Marion Ingle and Simon Clarke, from materials consultancy firm Sandberg LLP, UK, examine some causes of reduced performance of phenolic foam in construction insulation applications, and explain how these can be prevented and resolved.
Phenolic insulation has seen a significant increase in use in construction applications. The largest use of phenolic foam is in insulation panels, for example in cladding, roofing, underfloor heating systems and balconies, while other applications include pipe and cable insulation. There are numerous factors that influence the choice of insulation material for a particular application and there is a wealth of readily available published data dealing with the performance properties of insulation materials.
In the majority of applications phenolic insulation performs as expected without any complications. However, in a small minority of applications unexpected problems have occurred due to unforeseen chemical or physical interactions with other materials. Phenolic insulation materials are typically used in contact with other materials, and like other polymers the properties may be affected by interaction with other materials in direct or secondary contact with the material, as well as those of other materials it is used with – sometimes with catastrophic effects.
The combination of materials and service environments is practically infinite, so to avoid any potential problems it is important to consider the foam’s potential interactions with other materials in a particular design application. Interaction with other materials after end of life is also an important issue, particularly since the expected life of modern buildings can sometimes be relatively short, meaning these materials can end up in landfill or as contamination in brownfield sites. To understand the potential interactions between materials in each application, it is necessary to consider:
- how the foam is made
- what chemicals may be present in the foam
- what materials the foam is likely to come into contact withhow variation in ambient conditions, such as heat and moisture, may affect interactions between materials
Phenolic resins are prepared by reaction of a phenol or substituted phenol with an aldehyde, typically formaldehyde. Urea may also be present, as it can be used as a formaldehyde scavenger. Other additives include surfactants (to control the cell size and ratio of open to closed cells), blowing agents (which can be hydrocarbons, fluorocarbons or ethers) and catalysts. The catalysts used are of particular interest as they are typically acidic – the choice of acid catalyst may require some degree of compromise. The exotherm from catalysis of the polymerisation reaction causes the blowing agent to evaporate and so the foam expands. However, using a less aggressive acid to avoid potential reaction with other materials can have a knock-on effect on other properties of the foam.
Where phenolic foam is used with concrete or other cementitious materials, such as in underfloor heating and under-soffit or underrender applications, the concrete surface will be highly alkaline, with a pH of around 13–14. Since the foam will normally contain acidic components, some chemical reaction is possible – a pH of 1–2 has been observed under damp conditions. The foam may absorb some moisture from the wet concrete as it dries out after being laid, unless there is a waterproof barrier. In more severe conditions, following any water leak – be it from groundwater breaching a damp-proof membrane, a damaged underfloor heating pipe or long-term ingress of rainwater behind rendering – the alkaline salt solutions produced by the concrete may be absorbed by the foam, which can cause a chemical reaction with any acids present. While any elements absorbed from the concrete into the core of the foam can be identified using SEM analysis, the effects are normally obvious. For example, dark brown discolouration and loss of stiffness in the affected area is observed in wet foam that has been exposed to alkaline salts (although brown discolouration is also caused by heat and humidity). When the damage extends far enough into the foam, in the long-term it could result in cracking in the concrete due to the variation in level. Where concrete is laid directly above the foam (such as in underfloor heating), using a vapour-control layer above the foam reduces the risk of moisture absorption. Suspected water ingress can be investigated by various methods, including thermal imaging survey, which can show cold spots due to water penetration.
When polymers come into contact with phenolic insulation, the potential for migration of small molecules from one material to another also needs to be considered. For example, an organic additive leaching into a plastic fixing pin could result in plasticiation or environmental stress cracking, leading to premature failure. While review of the material properties may be sufficient to indicate whether any interactions are possible, various analytical techniques can identify the additive content of materials if this information is not available.
The potential effects of phenolic foams on metals are more widely known in the construction industry, due to a wealth of publications in the USA following problems with metal structural components attacked by the acidic additives. However, problems can still occur, for example when foams are used in direct contact with metal pipes. In one serious case, reaction between phenolic foam and electrical wiring was identified as the cause of a fire in a public building.
This is a particular issue for chilled water systems with copper pipework, where failures can occur within 2–3 years. Although a vapour barrier is normally provided for the pipe insulation, special care is required to ensure that the barrier is fully sealed, both along the length and at the ends of, or connections to, other materials. The problem is associated with large take-up of moisture from the atmosphere, which can be several times the weight of the original foam. The associated release of the acidic components can lead to the development of minute pinholes that penetrate through the pipe wall in a network of passages. Since the resulting leaks are so small, the water coming out can lead to further saturation of the pipework/ foam for some time before the water eventually starts to escape from the lagging. Some measures have been introduced to reduce this effect, such as the incorporation of sodium silicate coatings in the bore of the insulation. However, these have only limited effect and the protection will be overridden as the moisture continues to collect. It is understood that the formulations of foam containing the sodium silicate as an additive are being introduced, however their effectiveness has yet to be demonstrated.
Phenolic foam consists of open and closed cells. Because water is readily absorbed into the open cells, for insulation applications the proportion of closed cells should be greater than 90%, according to BSEN13166: 2008. In phenolic foam with open cells (a good example is florists’ green oasis phenolic foam) the high potential for water absorption increases with temperature. In insulation panels, for instance, absorption rates of 24% have been observed under controlled hot and humid conditions of 70°C and 90% relative humidity. This is important when designing cladding or rendering systems – if it is possible that the panel could absorb moisture, the increased weight of the foam needs to be considered during testing and design calculations to minimise the risk of degradation or personal injury. Cutting an insulation panel allows more open cells to be present at the cut surface, hence minimising the use of off-cuts is one way to help reduce moisture absorption. The cell structure can be examined using SEM, although voids in the foam, which also facilitate water absorption, can often be seen with the naked eye.
If the insulation supplier is unable to guarantee that the combination of materials proposed for a particular application is appropriate, literature research, materials analysis and product testing is necessary to assess suitable compatibility of materials. Any testing would need to consider the environment in which it is likely to be used, including service stress and extremes of temperature and humidity.
A case in point
Left: The litmus paper indicators show acidity (red) at the core and alkalinity (blue) at the surface.
that was drilled out following identification of a cold spot on the
thermal imaging survey. The dark brown area running up between the
panels shows how water spreads around the edges of the foam and has
travelled upwards toward the concrete surface.
Phenolic foam was used beneath a concrete floor containing an underfloor heating system in a prestigious residential extension in northwest London. During the building works, there was a breach in the damp-proof membrane below the foam and groundwater entered the system. The architect required expert input to investigate and comment on the effect of the water on the foam, and to advise whether the floor would need to be taken up and the insulation replaced. They wanted to avoid this, as taking up a concrete floor along with the underfloor heating system, insulation and damp-proof membrane, then replacing the system, would be inconvenient as well as expensive and incur a significant delay on project completion.
An initial examination of the insulation at the edge of the floor where the concrete had been removed revealed that the foam was in a poor condition. It was dark brown, with discolouration extending around the edges of the insulation panels, and there was significant loss of stiffness and flatness where the foam was in contact with the concrete. Although the water had mainly ingressed underneath the insulation, it was absorbed around the edges of the foam and upwards to the surface facing the concrete, due to the higher proportion of open cells around the surface of the foam (see images above). The pH of the foam in the damaged area of the surface in contact with the concrete was alkaline (pH 10–12), compared to highly acidic (pH 1–2) at the core of the foam, showing ingress of components from the concrete into the foam. While the condition of the concrete was not examined, if the floor were to be retained then a petrographic examination would have been carried out to assess any damage to the concrete resulting from prolonged contact with acid. If the damage to the foam was restricted to the edges of the floor, it may have been possible to take up the concrete around the edges and replace the foam. To investigate the extent of the water ingress, a thermal imaging survey of the floor was done to identify any unexpected cold spots. Several areas were identified in the central region of the floor and the concrete was drilled out at one of these locations, taking care to avoid the underfloor heating pipe circuit, which was located via thermal imaging.
Examination of the underlying foam revealed that it was in a similar condition. It was recommended that the floor was taken up, as not only would the insulation properties of the foam have been reduced, but the damage would have resulted in an uneven load distribution that could lead to cracks in the concrete.
For more information, contact Marion Ingle email@example.com