24 EXTERNAL WALL FINISHES Fit for purpose? By Alan Thomas, BBA
EXTERNAL WALL INSULATION IS A METHOD OF IMPROVING THE INSULATION OF A BUILDING AT THE SAME TIME AS ENHANCING ITS APPEARANCE.
T
he early systems in the UK were either developed on the Continent or based on such systems but since then a number of UK-designed systems have been introduced.
The systems are sophisticated and as in so many instances in the construction industry, the devil is in the detail – particularly the details at reveals and corners – and considerable emphasis is placed on the installation details during BBA assessments.
Systems are usually mechanically fixed to the substrate, or bonded with adhesive, or a combination of both and in all cases the suitability of the substrate must be taken into consideration.
The BBA’s assessments look at the strength of the fixing methods, so as to evaluate the resistance of the system to suction by wind – often a problem in high-rise buildings. This assessment is accomplished either by use of the BBA’s Tempest Rig, cyclic wind loading test apparatus and/or by analysis of installations elsewhere in the world.
The BBA’s links with approvals organisations on the Continent and further afield are invaluable in this type of work.
The next consideration is the insulant. Will it allow the requirements of Building Regulations to be met? It is suitable for the application? Will it react to the adhesive? If this is used, is it stable (not likely to deform?) And a range of other questions. These all have to be answered satisfactorily during the assessment.
On top of the insulant is the mesh that reinforces the render but some systems incorporate brick slip finishes to create a more traditional looking facade. Assessment of the mesh is not so detailed although it is important to note – another important detail – that a heavy duty mesh should also be used at ground floor level, particularly for public buildings or other area where impacts may be expected – even the bike handlebar can be a danger!
The top layer is usually a render finish – either acrylic or heavyweight cementitious with coloured aggregate. Both options may include polymer additives and these incur a requirement for an assessment of the behaviour in fire, as well as evaluations of other important characteristics, such as adhesion, resistance to cracking, weathertightness and many others, as set out in the relevant Agrément Certificates.
Much of the testing of external wall insulation systems is undertaken on the BBA’s hygrothermal rigs. Which reproduce the effects of rapid changes in temperature by heating a test panel (2.2m x 2m) and then dousing it with a water spray.
BBA Sun/Rain Test
This is like a system on a building heating up on a sunny day and then the rain comes, and then the sun comes out again: it does happen in real life but perhaps not to the extremes we put the systems through – but it is because we do this you can have
Alan Thomas, BBA Sales & Communications Director
confidence in BBA Approved Systems Interstitial condensation risk is also evaluated by the BBA’s Technical experts. Where there are different layers there may well be the possibility of interstitial condensation between the layers, or even within a layer. Typically this is assessed using computer modelling and simulation, although performance can also be checked by making observations on samples in the BBA test laboratories, where freeze-thaw tests are conducted to look at the implications of low temperature thermal shock.
Installation should be undertaken by installers trained and approved by the Certificate holder and it is a necessary part of the installation process for a proper preliminary survey to have been undertaken. This will of course include an analysis of the detailing required around windows, doors, eaves and corners, the position of the starter bead or expansion joints, if required.
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