EXTERNAL WALL INSULATION: The Defects are Often Built in. Part 2

The First Visual Signs of Premature Failure Explained

Underlying Rockwool Insulation Slabs Now Visible
Underlying Rockwool Insulation Slabs Now Visible

Some time back we blogged about a Midlands high rise EWI scheme that we’d monitored and noted that defects were being built in. Interestingly we have now noted very obvious visual signs of premature wall cladding failure. In fact we recently investigated a failed EWI scheme that exhibited a number of visual problems that were also noted on this scheme. When you consider that it generally costs circa £1m to install EWI to a high rise block then system failures become extremely expensive to remedy. In this particular case I drove past the building today and immediately noted early visual signs of system failure, in particular, what we call ‘thin coat failure.’ The white finished render should be ‘in plane’ and present a smooth finish across the whole facade but in this particular case you can now see that the underlying Rockwool insulation slabs in the Alsecco system are clearly visible; so what causes this and what are the implications?

Pillowing & Mattressing

We have previously written about the effects of pillowing and mattressing but this is something generally seen in Phenolic insulation boards, it isn’t a problem you see in Rockwool slabs.  Please read http://buildingdefectanalysis.co.uk/solid-wall-insulation-ewi/a-case-of-pillowing-or-mattressing/ for more information.

Technical Issues

Since we had initially commented that insufficient hammer fixings were being installed to the Rockwool slabs then we also believe that there is risk of structural failure at the interface between the Rockwool slab and the underlying substrate. However, experience tells us that the visual evidence suggest something far simpler… The render base coat has probably been applied far too thinly.  It is a problem that we are commonly seeing and we’re unsure if this is a result of the construction bean counters short ordering materials or whether it is simply down to poor site quality management?  The problem with overly thin render coats stretches well beyond the obvious aesthetic problem and raises two key technical issues.

  1. An overly thin render coat does not have the same impact resistance.
  2. An overly thin render coat is more permeable and hence far more susceptible to penetrating damp and subsequent saturation of the underlying insulation.
Another elevation with underlying Rockwool slab insulation clearly visible.
Another elevation with underlying Rockwool slab insulation clearly visible.

Thin Coat Failure

So we believe that this system is suffering from thin coat failure and has failed prematurely on both aesthetic and technical grounds and we predict significant technical problems in the near future. The only subjective discussion is with regard to the degree of failure exhibited.

You may wonder why these thin render coat failures are not immediately obvious on works completion and we think that is down to simple curing and shrinkage of the render, which can occur over a number of months; this project was only completed in October 2015 but visual system failures had to be evident months before we observed them. As the render coat shrinks back, it thins to reveal the underlying insulation boards, usually first noticeable when sunlight hits the elevation concerned.

Solution to Failure?

So, is there a remedy for this? If caught early enough before water penetrates and saturates the insulation, is there a quick fix? Sadly no. The problem is that whilst it may seem reasonable to offer additional coats to increase the render thickness, this  system would no longer have BBA approval. The reason for this is simple… You would be applying further base coat over an existing topcoat but the system was never tested with base coat applied over topcoat and therefore would no longer be BBA approved. We have written correspondence from the BBA stating precisely that fact.

Poor installation of Rockwool slabs.
Poor installation of Rockwool slabs.

It is easier to contextualise this system failure when comparing the underlying defects noted as the system was being installed and here we can see the poorly installed rectangular insulation slabs that are now showing through the render coat.

In our opinion both Rockwool slabs and EPS boards offer greater flexibility and potential for at least partial recovery of the EWI system but even retaining the insulation needs careful consideration based on a detailed investigation and recovery of site evidence.


6 responses to “EXTERNAL WALL INSULATION: The Defects are Often Built in. Part 2”

  1. Mitch Gee avatar
    Mitch Gee

    Seeing the boards in shallow lighting does not necessarily mean that the base coat has not been applied to an adequate thickness. The thickness may well be sufficient to prevent the risk of cracking or water ingress, but has resulted in an uneven finish that is more prone to the collection of organic matter and resulting microbial growth. This may cause the facade to become unsightly sooner than it should. Although undesirable it would not impact on the thermal or weatherproofing characteristics of the system.

    More care should have been taken to ensure the face of insulation slabs are flat when being applied. With Rockwool, slab thickness tolerances need to be considered as it is not possible to rasp down the surface of the insulation as it is with expanded polystyrene. Base coats are available that are designed to be applied slightly thicker which help to accommodate steps in insulation boards.

    1. Joe Malone avatar
      Joe Malone

      Thanks Mitch and yes, I completely agree with your comments in theory but what I state as probable ‘thin coat failure’ is based on a very obvious premise that the boards were not immediately visible on completion of the project. If as you say, this is an issue with levelling of the insulation slabs then the slabs in all likelihood would have been immediately visible on completion of the project; in this case they weren’t. We investigate a lot of system failures and are consistently seeing this problem with base coat being applied far too thinly. One consistently strong indicator for thin coat failure in Rockwool or EPS systems is where an acceptable finish was achieved on project completion that declines with passing months as the render dries and shrinks back. Where I don’t agree is where you describe this mode of failure as simply “undesirable”. It is an obvious aesthetic problem that may well impact on the value of the building and of course it is not what the client paid for so you already have an issue with potential breach of contract. I feel sure that not many EWI systems would be sold if the manufacturers brochures contained images of systems with insulation boards showing through and more often than not where boards are visible then it is a strong indicator of significant technical problems and impending premature system failure.

  2. A Dutton avatar
    A Dutton

    With regards to the above article I have been experiencing serious issues resulting from poorly installed external wall insulation
    My most concerning issue is in relation to a rental property I own in South Wales
    The insulation was funded by the government’s Green Deal scheme and was supposedly carried out by a company a credited to this scheme and registered with the Green Deal governing body
    A year since the installation the company in question South Wales Plastering has taken voluntary liquidation and are now trading under a different name with the same director
    My property is suffering from severe damp issues which is resulting in internal damage to the plastered walls
    I am trying to hold someone responsible for the failing and costs of removing the external insulation and making good the stone work beneath
    Any advise would be greatly appreciated

  3. P K Paul avatar
    P K Paul

    With reference to the above, I would presume it is due to uneven thickness of base coat applied over rock wool insulation to maintain external levels. Polymer based (PB) EIFS system should have uniform thickness of 2 – 3 mm thick base coats, there are situation contractor used 5 – 10 mm thickness of base coats to cover undulation in the rock wool surface. Due to variation in thermal expansion coefficient (CTE) between rock wool and cementatious base coats, can cause certain extend of delamination or sagging for the system from the rock wool.

    It is very unfortunate situation and remedial measure is to strip out base coat / reinforcing layer and finishes and redo it incorporating aesthetic groves or trims which will hide certain extend of undulation in the wall.

    EIFS Inspector

  4. James avatar

    A very interesting article. I wonder if using cork render such as ******* would allow for more flexibility rather than this thin coat shrinking back problem?

    1. Joe Malone avatar
      Joe Malone

      Hi James,

      I try to keep this blog site free of marketing and whilst I appreciate most questions and comments, it seems clear that your being the sales manager for the product mentioned is what prompted your question.


      Joe Malone

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