Ground Gas Protection

Protecting Buildings On Radon Affected Land

As part of its objective of constructing 300,000 new homes per year, in late 2020, the UK government issued reforms to fasttrack planning for brownfield land regeneration.

The A. Proctor Group has over twenty years of experience in providing solutions for the safe development of brownfield land and an extensive range of products and solutions that will cater for most scenarios from typical ground gases, including Radon.

Radon is a naturally occurring, colourless and odourless radioactive gas. It was discovered in 1900 by the German physicist Friedrich Ernst Dorn, who was studying the radioactive decay of Radium, itself only discovered by Marie Curie two years earlier. This work was expanded by the Nobel prizewinning Scottish chemist Sir William Ramsey, who identified Radon as the heaviest gaseous element, 9 times heavier than air:

In Ground Gas terms, Radon is produced by the radioactive decay of trace amounts of uranium found in specific soil and rock types. Therefore, the site geology determines the level of Radon present as opposed to historical usage or pollution. This type of geology is most common in areas with granite bedrock, such as Devon, Cornwall, the Peak District, Northamptonshire, south Wales and northeast Scotland.

Radon is harmful to humans as the radioactive elements formed by its decay can be inhaled into the lungs, where they continue to emit harmful alpha particles. These alpha particles cause localised damage to the lung tissue, leading to the development of lung cancer. The British Medical Journal estimates around 1100 deaths per year are directly linked to in-home Radon poisoning, making Radon the second most significant contributor to lung cancer rates behind smoking. Legislation requiring action against Radon in UK buildings has been in place since 1985, initially for workplaces, but since expanded to include housing.

Radon in Buildings

The average UK household radon level is 20 Bq/m³. The current “action level” for residential buildings is 200Bq/m³, and for commercial buildings, it is 300Bq/m³. If the Radon level is at or above the action level, protective measures must be taken to reduce the ingress of the Radon.

Some sources suggest at least 1200 new build homes per year will be above the safe level of exposure and should have remedial action taken. In addition, the risk of potential exposure to Radon has led to suggestions that building regulations should be updated to include more widespread Radon protection.

The first step to ensuring unsafe concentrations of Radon is not present is understanding how the gas enters buildings. There are several main routes of ingress for Radon or any other ground gas. These include cracked or porous solid floors, junctions and joints between components, cracked or porous below-ground walls such as in basements, gaps in suspended floors, porous walls and foundations, gaps at service penetrations and pipes and through cavity walls

Due to the geological origins of Radon gas, there is generally no scope to remove or neutralise the source of the gas, so barrier and venting solutions are most commonly employed. However, limiting the extent of the ingress paths here can help by reducing the long-term reliance on a barrier and providing additional protection.

Guidance on Radon

The principal source of Radon guidance in the UK is the BRE document “BR211 Radon: Guidance on protective measures for new buildings”. This document is referenced in Approved Document C for England and Wales, Section 3 in Scotland and Technical Booklet C in Northern Ireland. There is also extensive guidance available from the UK Health Security Agency’s website, www.ukradon.org. The website offers help and advice for householders, employers, professionals, local authorities and housing associations.

The distribution of Radon producing geology is illustrated in Radon maps distributed by the UK Health Security Agency for the UK and the Irish Environmental Protection Agency. These maps indicate the Radon risks associated with a specific location.

The basic mapping will typically provide Radon levels based on a 1km by 1km grid. For most areas of the UK and Ireland more detailed geographic guidance is available on request. In the UK, the lead agency responsible for this is the UK Health Security Agency, which also provides this information for Scotland, Wales and Northern Ireland. In addition, the Irish Environmental Protection Agency can provide guidance specific to the Republic of Ireland.

Low-risk areas with less than a 1% chance of exceeding action levels do not require any special measures to be taken. Sites between 1 and 10% are classed as medium risk and require a basic level of protection, with sites over 10% being high risk and requiring full protective measures. Basic protection typically comprises a Radon barrier membrane within the floor or solum of the building, linked to a damp proof course in the wall. This barrier system should extend across the entire footprint of the building, including the cavity, and all overlaps and service penetrations should be fully sealed. Full protection requires a Radon barrier membrane but supplements this with additional ventilation to increase the rate of gas exfiltration. The ventilation can be a ventilated void. If there is no void, then a Radon sump should be installed. Ventilation systems are usually passive, with the option to convert to active by adding mechanical extraction fans. Active ventilation is only necessary if Radon levels are above the action level, even with a barrier membrane and passive ventilation.

Selecting and Installing a Barrier Membrane System

Ensuring a barrier membrane system works effectively requires the material used to be suitably robust and the system to be well installed. Like other ground gas protection measures, the system is only as good as its installation, and if joints are not properly sealed or the membrane is punctured, then adequate performance cannot be guaranteed. However, a properly designed and fitted barrier will also function as a damp-proofing membrane. The first point to consider here is the specification of the membrane itself. The BBA requires Radon barrier membranes to be at least 0.4mm, or 400 microns in thickness. It’s important to remember, though, that membranes, if stretched, will become thinner, so ensuring that the material has good tensile strength and elongation characteristics will ensure that the membrane retains its properties

When installed under a slab, for example, concrete poured on top can exert forces on the material, causing it to stretch, and become thinner, creating a zone where the permeation of gas becomes more likely, even if the membrane is still intact and unbroken. This is the risk of using unreinforced DPM-style membranes for Radon protection. Reinforced membranes made from virgin material typically have the best mechanical strength properties thickness for thickness. The A. Proctor Group’s Protech Radon 400 membrane is manufactured from 100% virgin polymer with a polyester reinforcing scrim to provide excellent robustness and minimal elongation under load.

Once the membrane is installed, it should be thoroughly inspected prior to covering, as fixing any issues after the membrane is covered is often effectively impossible. This inspection should look for continuity issues such as unsealed joints, unsealed penetrations such as waste pipes, and punctures. Finally, although it may seem obvious, it’s also essential that the protective membrane and any associated accessories, such as DPCs, extend across the entire footprint of the building, from external to external, closing off any potential gas ingress points. This includes closing off any cavity walls with a suitably sealed and gas-resistant DPC.

Ventilation Guidance

Where it’s necessary to move beyond a basic level of protection, ventilation should be introduced below the barrier membrane. Where there is a ventilated void, a block and beam system, for example, the void depth should be a minimum of 150mm. The void can be ventilated with periscopic vents and airbricks, ground-level gully vents or several other options depending on the requirements of the occupiers or developers.

Where there is no void, a ground bearing slab, for example, Radon sumps, should be installed to provide subfloor depressurisation. The sumps and pipework are installed under the slab. A single, centrally located sump will provide extraction over a radius of 9m from the sump, around 250m². If the area under the slab is subdivided, for example, with foundation beams or other compartmentalisation, a separate sump may be required for each compartment. As a minimum, pipes or air bricks in the subdivisions should be provided to ensure the sumps can provide adequate extraction. If multiple sumps are used, they can be connected with pipework to form a manifold system and ducted to a single extraction system.

The A. Proctor Group has a comprehensive range of venting accessories, from simple periscopic vents and airbricks to ground-level gully vents.

Protection Of Basements

As well the above-ground living space, it’s crucial that any basements or occupied underground spaces are protected, particularly as these areas are at greater risk as Radon can permeate not only through the floor but also through the retaining walls. For a typical domestic house, the areas in contact with the ground could be several times higher than for a similar house without a basement. This risk is further compounded by the lack of natural ventilation in spaces below ground level. Alongside this, it’s also important to consider waterproofing when creating a new basement or converting an existing cellar. Guidance and recommendations for basement waterproofing are given in BS8102, which also advises on the importance of considering Radon mitigation when designing basements. In these situations, it’s usually best to appoint a specialist to design and install the combined system, as Radon and waterproofing requirements can sometimes conflict.

Radon Protection For Extensions

While providing protection to an extension when there’s none in the existing building may seem pointless, a subfloor depressurisation system can help provide some degree of protection to the existing structure. Space for a sump under the existing structure can be created by locating the new sump and extraction pipework adjacent to the existing structure and knocking through the existing wall. The new depressurisation system can therefore extract from the fully protected extension and the existing unprotected structure via the two linked sumps.

The A. Proctor Group’s experienced technical support team can assist with the specification of Radon protection systems, reviewing drawings and site investigations to ensure a suitable and safe solution optimised to fit the individual project circumstances.

Protech VOC Flex - East Calder

The new development of 2 residential properties at Lime Kilns, East Calder in Scotland has been provided with a high-quality VOC protection barrier in the form of Protech VOC Flex from the A. Proctor Group.

Prior to construction and in line with the guidance and legislation for brownfield site developments, an investigation of the site revealed the presence of historic VOCs. In full compliance with CIRIA C735 and the guidelines laid out in the British Standard 8485:2015 (+ A1 2019) and CIRIA C748, the geomembrane was installed by the specialist gas membrane contractor Structureseal Services. As laid out in CIRIA C735 the VOC geomembrane was thermal heat welded and all joints air lance tested in accordance with ASTM D4437-08 2013 in conjunction with the third-party validation engineer. The thermal heat welded joints were carried out by approved NVQ Level 2 gas membrane installers, including the installation of the A Proctor Group’s Provoid 25 cuspated geocomposite subfloor venting blanket to form a passive dispersal layer in conjunction with Provoid ground mounted gully vents to the perimeter of the houses. This combined achieved a “very good performance” passive venting installation fully compliant with BS8485:2015 (+A1 2019) Table 6.

Martin Taylor, Commercial Director of Structureseal Services commented, “we have successfully used Protech VOC Flex on several VOC vapour protection projects. It offers high-performance protection in compliance with CIRIA C748, is extremely robust and has many benefits which make it easy to install on site. One of these being that when the geomembrane is folded the product has “memory”, which enhances the speed, finished appearance and quality of the overall installation.”

The A. Proctor Group has over twenty years of experience in providing solutions for the safe development of brownfield land, and a range of ground gas protection products to make the development of these sites safer and easier. Following extensive research and development, Protech VOC Flex was introduced as a high-class performance VOC membrane, part of the Group’s comprehensive range of ground gas protection products. Protech VOC Flex is a 6-layer flexible proprietary reinforced VOC gas barrier suitable for use on brownfield sites that require protection from dangerous contaminants.

The A. Proctor Group’s expert design and specification advice allowed the project at East Calder on behalf of Concept Completed, to achieve a fully compliant design for ground gas protection measures for the project. Working with developers, specifiers and contractors the A. Proctor Group can provide safe, cost-effective and value-engineered solutions. Technical services include CAD detailing and project-specific recommendations utilising the latest guidance and ensure compliance with current legislation and standards including the BS8485:2015 (+ A1 2019) and CIRIA C748. Site Investigation Reports and Gas Monitoring Results can be reviewed and specialist advice can be provided to ensure full compliance and correct product selection.

Is it true diluting gas below the slab?

In this series, the A. Proctor Group will examine some commonly held myths surrounding the contracting world concerning thermal and acoustic insulation, condensation control, fire protection, and ground gas protection. In this article, we ask the question, is it true that the only way to dilute gas below the slab is using pipes and gravel? The increasing focus on sustainably resolving the UK housing shortage continues to drive the need for the redevelopment of brownfield sites and achieving effective site remediation, mitigating risk and protecting building occupants from soil contamination.

The accepted industry methodology of determining the risk to an end-user of a building, in line with guidance and legislation, comprises ‘source,’ ‘pathway’ and ‘receptor.’ Current legislation and advisory documents stipulate levels of protection required, depending on the specific permanent ground gas or VOC risks associated with the site. Volatile Organic Compounds (VOCs) and naturally occurring gas (such as Radon, Carbon Dioxide and Methane) can enter buildings through:

• Gaps around service pipes
• Construction joints
• Wall cavities
• Cracks in walls and ground slabs

In most buildings, measures to protect against ground gas are constructed below the ground floor level. A permanent ground gas (or VOC) protection solution consists of several individual elements combined to form an integrated system. This is done to limit the reliance on one individual component. These components are separately designated concerning gas protection in BS8485:2015 + A1 2019 (permanent ground gas), CIRIA C748 (VOCs) and BRE 211 (Radon) as:

1. Structural barrier (floor & substructure design)
2. Ventilation protection (floor slab type)
3. Membranes
4. Monitoring and detection
5. Pathway intervention

Ventilation types (Active or Passive)

The importance of ventilation is crucial, whether it is through an active system or a passive system. An active system will require mechanical extraction to enable the gases to safely ventilate into the atmosphere. Active systems can be more costly to run, leading to increased energy costs for the operation of the building. In contrast, a passive system depends on pressure differential to ensure that the gases are safely removed from beneath the building.

A better way of venting the ground floor slab

On some sites, the traditional ventilation approach by constructing a layer of gravel in combination with a network of pipes for venting is still adopted. This may require significant excavation of the ground in readiness and a typical depth of 200 mm to 300 mm to accommodate the gravel and pipes.

Increasingly, contractors are installing Ground Gas Protection Systems designed using a high-performance geocomposite void former that provides an effective void beneath the floor slab. One example of this is the Provoid system specifically developed by the A. Proctor Group.

Provoid comprises a high-density polyethylene single cuspated core with a non-woven, needle punched polypropylene geotextile fabric bonded to one side. This provides an internal space for potential gas to flow through without itself occupying more than a minimal percentage of the internal void space. When connected to air inlets and outlets, it allows sufficient air changes to dilute gases to safe concentrations when designed correctly. Being a shallow product means there is a reduced dig compared to the alternative of 200 to 300mm of clean stone or gravel. When installed as a full blanket Provoid achieves 1.5 to 2.5 points as per table 6 of BS8485:2015+A1 2019. The Provoid membrane can be laid in strips at predetermined centres or in a full blanket, depending on site requirements.

Provoid is also extremely strong and flexible and can be supplied in rolls of up to 45m2; therefore, large areas can be covered quickly. The membrane can be laid horizontally and vertically to deal with awkward foundation arrangements. Because of its flexibility, it will cope easily with settlement under the slab without compromising the system. In combination with the membrane, the Provoid Gully Vent Box is designed to effectively disperse passively vented gas safely into the atmosphere. The vents should be positioned at the building perimeter at centres to suit both the build and the gas regime onsite.

The A. Proctor Group offers specialist advice on Ground Gas Protection, including site investigation reports, gas monitoring results and full system design in line with CIRIA, NHBC and Building Regulations.