Use of radon sumps or sub-slab depressurisation (SSD) is one of the most common methods of reducing radon levels in buildings. The term SSD is used more in North American literature, and fairly represents how the systems work in many American and Canadian houses.

Early expectations for performance were centred around the idea that reversal of pressure difference across the entire floor area would prevent entry of most or all radon gas emanating from the ground. In ideal situations this is exactly what happens. One or more small suction points can be drilled through a concrete basement floor slab, and using only a small fan (30 to 50 watts power) a negative pressure of a few Pa can be maintained around the perimeter of the slab. Since radon entry is often associated only with the edge cracks in full-house concrete basements, the importance attached to pressure reversal in these areas continues to be relevant, in some cases.

In houses that seem almost purpose-designed for SSD systems, a high effectiveness of radon remediation (90 to 95%) can be achieved routinely.

Despite the simplicity of these systems, problems arise as a consequence of air being sucked from the interior of the house or basement. The location of these sources has proved crucially important in understanding detailed system behaviour.

In North America also, some houses were found to be of less than ideal construction. Two key problems are lack of an extensive hard-core layer and the presence of cross-walls. The usual solutions are still a combination of more suction points (16 in one large house, although probably far fewer would have sufficed) and use of higher power fans. In extreme cases of low ground permeability 'vortex blowers' or pressure generating fans have been utilised. These may have power ratings exceeding 200 watts, and problems of noise and running cost have arisen.

In contrast, some early test systems used in houses built on highly permeable ground gave equal or better performance when used with the fan reversed - thus acting to pressurise the ground under the house. However, diagnosis might support the view that the effectiveness owed more to ground dilution than to any pressure effect across floor slabs. Similar arguments can be applied to area-cure systems as reported from Sweden (see Section 60). However, pressurisation systems are not universally successful in these circumstances, and different houses may give different answers depending probably in part upon the closeness of the mitigation system to areas of high local radon entry potential.

More recently, three other factors have begun to concern designers of SSD systems for non-ideal houses. Some of the detailed experience was obtained in the UK. The three factors are:

1. that pressure field extension could not be attained to slab edges, but that nevertheless often the systems worked well,

2. that sometimes the systems did not work and especially not in rooms away from the main (often sole) sump,

3. that leaving edges of rooms unsealed sometimes appeared to produce a better result in some rooms than did the more usual practice of sealing all visible cracks and gaps in the floor.

Further factors have included impaired performance of solid fuel fires served by underfloor air vents (in the UK these are often known by a proprietary name, the Baxi). A more common problem is that noise levels from fans and exhaust points have proved noticeable in quiet rural situations.

Already therefore it is clear that there cannot be any unique most appropriate design for an SSD or radon sump system. This is particularly so in the UK, where variability of house designs and layout are more marked than in standardised 'concrete box basement' houses in the USA.

Design parameters range between wide limits. Examples from the UK are detailed in Section 60.

 Fan sizing:

Power requirements can range from 10 watts (a fan small enough to fit in the palm of ones hand) up to the usual 65 to 75 watts of in-line duct fans, and beyond to large pressure developing fans of several hundred watts power and producing many hundreds of pascal depressurisation at low flow rates. Energy costs are typically £7 per year for each 10 watts.

Historically, and on balance, fan sizing appears to have been over-generous, probably because of the (false) assumption that overall depressurisation had to be achieved and because it has been assumed that an ineffective system could be made more effective by increasing the speed or number of fans. Often, only marginal benefits accrued, depending very much upon the source of inlet air.

In ideal situations, very small fans may produce substantial reductions in indoor radon concentration, but unfortunately such conditions are found infrequently in high-level and old UK houses. This problem is likely to limit the use of passive stack vents also (see Section 63).

It seems unlikely that design rules in terms of 'watts per square metre' or similar parameters will ever prove useful. Ideally, fan sizing would be a secondary parameter - to be determined only once basic information about the building and its entry routes had been obtained. In cases where little or no diagnostics is undertaken, multispeed fans may be recommended, if only for a test period. If a low speed proves satisfactory, a smaller fan may be substituted in those cases where appearance is important.

High suction fans should be installed only with care and in situations where diagnostics indicates extensive and low permeability and with few short-circuits. Here they can be effective, and the higher running costs may be justified. Use of these fans to serve an area of floor where there are high radon levels and low permeability may prove disappointing if a higher entry potential exists elsewhere, but at lower radon concentration. Assessment only of radon concentrations underfloor can prove misleading, especially in 'mixed floor' houses.

Sump size:

Successful systems have used very small sumps (simply setting a 110 mm pipe into an existing hard-core layer, and with little or no attempt to form a void beneath the slab) up to about 10 m³. The latter was not designed as a radon sump, but was found beneath a house in the UK when the floor was excavated on the author's instructions. It had been a rain water store in years past, but had fallen into disuse.

Radon sumps in typical houses and where a large depth of hard-core is present need only be small. Preferably they should be centred upon an area of high entry potential (see Section 59) but these are sometimes difficult to detect, not present or inconvenient to address. Increasing the size of a radon sump will not substantially improve performance except where the limiting factor is low permeability in the neighbourhood of the suction point. In these cases multiple small sumps or edge suction can work better. Care needs to be taken when forming large sumps beneath thin old concrete floors, unless excavation and construction of an old-style 'BRE' sump is specified. These bricks and paving-slab designs are entirely adequate but often unnecessarily large. In situations of infinite source (see Section 59) they are as ineffective as any other design.

Care would also be needed if using high pressure water jets for extending pressure field. The technique has been used with some success in the USA but not under thin and structurally inadequate floors.

Pipe sizing:

Almost universally, pipe sizing in radon systems is not a matter of design. It is a matter of purchasing 110 mm upvc pipe from a builders merchant, and for two good reasons: this is usually in stock, and has been known to work well in the past. This approach actually has much to commend it, as detailed design would require system diagnostics and design of a standard unlikely to be cost-effective for all but the worst affected and difficult-to-cure houses. Nevertheless, it may be mentioned that smaller pipe (50 mm diameter) or rectangular duct work can be satisfactory in systems where low flow rates are predicted by diagnostics. In some houses this can prove much easier to install.

Again, no design rules exist that are applicable to every house. In the USA emphasis was given in many systems to ensuring that depressurisation reached all points of the floor slab. Indeed, diagnostics equipment is still advertised on the basis of the need to ensure such complete 'coverage' at time of system commissioning. It has proven unnecessary to achieve this.

Another 'rule of thumb' is one sump per 250 mBq/m², but derives solely from limited experience in schools where often there was a larger than normal hard-core layer, and with few cross walls because of the room sizes. In some small houses, even a large sump has proved inadequate. It is accepted that such 'rules of thumb' should not be relied upon to the extent of being included in design guidance for disparate existing houses. Unfortunately however, once promulgated they persist, and especially if incorporated in guidance from organisations whose past standards of work lend credence to present output.

The problems here are not centred upon the correctness of any arbitrary rule but (simply) upon recognition that radon sump systems can behave in wholly different ways depending upon the house design and construction and upon the underlying ground.

Design rules as existing at present may appear so generalised as to be unhelpful, but this is not the case. There is little point in trying to produce fine-structure designs for systems that by their nature in most real situations will always include an element of 'try it and see'. Additionally, given that components are available only in discrete sizes (50 or 110 mm pipe for example) it would be of little use for a computerised design programme to specify 68.5 mm.

A number of facts are known with certainty:

In houses where the design is ideally suited for an SSD system usually only one small suction point and a small fan may be needed.

To some extent, but without bothering too much with marginal areas, fan size and running speed may be selected either on the basis of vacuum diagnostic tests (see Section 59) or once the fan has been installed. This is an attractive option if multispeed fans are used, especially as at low speeds large fans are commendably quiet, but running costs can be greater than for a smaller fan running at nearer its rated speed.

It is often unnecessary to ensure depressurisation across full slab areas.

The reasons for this include that if key entry sources can be addressed, residual levels may be low enough in principal rooms and of little concern elsewhere. Also, many SSD systems operate not only by reducing pressure underfloor but by lowering radon levels in the ground immediately below the house, but depending much upon air flow pathways.

The problems of determining exactly what is likely to happen in real houses (as opposed to idealised test cells such as are in use at research establishments) are such that detailed diagnosis is unlikely ever to form a part of commercial radon remediation. The exception is where an experienced consultant can be employed.

In houses with solid floors, underfloor radon levels may reduce markedly when a system is installed, may increase, or may remain largely unchanged. There is no simple correlation here with flow rate, since air may be drawn to the fan either from deep underground (and with a maintained high radon level) or essentially from outdoors, but possibly via indoors. Radon concentrations in exhaust streams can provide some guidance as to what is happening, but use of tracer gas equipment is preferred.

To some extent the problems here mirror the difficulties of predicting what will happen when timber floors are depressurised: so much depends on the source of air and on localised entry potentials.

In houses having extensive cross walls it may be necessary to use multiple suction points.

The need for such complications does not depend necessarily upon the starting radon level or indeed upon the results of simple diagnostics, because of 'ground dilution' effects. Also it is relevant to question whether all of the house area needs to be 'cured', given that some rooms may be used only infrequently, and can be kept essentially isolated from the rest of the house by use of internal doors. An analogy would be installation of localised heating, rather than full central heating.

In those cases where simple sump systems fail to work as expected the cure may include a larger fan, but is more likely to centre upon recognition of the compartmentalised underfloor structure, and that air may be being drawn from the house to the system inlet.

Radon sump systems can be highly localised in their remediation, and in extreme situations may actually cause radon levels in adjoining or upstairs rooms to increase.

These effects arise because suction systems can draw large quantities of air not only from underground (or effectively from the outdoors) but from within the house. Occasionally, the systems can be dangerous for this reason (see below). The most curious behaviour of these systems has been seen in houses and schools that sit essentially on top of 'infinite sources' of radon, an old mine shaft for example, and where different parts of the building date from different periods. This is common where substantial alterations and additions have been made. Examples are given in Section 60. Air flow may sometimes be detected moving from room to room (via very sensitive pressure measurements), or whole house depressurisation may be detected when the system is operated. These effects may be manifest at the limits of measurement and tests should only be undertaken by experienced personnel. Unfortunately there are very few days calm enough for convincing results to be obtained.

Cases of radon levels increasing are not common, but may be explained by reference to the diagram below.

Suction is applied in area A (underfloor) but a pathway exists via internal or external walls, or underfloor, to area B. Air is drawn in around area B, but because of closed doors and/or windows is replenished not from elsewhere in the house or from outdoors but from another (but independent) source in communication with radon-rich ground (point C). Points A and C may show no direct communication.

Remarkably, point C may be in a first floor bedroom, and located where joists pass into old thick walls, and may previously not have been a major entry route. The situation as described may be expected to occur most often in old extended or altered properties, or perhaps in semi-detached housing, because of communicating walls.

It is likely there are other, as yet unrecorded examples of 'curious multi-cell behaviour'.

Radon sump systems may influence radon levels in adjoining properties, especially if terraced or semi-detached.

Usually the effect will be beneficial, but may be deleterious. No such effects have yet been observed with certainty, because so little work has been undertaken in semi-detached or terraced houses. In any case results may be site specific and anecdotal. This applies as much in the USA as in the UK. Newer houses are probably less likely to suffer from the idiosyncrasies of some of the older properties investigated in detail by the author, but in all cases greater attention to draught-proofing may increase the importance of 'uncharted' pathways.

Edge sealing may prove unhelpful especially where system air flow is very low.

At first acquaintance, this is a curious phenomenon. It may possibly be explained by the balance between depressurisation and ground dilution. Beneath marginally affected buildings especially, ground permeability may be low. A system installed to depressurise beneath the floor slab may fail to extend to edges of the slab. Remediation may be only partially successful. If some of the edge sealing is removed (as has been done in a few houses in the USA) small volumes of air may be drawn into the underfloor zone from a few points where the pressure field extends to the slab edge. This may reduce the underfloor radon concentration sufficiently effect an improvement in overall system performance.

The trick is probably to seal only those cracks in areas where there is no depressurisation, and to leave a few openings in areas where suction is thought (from diagnostics) to extend to the perimeter.

Dangerous side-effects of radon sump systems can occasionally occur. Whilst uncommon, they need to be appreciated by both installers and householders. Fatalities have already occurred in the USA.

The possible dangers arise from the fact that air can be drawn by the fan from within the building, and that this may affect heating system operation.

It has been known in the international literature of radon for more than a decade that often more than half of the air extracted by radon sump systems can be from the inside of the building. In typical houses in the northern States of the US, timber frame houses are constructed on a poured concrete basement, and radon sump systems have worked well and without deleterious side-effects.

In these houses little if any of the extracted air comes from inside the house, because the poured concrete basement is an almost airtight structure. The performance of these houses and the ease of system installation, has led to suggestions that sumps are the method of choice for all houses.

The construction of many solid-floored radon-affected houses in the UK is markedly different. This should be sufficient to induce caution. Pressure field extension may be blocked by internal walls, and air may be drawn from the building interior or from the roof space down hollow internal walls to the suction point. It is easy to confirm this by using tracer gas. The author remains grateful to householders who tolerated days of his tedious (and to him interesting) experiments.

The consequences of these multiple pathways can include increased condensation in radon systems, increased heating bills, and depressurisation of parts of the building interior.

Especial care may be necessary if applying systems to houses with cob walls because of possible drying out of the probably shallow foundations. However, cob walls are rare, as are buildings of other types having little or no foundation depth.

The most extreme (but least common) problem is that carbon monoxide from a boiler can be drawn into the building. This has occurred in buildings known to the author both in the USA and UK.

Interestingly, the common features have been a combination of a well sealed building and thick old walls. By some curious route, air was drawn from the inhabited volume to the underfloor suction area.

There are several recommendations for systems using large fans especially:

care should be taken to ensure that fossil fuel boilers have an adequate supply of fresh air, and that this supply will not be affected by the radon fan. It will probably be sufficient to ensure that the fresh air requirements for gas or oil appliances are met, but recognising that air bricks will continue to be blocked up by householders concerned by draughts and heating bills.

householders should be warned, and a warning notice should be installed also, to the effect that the fresh air provision for boilers should not be obstructed.

balanced flue heating systems are to be preferred in houses affected by radon.

These cautionary notes apply not only to houses with solid floors and radon sumps but equally to those with unventilated part or full timber flooring under which air is drawn using a fan for radon extract purposes. In the case cited above from the USA, the house had well sealed timber floors, but without provision for air-bricks because of the dry desert climate.

In other States fatalities have occurred apparently related to 'back drafting' - the term used in the USA to describe combustion products being drawn from heating appliance flues into inhabited areas. In some houses the fumes may then be distributed by the air conditioning system.

The author may be consulted in any case of a suspected problem of this type, or reference made to a qualified heating specialist - one who understands radon!

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