and the holes re-filled with a colour-matched mortar to restore the integrity of the external masonry skin. This allows the beads to fill hard to reach areas of the cavity, without the need for working at height or for extensive drilling to be undertaken.


What issues relate to blown CWI?

However, there are a number of building defects which can be caused by blown CWI. This is typically the case where the brickwork is in poor condition (e.g., poor pointing, spalled bricks or cracking) or CWI is installed inadequately or incorrectly. This can lead to water penetration through the cavity and subsequent excessive internal moisture (dampness). There may also be cold spots (bridging) internally if the insulation beads were not distributed evenly throughout the cavity or if the cavity was not sealed at the top before installation took place.


It is always recommended to instruct a reputable installer, such as a contractor registered with the Cavity Insulation Guarantee Agency (which applies for 25 years). Any CWI installation should also follow the best practice guidance in BRE Good Building Guide 44 Part 2 Insulating masonry cavity walls: principal risks and guidance.


Foam is another alternative material to be used for CWI. Modern spray polyurethane foam can be used, although we have already covered it’s use in another blog article. This is different to urea formaldehyde (UF) foam, which was injected into many cavity walls as a form of CWI during the 1970s and 1980s. Over time, UF foam degrades and can slump to the bottom of the cavity, as well as being a health hazard and potential carcinogen.



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Stay tuned for our next blog post to help build a better you.

N.b. Nothing in this article constitutes legal, professional or financial advice.


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