Damp management and remediation
Chemical damp coursing
Sometimes a band of masonry will receive the chemicals, and sometimes a targeted mortar course.
Chemical injection dpcs are usually injected into walls about 150mm (or 2 courses of brickwork) above ground level. By injecting chemicals into walling, usually silicones or siliconates (a new range of Thixotropic preparations are being introduced), a band of masonry is produced that has water repellent qualities. The chemicals used act as pore liners, preventing capillary movement of moisture through the tiny pores of the brick, stone or mortars. The chemicals are either solvent or water-based solutions. The solutions are applied at high or low pressures by dedicated pump equipment linked to multiple lances. The lances incorporate special pressure-tight seals to help force the fluids into the substrate.
Evaluation of physical dpc option
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Many older properties have at some time had walls drilled to accept the chemicals. So you will see lines of drill holes at the base of masonry walls on a regular basis during survey work. If you can't see any injection drill holes, the property may have been injected from the inside faces of walls. Some properties have been treated on multiple occasions, as the original chemical dpc may have been deemed to have failed, or a new chemical dpc with the guarantee attached is required by the client for procedural reasons.
The presence of holes does not guarantee that anything has been injected into them, so be careful how you word survey reports. You can insert a pencil to find out the drill angle and some indication of hole depth. Holes for chemical injection have often been drilled downwards at an angle to enable the chemical to soak into masonry and jointing material. This is a risky operation, as existing slate damp courses have been known to be damaged in the process.
Look carefully for surface irregularities of the plaster finish at around 1m above floor level – this will nearly always be evidence of replastering. Chemical injection dpcs are most often part of a combined remedy approach, with plasters hacked off and replaced along damp-proofed walls. Your moisture meter pins may be more easily inserted above the damp-proof plaster line because the original plasters will usually be softer than the new hard cementitious waterproof renders.
Damp-proofers' reports usually advise that internal plasters are taken off because they may contain hygroscopic salts (i.e. nitrates and chlorides that are said to be sourced from the ground). These salts collect at or near the plaster surface and attract moisture from damp air. So even if the new dpc is effective, salts remaining in the plaster will prevent it from remaining dry, and decorations will fail.
The 1985 edition of British Standard 6576 (latest edition is BS 6576:2005+A1:2012) stated that the function of new plaster was to hold back the hygroscopic salts resulting from rising damp. But the plaster is really there for 2 main reasons. True, there is a case for replastering if ground salts (e.g. chlorides and nitrates) have built up significantly in existing plasters, but some product manufacturers tell us that the plaster is also there because the chemical injected dpc is not reliable. But the British Standard was quite cautious concerning removal of plasters in conjunction with dpc injection. In section 7.3.1 it advised:
‘Where the plaster appears to be sound and unaffected by salts, the extent of replastering may be minimised by deferring any decision to replaster until the drying out period is complete.’
Few damp-proofers would take this advice to heart, as they would consider it a risk to install a chemical injection dpc without the added protective waterproof plaster shield, especially if the walls were found to be wet on removal of existing plasters (and waiting for a dry wall means waiting for payment). Plasters are quite automatically hacked off in practice, whether salt-damaged or not.
The reality is that whether or not hygroscopic salts are present in the existing plasters, the chemical injection dpc will be described in its BBA certificate as part of a combined remedy, with the plastering system crucial to ensure that a dry decorative surface is achieved
The BBA certificate will describe in detail the plaster specification, and if the plaster specification is not adhered to, the dpc has not been installed in accordance with the relevant BBA certificate, and any guarantee may be invalidated. So to an extent there is a conflict between the British Standard and the damp-proofing method as set out in a BBA certificate.
The plaster system applied in conjunction with the chemical dpc injection is nearly always a strong cement-rich render mix, able (if correctly applied) to keep any underlying dampness at bay. This means that, even if the injected dpc is not fully effective, a dry wall finish may still be achievable because of the protective plaster shield.
So we have in effect a damp-proofing system, rather than a dpc. Injection plus plastering is a 'combined remedy'. Some might suggest that the chemical injection, being probably an unreliable damp course, might just as well be omitted completely if an effective frontal masking by plaster is achieved.
When inspecting a property that has been subjected to a chemical injection dpc, always check whether a damp-proofer or a general builder has installed the plasterwork. If the plaster has failed, find out if the recommended plaster specification was passed on to the plasterer by the damp-proofer. If the plasterer has received knowledge of the plastering specification, the plasterer rather than the damp-proofer might be liable for making good plasters.
Types of chemical dpcsPressure injection: commonly pressure injection courses have used organic solvent-based silicones or aqueous solutions of siliconate. Chemicals injected under pressure can damage friable mortars, and fail to fully impregnate masonry due to 'viscous fingering', whereby chemicals introduced under pressure advance in the material in fingers, leaving zones between the fingers untreated. This method is likely to be superseded by the more user-friendly injection creams. Gravity transfusion: this method is rarely used. Its main advantage is that you can carefully control the amount of chemical introduced to the masonry at each entry position, where a bottle will feed the chemical solutions by gravity into the masonry. The method, although potentially more reliable than pressure injection, is slow, and therefore not so attractive to contractors. Injection mortar: injection mortar is not commonly used. A cementitious slurry is injected into large (e.g. 19mm diameter) holes in masonry. It is sometimes used where standard pressure injected silicones or siliconates might not work (e.g. to rubble filled or flint walls). It can only really be relied on to 'control' dampness, as a discrete and reliable barrier will probably never be achievable. Injection creams: silicones are gunned into pre-drilled holes in the mortar joints rather than into bricks. A modified mortar joint acts to repel moisture, providing that the chemicals have satisfactorily cured. As for other chemical dpcs the zone of modified bed joint controls rather than stops damp, to varying degrees of efficiency. This method can be more suitable for the DIY enthusiast, as no expensive pumping equipment is needed. The drilling, however, requires professional standard masonry drilling equipment, and any replastering should be carried out by suitably qualified and experienced plasterers, for good results. |
Typical scope of works: chemical injected dpc plus replastering
- Stripping out all fixtures, fittings located on or near walls to be replastered. Kitchens and bathrooms may need to be taken out. Central heating radiators may need to be removed. Shelves are removed. Skirting boards along walls to be replastered are nearly always taken off, or sometimes sound boards are refitted incorporating joinery liner, and treated with preservative.
- Wall plaster is hacked off to typically 1,000mm or sometimes 1,200mm height.
- After hacking off plaster and installing a chemical dpc, ideally the exposed masonry should be left to dry out as much as possible before replastering.
- Solid walls are drilled and injected with dpc chemicals. External holes are plugged or made good.
- Some external making good may be executed, e.g. repair to external plinths. Sometimes external perimeter channels are formed to eliminate low-level damp penetration. Electric cables and circuits require renewing, rerouting, resecuring and/or protecting. Socket and switch boxes may need to be replaced.
- Any holes or unevenness in substrate are made good or dubbed out.
- Two coats of waterproof render plus a finish coat are applied, with a careful match made to adjacent plasters.
- Skirtings and any other linings or beads are renewed or replaced.
- Appliances are refitted, kitchens and bathrooms are reinstalled.
- Radiators are refitted and central heating recommissioned.
- Redecoration is usually not part of a dpc installation, but for a specified period water-based paints are usually stipulated to enable residual dampness in walls to more freely dissipate out.
Typical specifications: pressure injected chemical dpc
- Check carefully that plastering and chemical injection has been carried out or will be carried out to meet the relevant requirements (previously BS 6576 1985; check new edition), as well as the particular damp-proofing system’s BBA Agrément certificate. You may also usefully refer to the BWPDA Code of Practice The installation of remedial damp proof courses in masonry walls if a registered BWPDA contractor has installed the dpc system. In checking the effectiveness of an existing dpc system, it is probably the standard and suitability of the associated replastering that is easiest to assess.
- Generally, chemical injection dpcs are suitable for walls up to 600mm thick. The appropriate height for injection is selected so that injection is externally 150mm above finished external yard levels and internally close to any solid floor finish. Injection should be below timbers in contact with the wall.
- Holes 10mm in diameter are drilled, 2 per brick stretcher and 1 for each brick header.
- Vertical injection is necessary to isolate dpc treatment from effects of dampness from adjoining walls or to maintain continuity between horizontal dpc treatments at different levels.
- Drill solid walls 230 or 340mm thick from both sides, or by an approved drilling sequence from 1 side only. Solid walls of greater thickness should be drilled from both sides.
- Drill cavity walls from both sides, or sometimes using progressive drilling techniques from 1 side only.
- Injection pressures should be specified. Continue injection until fluid exudes from the substrate.
- Leave treated walls 230mm thick for a minimum of 3 days, in dry warm conditions, and 10 days, in wet, cold conditions, to allow initial drying out (a clause included in BBA certificates).
- Use patent plugs to cap off the holes, or a suitable mortar.
- Reduce ground levels where necessary.
- Externally, expose the proposed damp course line by removing facing material.
- Check that timbers in the vicinity of the treatment are free from wet/dry rot. Take appropriate action to make good, protect and preserve timbers.
- There should be continuity between an injected dpc and any solid floor dpm.
- Avoid dpc bridging; for example, by design and installation of a suitable plinth and render detail, and typically by finishing internal plasters above finished floor level.
- Health and safety – some fluids give off a flammable vapour and sources of ignition must not be present during injection or for 2 days after. Full ventilation is required during this period.
Typical specifications: associated dpc replastering
- Remove/hack off plaster as necessary.
- Disconnect electric circuits as required for safety reasons.
- Check positions of pipework, and be aware before hacking off plaster that cables and pipes may have been run incorrectly, and could be vulnerable to damage.
- Replaster internally using either renovating plasters or sand/cement renders. Renovating plasters contain cement, lime and lightweight aggregate.
- Replastering should be carried out up to no less than 1m from dpc height up the subject wall and 500mm above the maximum level of the rising dampness or salt contaminated plasters.
- Rake out mortar joints to 15mm depth.
- Timber fixing grounds should be removed.
- Three parts sand to 1 part cement should be used (sand should be washed sharp sand, loam- free and to BS 882).
- Two coats each 12mm thick are required, finished with a 3mm skim of Carlite or 'Multi-finish'. The plaster finish should not be polished (i.e. over-trowelled using a steel trowel).
- Waterproof additive is often specified, incorporated in the first render coat only.
- Do not use bonding plasters to bond metal corner beads.
- Joinery may be fixed using masonry nails or inorganic fixing grounds. (Although a typical spec. item, fixing skirtings using masonry nails is not considered good practice by this author. Masonry nails may split masonry as they are driven home, as well as potentially splitting joinery due to their pointed ends.)
Advantages and disadvantages of chemical dpc injection plus plasteringAdvantages:
Disadvantages:
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General issues of chemical injection dpcs
Remember that chemical injection dpcs are only rarely used as dpcs on their own. They tend to be part of a damp-proofing system that would usually include the chemical injection dpc plus a waterproof plastering application.
As highlighted in Damp management and remediation, a water repellent type dpc is more likely to control rather than stop rising moisture. So if there is a severe moisture load, a chemical injection dpc may not be the best remedy.
If you were to inject a chemical dpc in an obviously damp wall, hack off existing plasters, allow the wall to dry out, neutralise any remaining hygroscopic salts and then replaster using lightweight (non-waterproof ) plasters, the dampness would probably return and ruin decorations. The effectiveness (or lack) of chemical injection damp courses would be finally found out. Damp damage would arise because the chemical injection dpcs had failed to stop rising damp, or would fail because they were not able to stop a lateral damp penetration – or indeed dampness from any other source.
There is rarely a full and independent evaluation of low-level dampness in buildings. In the To manage or cure? case study you will see a physical dpc insertion was proposed to solve low-level dampness, but was thankfully not ultimately selected as the preferred remedy because the author was commissioned to carry out an independent dampness investigation, which led to a more relevant remedy design.
Some types of chemical injection dpc (e.g. injected creams) create a modified mortar bed that offers at least some resistance to the vertical passage of moisture. However, if a band of masonry is chemically injected, then the band of treated masonry could also be able to offer some resistance to a lateral damp penetration. Application of an internal waterproof render provides what is in effect a vertical barrier.
If a dry decorative finish is achieved by virtue of a reliable plaster masking system (for however long), it could be erroneous to state that the chemical injection dpc has worked, because it may well not have. We would be witnessing the effects of a combined remedy, with possibly just the masking plaster having worked. But beware, moisture could eventually find a way out from the wall to the habitable space laterally, vertically, or via any imperfections in the plaster system, whether from its original installation or by subsequent damage by following trades or longer-term deterioration.
Even if it were possible to prove that a chemical injection dpc can be fully effective in stopping a rise of moisture, you could only prove a particular application to have worked, as successful injection of a dpc depends more than any other type of dpc on a carefully planned and executed introduction of the chemicals (i.e. workmanship) and into a suitable wall (in terms of its materials and make-up).
The only person who will know whether or not a physical dpc has been carefully and properly installed in a new building is the bricklayer. A physical dpc is exposed for inspection often for minutes only before the first layer of brickwork or masonry is laid upon it. All we see then is the edge of the dpc. It is likely that fewer dpcs are fully inspected these days, due to shortfalls in the resources of building control. There are critical ground rules concerning how lengths of dpc are lapped, and the actual width of dpc used for the subject masonry units. Dpcs that are too narrow, for example, allow bridging of dampness via jointings, while dpcs that are too wide could collect mortar droppings to become bridging positions, or be themselves water collection positions.
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Example Consider a length of masonry wall that we are sure has been chemical injected – whether a cream application into a mortar bed or a pressure injection of water-based siliconates to create a band of resistant masonry. Imagine that what we have is a discontinuous water-resistant bed joint or band of masonry. There were too many voids for the chemicals to completely treat the whole course or the whole volume of masonry. So we have pathways through the resistant masonry – rather like openings through a cloud layer. Moisture can then soak upwards through the less resistant masonry and eventually sit on the top of the resistant masonry. The moisture may not find a way out to evaporate. The wall faces could be cement rendered – at least on 1 side. The moisture may collect in this way and disperse out via evaporation off the outside wall or an exit route at the base zone of the wall internally, where the protective plaster shield terminates – and there will so often be a band of very wet masonry at the base of the wall internally just below the protective plasterwork (particularly in a solid floor scenario). This creates a damp environment behind a timber skirting. So in a way you allow some water upwards through the 'filter' the damp-proofing has created – but allow far less back downwards. The author believes this could be what you achieve at the base of a wall by chemical injection of a dpc. Imagine something akin to a plastic sheet – say 225mm wide, but with 100mm holes in it 200mm apart – this might help you understand better what is happening inside the wall. |
Can retrofit chemical injection dpcs work?
The preceding discussions have enabled us to see behind the smoke screen; that is, that we rarely ever evaluate the dpc itself, but are usually faced with a frontal masking system that has either produced reliably dry or frustratingly damp plasters for decorating on.
The short answer to the above question is that nobody really knows. There has been little, if any, independent and published research of the effectiveness of chemical injection dpcs in real buildings, until more recently with the Camberwell Pier experiment (see above). The system may have produced a dry decorative surface – but this does not mean the chemical injection itself has stopped any damp. Experts within the industry usually advise that chemical injection damp courses control rather than stop damp – or to put it bluntly, on their own they are unlikely to produce a discrete and reliable barrier to the movement of moisture from below. So no – they actually rarely 'work', although the new age of thixotropic creams or pastes may be more effective than past chemical solutions.
If you read the small print of technical handbooks by leading manufacturers of chemical dpc systems, there will usually be mention of chemical injection dpcs 'controlling' rather than 'stopping' rising dampness. You could draw the obvious conclusion here that even the manufacturers admit chemical injection does not on its own 'work'. You may wonder, then, why so many house surveyors ask if it does.
There has been little by way of published research concerning the effectiveness of chemical injection dpcs. What trials there have been have usually been of specially built masonry panels – which do not match up in many respects to walls as found in real properties. If a dpc were proved to not work in a specially built masonry panel, this would be the more significant result – as the specially built masonry panel will have been most carefully and solidly built. Real walls with voids, or made of combinations of materials, are less likely to be successfully chemical injected. Test panel walls are also usually of modest thickness, and in real properties there are particular difficulties in successfully injecting the thicker wall zones, such as chimney breasts. Testing a panel of masonry in a covered workshop is of course testing damp-proof injection in a factory setting, probably undercover, etc. – whereas construction site conditions will be more difficult and more pressurised. When operatives know the subject wall will in any case be given a vertical barrier internally by way of waterproof plastering, they will have less incentive to carry out a thorough chemical injection.
It is believed, for example, that BBA tests these days are in most instances essentially bench tests of quite small pillars, rather than larger masonry panels.
The typical test pillar, described in MOAT No 39: 1988, is a strange combination of brick lower section (65 × 100 × 100mm) and limestone upper (100 × 100 × 250mm). This is not what you would expect to find in a common house wall. The brick bottom section is immersed in distilled water up to 50mm depth, and the upper limestone block is weighed at intervals to see if it is taking in any moisture from the brick base. Quite a clever test idea but in the author's opinion not actually replicating a real wall.
Why is it difficult to achieve a successful chemical injection wall dpc?
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Sometimes brick panels are constructed to test damp proofing. But the panels are built using specially conceived mortars that you would not find in a real wall. The mortar concoction is made up using, for example, 'Ham river washed sand', 'snocal', slaked lime, Fossasil and water. This combination of mortar constituents means it would be difficult for those outside the BBA laboratory circle to carry out any comparative tests.
BBA testing is paid for by manufacturers, and the results are not thought to be publicly available. There is some site appraisal of the damp-coursing system, but this may be more concerned with method of installation and COSHH safety assessments rather than any long-term monitoring of walls before and after treatment to check effectiveness of the damp-proofing.
The author made contact with numerous manufacturers of damp-coursing products as well as dozens of remedial treatment surveyors in the preparation of this text, but was offered just 1 set of unpublished test results, from some cottages damp-proofed in the early 2000s (the manufacturer requested that the source remained confidential).
Field research typically involves consumers completing evaluation questionnaires after the damp-proofing. It is likely that a dry decorative wall finish would be achieved with any damp-proofing system that incorporates a waterproof render internal replastering – so the consumer would get what they want: a dry internal wall. Consumers also rate highly the efficiency of the damp-proofers, whether the operatives were courteous, that the treated rooms were reinstated neatly, etc.
But the downsides can become apparent over the longer if not the shorter term.
| Table 2: comparison of physical and chemical dpc work operations | ||
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| Operations | Physical | Chemical |
| Set up site | Yes | Yes |
| Remove furniture, units | Only when necessary | Yes |
| Remove, set aside radiators | May not be necessary | Yes |
| Take off wall plasters to subject walls | Only make good when necessary | Usually to all walls injected up to 1-1.2m |
| Take off skirtings, set aside | Only where necessary | Usually taken off prior to hacking off plasters |
| Replace or renew skirtings | Only where removed | Usually to all injected walls |
| Insert dpc, make good after | Yes | Usually only drill holes to plug |
| Reinstate furniture, units | Only where necessary | Yes |
| Reinstate service/refit radiators | May not be necessary | Yes |
| Table 3: cost comparison of physical insertion versus chemical injection dpcs | ||||||||
|---|---|---|---|---|---|---|---|---|
| Hack off plaster (soft) | Hack off plaster (hard) | Plaster repair (small patches) 1:1:6 | Waterproof plaster (to 1200mm) 1:3 cement: washed sharp sand + waterproofer | Take off skirting (150mm) | Renew skirting | DPC - install 225mm | DPC - install 100mm | |
| Physical | £20 | £30 | £50/m2 | N/A | £5/m lin | £20/m lin | £110/m lin | £55/m lin |
| Chemical | £20 | £30 | N/A | £45/m2 or £60/m run | £5/m lin | £20/m lin | £30/m lin | £15/m lin |
Analysis of Tables 2 and 3
From Table 2 we can appreciate that insertion of a physical dpc (in most cases) involves far less internal stripping-out work, and then inevitably far less reinstatement work, of plasters, skirting boards, kitchens, bathrooms, services, etc. But set against this is the much slower installation time of the physical dpc insertion.
These differences in the scope of required works is, of course, reflected in the costs of the 2 damp coursing methods. In, for example, a 225mm thick masonry wall, a physical dpc insertion could cost over 4 times the chemical alternative – as the respective rates of £110 and £30 per metre run testify. But in any remedy appraisal, this needs to be set against savings in replastering possible using the physical dpc option. When both sides of a solid wall need replastering, the balance can tip in favour of physical dpc insertion.
There must have been thousands of disputes where chemical injection damp-coursing above ground had failed, but the damp-proofer has escaped liability when associated plastering was carried out by others. Damp-proofing, if actually needed, depends more than anything else on the standard of internal remedial plastering, which acts to mask damp behind. (We can never assume that chemical injected dpcs actually stop damp.)
Many chemical dpc systems fail, due to unsatisfactory specialist plastering. The costs of hacking off and plastering again to remedy the damp problems can be high, disruptive and frustrating for property occupiers and owners. Physical dpc insertion has a much higher chance of success in the author's opinion – as long as the dpc has been correctly positioned and installed, and was actually needed in the first place to solve the damp problem.
Electro-osmosis dpc (passive or active)
This method is rarely used today. Some systems failed in the past and were often made good by a chemical injection. But the chemical dpc that followed will probably have been used in conjunction with a plaster protective shield, and may in reality have been no more effective than the 'failed' electro-osmosis system. There seems to be a lack of confidence in electro-osmosis among damp-proofers, and the Building Research Establishment has found these systems to be ineffective (BR466, 2004).
Evaporation improvement methods
Siphon tubes - sometimes referred to as atmospheric siphons:
Various types are often marketed rather optimistically as the panacea for all damp problems. What is true is that any method (carefully installed) that encourages a wall to evaporate out moisture from within must at least reduce the wall moisture content while the devices function.
Atmospheric siphons are not in themselves a damp coursing method, but are really a support measure to help moisture evaporate from walls.
The effectiveness of these systems remains controversial, given that they could be capable of inducting rainwater penetration into the core of the wall, especially if they have not been drilled into the wall correctly. That can cause cold spots surrounding their installation, which could create the conditions for interstitial condensation. The writer has seen many examples where these systems have been painted or rendered over, blocking the effectiveness of these systems to openly ventilate to evaporate moisture from the wall.