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Last edited 23 Feb 2018
The benefits of specifying expanded polystyrene for external wall insulation
External Wall Insulation (EWI) systems have been used for many years as a popular solution to the problem of thermal inefficiency in housing. However a number of high profile failures have made specifiers take a closer look at system designs and their individual components. Often it is the choice of insulation that results in a failed system exacerbated by damp or water ingress.
EWI systems are made up of several different layers bonded to the insulation board, to form the finished externally rendered façade. This is designed to protect the building against weather or ingress of moisture.
 Reasons for failure
 Wet Boards
The integrity of the system becomes quickly compromised if the boards used to bond the whole system together are even slightly wet when installed. Many cases of system failures have moisture sodden Mineral Fibre or Phenolic insulation boards at the heart of the issue. Boards become saturated due to adverse weather conditions which can affect the thermal performance.
The photographs below show the failure of a system where wet boards were installed followed by render. The system could not dry out sufficiently preventing the render from bonding securely to the boards, as a result the heavy render de-laminated from the insulation creating a risk to the public.
If Phenolic insulation boards become wet an acid can be produced which can corrode any metal fasteners or fixings, causing system failure. If Mineral Fibre insulation is installed while wet then the moisture cannot escape and will accumulate around the metal fixing and oxidise, again causing corrosion and system failure.
Some types of insulation material are known to have unfavourable attributes which lead to degradation. Under some circumstances the degradation can be expedited by moisture. Phenolic and PIR insulation boards are installed in EWI systems with an initial declared thermal performance, but these products naturally degrade. This means that even if the boards are in perfect condition when installed the thermal performance deteriorates over time.
Shrinkage of insulation boards, particularly Phenolic, within EWI systems is another major problem (see photos below). With changes to Part L Conservation of Fuel and Power requiring lower U-values the demand for Phenolic insulation soared. In order to keep up with demand some manufacturers reduced the curing time of Phenolic which adversely affected the quality of the product. Insulation boards were installed prematurely within EWI systems while they were still curing. This caused the insulation boards to shrink, opening up visible gaps between the boards and cracking the render. These problems led to reduced thermal effectiveness of the insulation, unsightly façades and in most cases entire system failure.
Moisture is created naturally within buildings, especially residential properties, and must be allowed to escape otherwise serious problems with damp can occur. One of the ways that moisture escapes is through the external walls, so the choice of external wall insulation is important. Some insulations restrict the passage of water vapour through the wall, trapping moisture and causing damp on the inside of the property and speeding up the degradation of the insulation itself. This can be avoided if a truly hydrophobic and permeable insulation material such as EPS is used, this will allow water vapour
to escape but will protect the inner leaf wall from any ingress of rainwater.
By installing insulation on the cold external walls the dew point is generally moved to the outside of the building, reducing the chance of interstitial condensation occurring within the wall. However variable weather conditions and humidity can lead to the dew point moving inside the wall and causing interstitial condensation. If Phenolic or Mineral Fibre is used and condensation occurs then the insulation will get wet. Once wet the insulation will remain sodden increasing the chance of mould and degradation of materials leading to system failure.
 Gas leakage
Poor quality, open cell, Phenolic insulation boards leak out the blowing agent gas which is replaced by air. If these types of boards are installed in an EWI system the leaking gas can react to chemicals in the render finish and cause large areas of unsightly staining on the façade as can be seen in the photos below .
 Benefits of EPS
EPS has a closed cell structure which means its mechanical and thermal properties are unaffected by humidity. EPS has the capability to repel water whilst allowing water vapour to pass through it. These unique properties mean it is ideal for external wall insulation applications which need to be kept dry but can breath to allow moisture to escape preventing condensation.
 Chemically inert
EPS constitutes approximately 98% air and 2% plastic, making it chemically inert. The fact that no chemicals are present in the material eliminates problems associated with corrosion of fixings and leakage of gasses. EPS is also non-toxic, non-irritant and odourless making it entirely safe to use. These attributes help reduce on-site health and safety issues and concerns compared to other insulation materials.
EPS is a durable, rot proof and permanent material when specified and installed correctly. It will remain effective for the life of any application with no deficiency effects to be expected over a 100 year life cycle. EPS is non-biodegradable and so its physical properties will not degrade or deform, it does not lose any performance over time meaning it can be safely specified in an EWI system.
EPS offers exceptional sustainability credentials achieving a BRE Green Guide Rating of A+. It contains no CFC’s or HCFC’s, has Zero Ozone Depletion Potential and a low Global Warming Potential whilst also being 100% recyclable at many stages of its life cycle.
EPS used in EWI applications contains a Fire Retardant Additive (FRA) to achieve a Euroclass E rating for the reaction to fire as a naked product i.e. insulation with no covering material. However if a whole element performance approach is taken then EPS can be safely used in an EWI system where it is fully covered. This typically achieves Euroclass B-s1, d0 results via independent full scale fire tests which satisfies current building regulations and is a good rating which is comparable to other similar insulation materials.
EPS offers a proven and economic solution which helps specifiers maintain build costs and insulation budgets. The cost of EPS is much lower than that of other insulation materials and is up to 50% cheaper than Phenolic insulation.
--Styrene Packaging and Insulation Ltd 11:48, 18 Jan 2018 (BST)
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