The first contaminant we are going to look at is Methane gas.

Methane is a naturally occurring gas which mainly originates from the decomposition of organic materials. This gas production is most commonly associated with landfill sites, but peat bogs can also cause elevated levels of methane.

Naturally occurring methane is colourless and odourless.

Over the long term, any exposure to methane at concentrations over 1% can cause impacts on well being such as headaches or dizziness, and can cause difficulty breathing at higher concentrations.

The problem most typically associated with methane contamination however is the risk of explosion.

At certain concentrations in air, methane becomes explosive and will readily ignite. The upper and lower extents of this concentration are referred to as the upper and lower explosive limits. For methane in air, these are 15% and 5% respectively.

Below 5% concentration there is insufficient methane present to fuel and explosion, and over 15% there is not enough oxygen present for ignition. Concentration outwith these limits should not however be regarded as safe.

The most widely known incident arising form methane ground gas is the Abbeystead disaster, which took place in 1984 at an underground water pumping station in Lancashire.

This was the first real disaster caused by ground gas which helped start the ball rolling in relation to the guidance and legislation that we now have to follow.

Methane which had seeped into the building from coal deposits deep underground ignited and destroyed the facility during a guided tour for local residents.

At the time of the explosion there were 44 people in the building, of those 44 8 died instantly and 8 subsequently died due to their injuries.

Subsequent enquiries and appeals found the buildings designers liable for the accident by failing to exercise “reasonable care” in assessing the risk on site from gas ingress, highlighting the importance of desk studies and site investigations when dealing with potentially contaminated sites.

Fortunately there have been very few other cases of methane explosions in buildings in the UK since.

Carbon Dioxide

Carbon Dioxide, CO2, is another naturally occurring ground gas, most commonly originating from disused mine working where pockets of gas may accumulate.

CO2 is an asphixiant, as it typically reduces the concentration of oxygen in the air below critcal levels. From 1% concentration in air, CO2 will begin to cause headaches and drowsiness. 5% will begin to induce sleep or loss of consciousness, and 10% will lead to oxygen starvation, suffocation and death.

High concentration can cause sudden loss of consciousness, meaning victims may be rendered unable to escape before they become aware there is a problem. A recent example of CO2 problems occurred at Gorebridge in Midlothian. In this case site investigations revealed the presence of former mine workings under the site but detected relatively low gas levels, therefore no requirement for gas protection measures.

However, when housing was built on the site, incorrect foundations were used, and this, combined with construction of a nearby railway line disturbed the ground and released high volumes of gas from the mine workings. It’s also possible that the investigation boreholes themselves may have contributed to the release of gas from the mine workings.

Combined with a period of exceptionally low atmospheric pressure, this CO2 entered several properties on the site, causing various health problems for residents. In some properties concentrations of up to 20% have subsequently been detected.

The severity of the problems combined with the difficulties in retrofitting adequate gas protection has led to a number these properties being demolished, causing both significant cost to the local authority, as well as extensive disruption to residents. The fate of the remaining affected properties is currently uncertain.


Of all the common ground gases, Radon is considered the biggest killer, with Public Health England estimating around 1200 radon-related deaths per year. Radon is a radioactive gas produced by the underlying geology of some sites and is associated with the development of various cancers.

Everyone is exposed to radiation form a variety of sources at all times, and in the UK, the largest source is natural radon ground gas. That being said, typically these low levels of exposure do not cause any problems.

The trigger level for dangerous radon exposure is 200 becquerels, and this can be reached where ground produced radon gas concentrates in enclosed spaces. For comparison the background level of radon activity is around 8 Bq.

Radon levels will change across the country as it depends on the geology of the area, ie where granite is prevalent as the underlying rock. Radon maps are produced to indicate where the underlying geology is likely to produce significant amounts of Radon and require protection to be provided to any development.

In the white areas on this map it would suggest that no protective measures are required, down to the darker areas where barriers and/or ventilation would be necessary. A word of warning with these maps though – they are based on the underground geology and historical readings, but may not accurately reflect the precise situation on site, or how the development itself may have altered the pathways for gas flow.

There have been instances where properties were built on areas that displayed white on the Radon maps. Once completed monitors were added to these properties and it was found that rather than having the expected low levels of Radon indicated by the historical maps, there were actually extremely high levels of Radon detected. This confirms the importance of not relying solely on historical data and maps and that really all homes should be tested regardless of location and looking to install basic radon protection membranes in all properties. Current Radon protection measures allow the use of a basic 1200 gauge polyethylene membrane, such as would be used for damp proofing measure. Whilst this may be able to resist the permeation of Radon the ability of the membrane to withstand the build process must be considered.

Pollution Linkages

The approach to dealing with these type of contaminated land is based on the concept of “pollution linkages”.

The primary linkages present in every situation are the pollution source, the pathway, and the receptor. This source, pathway, receptor model forms the foundation of risk based design strategies for ground gas and VOC contamination.


Source treatment involves taking action to break the linkage at the source, the actual pollutant itself. In very simple terms this involves achieving one of the following outcomes:

Destruction/Neutralisation, whereby the pollutant is rendered harmless through the use of some form of treatment such as adding a degrading chemical agent or a process such as heat.

Removal, which can entail either removal of the polluted soil and disposing of it off site, or extracting the pollutant from the soil.

Immobilisation, or “trapping” the pollutant by adding chemicals to solidify the contaminants, making it impossible for liquids or vapours to escape.


A pathway management solution breaks the link between the contamination at the source and the building occupants by two primary means:

Barrier systems, which physically block the passage of harmful liquids or vapours either at the source or the receptor.

Dilution/Dispersal, where ventilation systems are used to reduce vapour concentrations below harmful levels.

Ensuring barrier and ventilation systems work effectively over the lifetime of the building is critical. Both specification and installation of these systems is more complex than it may appear, with an array of physical properties, tests and installation details to consider.

It’s also not advisable to rely on the membrane to provide a 100% barrier to contaminants, so these systems are often used as part of a wider ranging remediation strategy, particularly on heavily contaminated sites.


Receptor management is the most problematic of the three strategies, mainly as it can come into direct conflict with the purpose of the building or structure by placing restrictions on the occupants which may prove unacceptable.

For this reason, receptor management is most commonly used as a temporary measure while a longer term strategy is developed. For example, restricting access to buildings during decommissioning of an industrial site, or evacuating residential properties in response to a nearby chemical spillage.

Another more common example of receptor management would be health and safety requirements placed on contractors during remediation works, such as PPE or safe working practices.

Gas Protection Legislation

There is a great deal of legislation and guidance available in this area to cover all aspects of the source/pathway/receptor model.

In todays presentation we’ll be focussing mainly on barrier systems, so the pathway management part of the model, and the legislation dealing with that.

Ciria 665

Ciria 665 forms the underlying basis of gas design, by classifying contaminated sites into what are known as “characteristic situations”. These classifications, from the least contaminated CS1 up to heavily contaminated CS6 are based on detailed site investigations typically undertaken by specialist consultants.

Once the characteristic situation has been defined, further guidance can be referred to to determine the most appropriate gas protection measures to specify.


BS8485 is a code of practice for designing new buildings and conducting risk assessments on proposals. It was originally issued in 2007 and was a 28-page document, however after a revision in 2018 it has now grown to about 100 pages.

BS8485 bases its guidance on site classifications from CS1 to CS6. A CS1 site has no gas present, while CS6 sites are heavily contaminated, almost to the point of being unsuitable for development.

The site classification is then cross referenced against the buildings risk category. Type A buildings are domestic structures with small rooms and limited air movement, thus have a higher likelyhood of gas building up to a dangerous level, and increased exposure for occupants.

At the other end of the scale, Type D buildings such as warehouses have large volumes and poorly sealed or frequently open doors, leading to lower concentrations of gas in the internal spaces. Building of these types are generally less continuously occupied.

This assessment produces a points score for the building and site combination, and this sets the level of gas protection measures that must be applied. The first area in which points are allocated is the base construction of the floor and substructure. A poorly sealed element such as beam and block floors will score no points, while 2.5 points are given for a fully tanked, waterproof basement.

The main issues here are in the middle case, slabs with “minimal penetrations”. The degree of acceptable penetration is poorly defined by the standard, leaving it largely up to the designer what points score is acceptable.

Further points are awarded based on the underslab ventilation provided, which can be either passive or active.

The most basic form of ventilation is a pressure relief pathway, but little guidance is available on the degree of performance and robustness required. Beyond this are passive dispersal layers, which are classed as either good or very good based on how well the gas is dispersed within the layer.

The passive systems rely on gas pressure, atmospheric pressure and wind pressure to drive the gas flow out of the underfloor spaces. If properly designed and maintained, the passive nature of these systems makes them less prone to failure than active systems.

Active systems use powered fans to removed gas, which in theory makes the systems more consistent and potentially more effective at removing gas. but problems can occur if fans are switched off for any reason, such as during change or tenancy or sale. For this reason the points associated with active systems were reduced by the 2018 amendments so that in most cases additional passive protection would be required.

Finally points can be scored by a properly specified and installed gas barrier membrane. Again the guidance has been made tougher with the new revisions, and in order to score points the barrier membrane must fulfil all the listed criteria.

This ensures all membranes used on site are of sufficient quality and are properly installed. Under the old guidance barrier membrane could score points under BS8485 without providing any actual protection in practice. This could arise as much from damaged membranes or from poor workmanship sealing joints as from low quality material.

NHBC Traffic Lights

The NHBC uses a traffic light systems to remove ambiguity form the classifcation and streamline the process. This is not intended to remove the need for specialist assessment, but rather to simplify the implementation of measures based on their findings. A “green” site is roughly analogous to CS1, and no special protection measures are required.

Amber 1 sites site align with CS2 and can be dealt with by using a simple membrane and passive subfloor ventilation system, as the risk is comparatively low.

Amber 2/CS3 sites being higher risks require a specialist contractor installed membrane with additional verification of the membrane integrity.

Red sites are generally considered unsuitable for private housing developments, and if built on at all, will required a more in depth and specialised approach.

A requirement of BS8485 is that the membrane installation be verified in accordance with Ciria 735, another important document for designers dealing with gas protection.

Ciria 735

Released in 2014, C735 details requirements for validation and inspection, and means that more or less every gas protection installation will require fully independent inspection and verification of at least some percentage of the development.

This Ciria document also details the types of testing that can be undertaken, form simple visual inspections through to air lance and tracer gas testing. Not all of these testing methods are suitable for every site, and these limitations discussed in the Ciria documentation, making it well worth reading for anyone involved in these types of projects.

The purpose of mandating this testing is to ensure the systems as installed will perform as designed and ensuring that performance is maintained throughout the design life of the installation.

The requirement for inspection can be reduced if NQV Level 2 installers undertake the job, so in this way Ciria 735 aims to upskill the groundworks installation industry by reducing the associated costs of verification and inspection to offset the expense of installer training.

VOC Contamination/Ciria 748&716

A further consideration, dealt with primarily via Ciria documents, is VOC contamination.

VOCs or volatile organic compounds are defined in CIRIA 716 as: "organic compounds that are volatile under ‘normal’ environmental/atmospheric condition. They may be found in the ground in the solid,liquid and dissolved phase form as well as in gaseous phase”.

VOCs can be both man made or natural and have a wide range of applications and sources, being found in everything from paints and coatings to plastics and cleaning products. Hydrocarbon fuels such as petrol and diesel are also forms of VOC.

When specifying geomembranes for VOC applications, it’s important to remember that chemical resistance and vapour permeability are not the same thing, and membranes that are highly resistant to liquid VOCs may not be suitable in a VOC vapour barrier application. Conversely, membranes which resist VOC vapours effectively may lack the resistance to be used in the presence of liquid hydrocarbons.

That’s not to say the chemical resistance is not important, as the barrier must be able to resist exposure to adverse conditions in the longer term without negative effects to its performance or mechanical properties. It can’t always be guaranteed that the VOCs will remain a vapour as water table may rise and bring the membrane into contact with VOCs in liquid form.

The actual VOC resistance is only one facet of membrane performance that must be considered, the Ciria 748 document gives important guidance on the specification of membranes used on VOC contaminated sites..

Membranes must also be able to resist degradation processes such as oxidation, particularly if they are based around foils. Once again, there is a series of linkages that must be considered as the basis of good robust design.

Foil membranes are often considered to be a perfect solution as they offer high resistance to all types of vapour, but it must also be taken into consideration that the foil responsible for this resistance can degrade or breakdown over time.

Its not uncommon for a membrane to pass the vapour permeation tests then subsequently fail due to degradation association with oxidation where vapour has permeated through the cut edges of the membrane.

As part of an appropriate overall design, a membrane of lower resistance to a liquid contaminant may still be suitable provided exposure to that liquid form is minimised, for example by means of source remediation or by ensuring the barrier is located in the structure such that contact is impossible.

Membrane Installation

We’ll now move on to take a look at some common installation situations, and the difficulties that can arise with each.

We’ll begin with corner details, which are probably the most common detail, present on every site. This corner has been cut and resealed using flashing, but has been left unsealed at the corner. The membrane has also not been pushed into place properly, so any subsequent concrete or screed pours will overstress the membrane which may lead to tearing and subsequent gas ingress.

In this case the membrane has not been extended to close the cavity wall, leaving open a pathway for the gas. The membrane is also roughly folded into place, which may cause problems when it comes to installing further courses of brick/blockwork.

The most reliable and time efficient method of getting a good corner joint is to use preformed corner units. In most cases however, these corners will need to be adapted to fit precisely into place.

The important part of the corner, the integrity of the actual corner point itself can be maintained by carefully cutting the corner unit across the middle and sealing both the top and bottom sections into place as shown here.

This ensures the corner area remains fully gas-tight. Another detail found everywhere is the simple membrane joint. Apparently simple enough, joints between adjacent runs of membrane commonly cause problems The most obvious problem is missing sections of sealing tape, creating a path for gas to bypass the barrier, but tape joints can cause other problems.

Here we can see where the tape run has not adhered to the membrane where the membrane is not sitting flat on the substrate. The most effective method, and that used by the majority of NVQ2 qualified installers is to weld membrane joints. This not only saves on the cost of tape, but if far more tolerant of weather and site conditions as tape ahdesion is not an issue. Successful tape jointing requires a clean and dry membrane surface to joint, and temperatures within the tapes operating range, typically 5-30 degrees. Assuming these conditions are met however the optimal procedure for a successful taped joint is shown here. We developed this detail in response to the guidance regarding site inspections. By moving the butyl tape joints to the edge of the lap it becomes far easier to ensure the joint has been done correctly.

This positioning of the tape also ensure any concrete or screed poured on top of the membrane cannot enter the joint and put additional pressure on the integrity of the joint.

Detailing around stanchions is little more difficult, but does not pose too may problems if the method shown here is used. The most important thing to note here is that the flashing tape used is of good quality and matched to the membrane used. As manufacturers we recommend and supply tapes that we know to work well with our membrane systems, and this advice should be followed.

The flashing tape should be rollered into place without leaving gaps underneath. Flashing tapes are usually less resistant to elongation than membranes, so it’s important these areas are fully adhered to a properly supported substrate to limit the potential for movement.

The rounded corners shown here are also important, as these are less likely to lift than a sharp corner and put less stress on the membrane and tape interface. Although it may seem obviously, it’s also important to ensure the gas tight layer is continuous. If as shown here the stanchion is in the centre of a concrete column or pile cap, the entire area must be sealed.

Pipe penetrations are another area it’s important to get right as they are a common feature on many sites.

These images shown of the more common problems that can occur. In the first case the supplied top hats did not fit properly so the installers have got a little creative. While it’s possible this detail may work to provide a gas tight seal, it’s by no means a given, and would be almost impossible to verify.

In the second, the smaller pipe appears to have been relatively well sealed, but the larger is not sealed at all, and corresponds to a lap in the membrane, creating additional complications. The close proximity of the pipes to each other also makes detailing and sealing using proprietary top hats difficult.

Here we can see the correct procedure to provide a gas tight seal to a pipe penetration, as with columns, this is not particularly difficult to achieve. Again as with stanchions, ensuing the membrane is cut as little possible, and the top hat is smoothly seated onto the membrane is critical. The flashing tapes should be firmly adhered in place and properly supported.

The general conditions around a site are often as important as specific details when it comes to validation of membrane installations In these images you can see that patches have been applied to the membrane haphazardly and messily. The marker spray is still visible around patches, indicating that the applied patches are too small to give the required overlap.

In this case we can see the rebar laid directly onto the membrane with no protections fleece or board used. The fleece is not only important to prevent direct puncturing of the membrane, but also to prevent damage from cutting using saws and grinders.

These processes produce large amounts of sparks which, if they contact the membrane can melt holes and cause significant damage. So while a small increase in cost per square metre to specify a fleece or protection board may seem unnecessary the effect it can have in reducing remediation cost and delays is more than worth it in most cases.

Membrane Systems

The choice of material used on site must be informed by both the permeability, chemical resistance if applicable, physical properties, and expected installation procedure.

All the possible membrane types in gas protection have pro and cons, and it’s important the properties of the material selected adequately address the risk factors present on a given site. When it comes to building on contaminated land, there are no magic bullets or one size fits all solutions.

So now lets look at some of the most commonly used geomembrane types.

Polypropylene or PP

PP membranes have good elongation characteristics, and are highly flexible. This makes them a good choice where settlement is likely or where complex detailing must be undertaken.

PP is however relatively permeable to most gases and is expensive. PP membranes also have poor adhesion characteristics and therefore require specialist welded joints rather than tape jointing.

PP membranes are traditionally used in water containment applications, due mainly to its suitability for use on larger unreinforced areas.

High Density Polyethylene (HDPE) HDPE is cheaper, and offers good resistance to puncturing, but in comparison to PP is rigid and therefore both difficult to install on complex details and prone to stress cracking.
Any bending of the membrane to angles over 60 degrees is typically not recommended due to it’s detrimental effect of performance. Because of this mechanical weakness, HDPE is better suited to large uninterrupted floor areas, which is a good fit with the roll sizes they are typically supplied in.

Like PP, HDPE membranes should not be tape jointed and require specialist welding contractors and verification.

Low Density Polyethylene (LDPE)

LDPE membranes are “standard” DPM materials, which are cheap, easy to handle and readily available, but can be low-quality and are generally not suited for gas resistance applications due to their high permeability and limited resistance to damage and UV.

LDPE membranes resistance to VOC vapour transfer is particularly poor, and these membranes can degrade rapidly if directly exposed to chemical contamination.

Low grade LDPE can also have problems with manufacturing tolerances, resulting in a material that while water resistance if undamaged, will not deliver consistent and robust performance in gas protection applications.

Reinforced Virgin LDPE

Reinforced membranes made from higher quality virgin LDPE are inherently more consistent as good control of thickness and material tolerance is critical for achieving a good lamination with reinforcement. This reinforcement also greatly improves the membranes resistance to tearing and damage while allowing the membrane to retain its flexibility.

This results in a good balance of durability, performance, ease of installation and cost. It is however important when using these membranes to have a good understanding of the risks present on-site to ensure the vapour or gas permeability of the membrane is adequate for the application.

Foil Membranes

Adding a foil core to LDPE or HDPE membranes dramatically increases the gas and vapour resistance of the membrane without a correspondingly dramatic cost increase. This makes them a good option where elevated risks are present and it’s not feasible or desirable to use additional techniques such as source remediation to lower the concentrations present. There is a common misconception that because the membrane has an aluminum foil in it’s build up that this will offer good all round protection from contaminants. While that may be true elongation and foil degredation can be issues.

While the base plastic membranes are able to elongate to some extent, this is not true of the foil layer within them. If settlement occurs the corresponding stretching of the membrane can rupture the foils leading to a significant deterioration in gas barrier performance.

It's therefore critical when using foil based membranes to make sure settlement is minimised, and expansion joints are properly detailed to avoid overstressing the membrane.

Foil membranes such as the A. Proctor Group’s Protech GM Super are generally considered a better fit for common ground gas applications such as methane and landfill gases, when dealing with VOC contamination a dedicated membrane such as Protech VOC Flex will offer a more comprehensive solution.

Specialist Film Laminate Membranes These types of membranes are similar to foil core membranes as above, but replace the foil with a specialised plastic film such as EVOH. This allows the performance of the membrane to be adapted to meet application specific barrier requirements, so these membrane are typically more common where VOC or chemical contaminants are present. EVOH membranes offer good resistance to a variety of VOC contaminants while still allowing the membrane to retain good physical performance characteristics such as flexibility and elongation.

Liquid Applied Membranes

Liquid applied membranes are ideal for complicated foundation detailing and uneven surfaces. The downside to these systems is that it’s not always easy to determine the tolerance of application, such as continuity or thickness of coating.

Liquid applied systems do however make a good companion to sheet membranes where complex details must be sealed


So we’ve seen throughout todays presentation that the most important factor in dealing with contaminated sites is good design based on good information. Whether it’s conducting a thorough investigation into the historical uses of the site or ensuring the correct membrane for the job is well installed by contractors, a good understanding of the risk factors involved and a well developed holistic remediation strategy is key to achieving a safe and good value solution.

Technical Support Services

With over 20 years experience in ground gas protection, the A. Proctor Group have always taken this approach, supplying both a range of fully tested membranes, and comprehensive technical support and design consultation throughout the specification and installation process.

Perhaps the most important aspect of this is providing hands on assistance with detailing of membranes and the production of drawings to allow correct installations to take place.

We also provide a full library of BIM data and template constructions for all our products, available in both Revit and IFC formats for compatibility with a wide range of software and systems.

Finally our on-site toolbox talks ensure the entire project team is confident working with our materials and has a full understanding of how they work and the benefits they bring to the projects.

That concludes our webinar, many thanks for your time.

This Webinar Includes
  • The sort of contaminants that need to be considered – including Methane, Radon & CO2
  • The impact on building occupants
  • The relevant legislation – including CIRIA 665, 735, 748, 716 BS8485, NHBC Traffic Lights
  • VOC contamination
  • Correct membrane installation, including jointing, corners, stanchions & penetrations
  • Gas protection systems available & their relevant applications