Protecting Buildings On Radon Affected Land

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 prize winning Scottish chemist Sir William Ramsey, who identified Radon as the heaviest gaseous element, 9 times heavier than air.

In ground gas terms, most radon is produced by the radioactive decay of trace amounts of uranium found in certain types of soil and rock, and as such the levels of gas present are determined by the underlying geology of the site as opposed to any historical usage or pollution.

This type of geology is most common in areas with granite bedrock such as south west England, the Peak District, north east Scotland and Northamptonshire. Aberdeen in Scotland, being known as the “granite city” has one of the highest background radiation levels in the UK.

Regardless of geology though, we are all exposed to a certain degree of radiation from a wide range of background sources both natural and man-made. The radon level in an average UK home contributes about half our typical exposure, and this level is quantified in Becquerels per cubic metre.

Legislation requiring action against radon in UK building has been in place since 1985, initially for workplaces but since expanded to include housing.

The average household radon level is 20 Bq/m3, and even at levels up to 100Bq/m3 individual risk remains relatively low, however if the level goes above 200Bq/m3 then remedial action is required.

Radon is harmful to human 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 largest contributors to lung cancer rates behind smoking.

RADON IN BUILDINGS

Some sources indicate at least 1200 new build homes per year will be above the safe level of exposure and should have remedial action taken. This has in turn led to suggestions that building regulations may need to be updated to include more widespread radon protection.

The first step to ensuring an unsafe concentration of radon is not reached is to understand how the gas enters buildings. There are seven main routes of ingress for radon, or for that matter any other ground gas.

• Through cracked or porous solid floors.
• Through junctions and joints between components.
• Through cracked or porous below ground walls such as in basements.
• Through gaps in suspended floors.
• Through porous walls and foundations.
• Through gaps at service penetrations and pipes
• and through cavity walls.

Because of the geological origins of radon gas, there is generally very little scope to remove or neutralise the source of the gas, so barrier and venting solutions are most commonly employed. Limiting the extent of the ingress paths here can however help by reducing the long term reliance on a barrier, and providing additional protection.

It’s also important from an energy efficiency point of view that airflow through the heated envelope is reduced as far as possible, so minimising the overall porosity of the building fabric should always be a priority.

RADON GUIDANCE

The principle sources of radon guidance in the UK is the BRE document 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 of Public Health England’s UKRadon.org website. There you can find help and guidance for householders, employers, professionals, local authorities and housing associations.

BRE RADON MAPPING

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

Its worth bearing in mind here though that there can be issues with the available resolution of the geological data. The basic mapping will typically provide radon levels based on a 1km by 1km grid. While it can be in a lot of cases be reasonably inferred that a “clear” grid square surrounded by other clear squares is unlikely pose much of a risk, in areas with more complex and varied geography it may be necessary to request a more detailed assessment of the local risk.

For most area of the UK and Ireland bodies more detailed geographic guidance is available on request. In the UK the lead agency responsible for this is Public Health England, who also provide this information for Scotland, Wales and Northern Ireland. The Irish Environmental Protection Agency can provide guidance specific to the Republic of Ireland.

This site specific “radon potential band” determines the level of assessment and protection required. The radon potential band percentage represents the chance of any given building in the area having a radon level at least as high as the required “action level” of 200Bq/m3

The Low risk areas with less than 1% chance of exceeding action levels do not require any special measure to be taken. Sites between 1 and 10% are classed as medium risk and require a basic level or protection, with site over 10% being high risk and require full protective measures.

On medium and high risk sites testing and monitoring are also advised, which again is due to the highly localised nature of radon requirements, particularly in areas where different classifications meet, or site which are on the borderline between risk categories.

We’ll move on to look at the various measure in detail shortly, but the most basic protection level 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 systems should extend across the entire footprint of the building, and all overlaps and service penetrations should be fully sealed.

Moving to full protection requires the same basic level barrier system, but supplements this with additional ventilation to increase the rate of gas exfiltration. Depending on the level of risk, and the precise configuration of the subfloor and foundations, this ventilation system may be active or passive, and may require the use of sumps and/or mechanical extract fans.

PROTECTIVE MEASURES

We’ll now take a look at some of the protective measures in more detail. While it’s certainly true that there are less variable to consider with radon protection than with some other types of ground gas or site contamination, it’s still important ensure the system is well designed and fit for purpose.

MEMBRANE SYSTEMS

Ensuring a barrier membrane system works effectively require 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. A properly designed and fitted barrier will however 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 be at least 0.4mm, or 400 microns, thick. It’s important to remember though that membranes, if stretched, will become thinner, so ensuring the material can good tensile strength and elongation characteristics will make sure that the membrane retains it’s 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.

Reinforced membranes made from virgin material typically have the best mechanical strength properties thickness for thickness. As while a thicker membrane can certainly work adequately, the additional weight and thickness can make detailing and handling more difficult.

Joints in the membrane can be either taped or welded, however in either case appointing experienced and suitably qualified contractors is recommended to ensure a good result is achieve first time without requiring remedial action to fix any issue arising from poor workmanship.

However 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 any continuity issues such as unsealed joints, punctures or areas when the membrane is not adequately supported, for example at boundaries where voids may cause excess elongation.

Seals at all service penetrations should also be inspected, as these details can be problematic onsite, particularly if pipes are location near foundations, or if installation took place under less than optimal weather conditions.

Finally, although it may seem obvious, it’s also important 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 material.

VENTILATION

Where it’s necessary to move beyond a basic level of protection, ventilation should be introduced below the barrier membrane.

This example shows a typical beam and block floor construction with a subfloor void, and the the gas resistant membrane placed over the beam and block subfloor, then screeded over.

There are a number of systems and solutions for introducing ventilation to this subfloor area, such as ground level vent boxes, telescopic vents though external walls, and simple air bricks. Exactly which of these solutions is most appropriate depends on the specific building envelope, and relative locations of ground and finished floor levels. Provided the vent openings have sufficient open area to deliver the airflow required they will all do a similar job.

Where to position these openings to effectively negate gas build-up, depends to a certain extent on the layout of the foundations, and any supporting structure. While the best course of action is to discuss the size and location of vent openings with a specialist, there are some simple rules we can apply as initial guidance.

Based on NHBC guidelines, vent openings should typically be placed at a maximum of 2m centres along external walls and not more than 450mm from corners. The relationship between voids separated by ground beams should also be considered.

In ground bearing slab applications where full radon protection is required, it’s necessary to provide radon sumps to facilitate subfloor depressurisation. The sumps and pipework are installed under the slab, with gas collected by the sump and ducted to an extract point above ground.

This enables a subfloor depressurisation system to be installed with relative ease at a later date should it be deemed necessary post completion.

In a typical house, a single central sump is normally sufficient, and will provide extraction over a radius of 9m from the sump, around 250m2. If the soils is particularly permeable, this may increase to a 15m radius, but this would require project specific assessment.

If the area under the slab is subdivided, for example with foundation beam or other compartmentalization, a separate sump may be required for each compartment depending on soil conditions. At the 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 a manifold systems and ducted to a single extraction system.

This extraction system creates the depressurisation that removes the gas. Most commonly this is a powered, or active system where an electric fan is used to drive the extraction. While it’s possible to use a passive system without a fan, active systems are more common.

This is because the depressurisation aspect of the ventilation system, is generally only used if post completion monitoring shows it is required. If this is the case, a fan can be easily attached to the built in pipework to provide this.

Passive systems typically require more forethought and complex pipework taken though the building to high level extract points to create a passive stack effect.

In either case however, it must be ensured that gas is extracted to a safe location away form opening windows and ventilation system intakes, with all pipe joints solvent welded rather than push fitted.

If a powered fan is to be used, it should be located as close to the end of the pipe run as possible to ensure the pipework is kept under suction, in order to prevent any gas leakage in unintended area. This is particular important if pipework is to be run through the living spaces in a dwelling.

Whatever the construction, if ventilation is required , it’s important to ensure it’s done right, as systems which are ineffective can lead to high gas reading and complex remediation. Conversely, an over engineered system can increase timescales and costs with no particular benefit.

At the A. Proctor Group our experience technical support team can assist in the specification of such systems, reviewing drawing and site investigations to ensure a suitable solutions is specified, optimised to fit the individual project circumstances.

RADON PROTECTION FOR EXTENSIONS

While it may seem pointless to provide protection to an extension when there’s none in the existing building, a subfloor depressurisation system can help provide some degree of protection to the existing structure.

By locating the new sump and extraction pipework adjacent to the existing structure, and knocking through the existing wall, space for a sump under the existing structure can be created. The new depressurisation system can therefore extract form both the fully protected extension, and also form the existing unprotected structure via the two linked sumps.

PROTECTION OF BASEMENTS

As well the above ground living space, it’s important that any basements or occupied underground spaces are protected, particularly as these areas are at greater risk as gas can permeate not only through the floor but also through the retaining walls.

For a typical domestic house, this means 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 available 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 relating to basement waterproofing are given in BS8102:2009, 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.

MONITORING & TESTING

If its necessary to conduct monitoring of radon levels in a home, this is typically done by using detectors supplied by Public Health England. These detectors are placed in the home for at least a three month period to assess the level of radon exposure for the occupants.

PHE typically recommend a year of monitoring for new homes in radon affected areas, and this data can be used to determine if active measures such as fans are required or if the basic level of protection is sufficient.

It’s typically not required for extensions, however if substantial structural work is undertaken or the dwelling is in a particularly at risk area it is worthwhile considering.

To conduct the testing, PHE supplied radon monitoring devices are placed in representative area of the occupied spaces then return to PHE for assessment at the end of the testing period. Generally the detectors are placed in areas like tv stands or bedside tables to give an accurate impression of occupant exposure.

They should not be placed in areas which or close to ventilation or windows, or in places away from the main inhabited zones in the dwelling, for example on top of a high wardrobe or on the floor.

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’ve discussed this before in our webinar on brownfield sites, but installation quality is a critical part part of ground gas protection, so it’s worth repeating here.

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 over stress 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/block work.

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 adhesion 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 structural reinforcing bars laid directly onto the membrane with no protection fleece or board used. This protection 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.