Last edited 01 Sep 2020

Defects in dot and dab


[edit] Introduction

Dot and dab is a technique that was developed to replace traditional wet plastering and it is now frequently used in new buildings.

It became prevalent in England during the late 1970s and quickly gained in popularity due to its lower costs, faster installation, quicker drying time and ease of finishing. Walls finished in this way can be painted almost immediately.

Dry lining, as the process is known, is widely used by builders and developers and is particularly useful because it can be used on both wooden and masonry internal partitions, ceilings and walls. The ability of plasterboard to be directly attached to masonry using dots and dabs of adhesive gives a considerable speed advantage over the wet plastering.

Wet plastering is a traditional trade that requires a high level of skill, takes longer and needs more drying out time and all of these issues make conventional plastering more expensive and less common.

Despite its benefits, Dot and Dab is not without problems.

[edit] Down sides of dot and dab

Block work walls are generally built by bricklayers ‘on a price’ and as a result they are of a less than perfect construction, with frequent gaps and cracks that remain unfilled. These are then covered over by the plasterboard linings. When dot and dab was first introduced, cavity walls were well established but cavity fill was almost unheard of for new build.

All these cracks and gaps allow outdoor air to whistle around behind the linings, cooling them and the home they are part of. In the worst cases there is so much thermal bypass, as it’s known, that the householders were effectively living in plasterboard tents.

More recently best practice dictated additional measures should have been used when dry lining. There was a requirement to use full ribbons of adhesive around the perimeter of each wall to try and prevent the worst of the problems mentioned above. However, the tradesmen again were generally on a price and as this method takes longer and uses more materials it was very rarely used. Adherence to the technique is very difficult to validate and so in reality tends not to happen.

Then block work was intended to be sealed with a parge coat before the linings are applied. Again this is rarely seen in practice. In addition there is often a gap left at the bottom of the wall which allows draughts to enter the rooms.

Recently air tightness testing of new homes has been introduced, but unfortunately a room or even a whole house can be reasonably air tight yet still have all the problems already highlighted above. The recommendation that the skirting boards should be sealed to the floor is questionable, even though it is one of the so called 'robust details'; others include sealing all internal visible cracks and gaps to give an airtightness layer inside the home, yet outdoor air is still able to waft around behind the linings and inside the insulation barrier.

[edit] Why this article was written

This article was first started in March 2013 after a diligent home owner discovered that warm air was emanating from gaps behind his plasterboard linings and leaking into his loft. The story got worse when it was discovered that cold air could travel from under the suspended concrete ground floor up the back of the ground floor wall linings and into the first floor void. From there it could continue on up behind the first floor linings, being joined on its way by cold air coming in from outside through many cracks and gaps. There were lots of these around the first floor joist ends, and also between the blocks that were used to infill between the joists. Shrinkage and movement of the joists magnified the problem.

Readers may not have noticed this air movement which even on still days is driven by convection currents, however you may have noticed draughts emanating from electrical socket outlets or perhaps you will do now. Most homes have these to a limited extent, even those that have been wet plastered.

[edit] Thermal bypass

Having discovered that air was circulating behind the linings, a spot temperature survey of the main bedroom was performed. It revealed that there was an eleven degree temperature gradient between the bottom of the walls and the top. The bedroom was partly above a garage but the floor had been well insulated. The bottoms of the walls were thought to be below dew point at least some of the time. There were some areas, mainly in the corners of window reveals, where mould was evident.

The main reason that there was no condensation at skirting level was because it was so draughty that even if droplets did form, they would evaporate away again almost immediately as the drier outside air passed over them stirring and mixing with the warmer air in the room. Not only were the draughts moving behind the linings but they were coming out under skirtings even on internal walls, through down lighter holes on the ground floor ceilings, round pipes, out of switches and even from behind architraves. The owner has described his house as a colander on several occasions.

It was also noticed during the temperature survey that the window reveals were a lot cooler than the walls nearby. The owner eventually decided to bite the bullet and removed a strip of plasterboard adjacent to a window to see what was behind it. More holes, a wavy plastic damp proof course with gaps both sides of it allowing draughts easy access. This vertical dpc could have allowed damp onto the plasterboard lining had the construction been more airtight.

Bricks instead of blocks had been used to close the cavity; these would normally have caused thermal bridging had there not been so much thermal bypass going on.

[edit] Extensions

Most houses get altered and extended during their lifetime and this house was no exception, although hopefully the way that it was carried out was not typical. The extension to the lounge involved demolishing the old rear wall and supporting the upstairs walls on steel beams. These beams were above ceiling level and there were gaps over, under, through and around them. The new ventilated roof of the extension connected to the ceiling void of the lounge.

[edit] How to set about rectifications

All this is very disappointing but some encouragement can be taken from the fact that there are plenty of remedial actions that can be considered.

[edit] Remedial action to walls

The brute force approach would be to rip off all the linings and wet plaster the whole house; however, this would be disruptive and expensive and the floor void would remain a problem. Re-plastering like this has been done many times and is the only full proof way to deal with the problem, but is only practical at the time of major renovations.

The following suggestions are intended to help those who want to try to improve their air tightness and are on a tight budget. A good first step would be to seal the gaps at the bottom of the walls. This could be done by removing the skirting boards and fully foaming the gap between the bottom of the plaster board and the wall.

A further action would be to similarly seal the gaps at the top of all the ground walls and those at the top of the first floor walls too. This would still leave the air ingress in the floor void as the biggest unresolved issue, but technically this is not a fault originating from blob and dob since it is generic to all cavity wall houses.

Problems would still exist behind the linings but their effect would have been significantly reduced if, and only if, there were no longer air paths between the voids behind the plasterboard and any part of the inside of the house or flat. We cannot emphasise how important it is that the void is completely sealed and that no draughts can possibly emanate from this area.

A more comprehensive solution would be to develop a system that could be used to fill this void. One idea would be to use a low viscosity delayed setting foam. The development of such a solution would be technically difficult, because it would need to accommodate voids that are quite variable in nature. It would also need to take account of holes leading into the cavity or other enclosed spaces that are significantly larger than the voids themselves. There could be problems with clogging, over pressuring and blockages.

Additionally, foam might escape into the foundations, under-crofts or even manholes or drains as well as into service ducts or electrical equipment and fittings, so these would all need to be considered and preventative measures taken.

[edit] How to deal with the floor void

On removing an upstairs skirting it will become immediately apparent that there are problems, draughts will be easy to feel and see, gaps can be observed sometimes right into the cavity, and fingers can be pushed into the gap between the plasterboard and the blockwork. Gaps, whether visible or not, will be present around the joist where they enter the outside walls; these must be filled and because rodents can chew through foam, the best material for this is mortar. It is not possible to do this without taking up flooring sheets or floor boards.

Once the first floor skirting and a floor board have been removed there will now be relatively easy access to the top of the ground floor wall/ceiling junction, and it is a good opportunity to seal this and foam can be used this time. Once good access is available we strongly recommend sealing all the outside wall masonry between the ceiling and the floor.

Several approaches to this are possible: plastering it, fully buttering a right sized piece of board with bonding plaster or even with fixing foam and sticking it to the wall and sealing round it later with foam again. The sealing should continue up behind the first floor plasterboard lining to ensure that no draughts can get either into or out of the void behind the linings.

[edit] The edge of the first floor ceiling

This joint is more difficult to access and is generally visible when lying on a plank in eaves the loft having pulled back the insulation. It should be possible to see the gap between the wall or wall plate and the ceiling plasterboard. In dot and dabbed homes there is usually a gap big enough to almost see down between the edge of the ceiling and the wall plate or blockwork, it is easier to see on gable walls. It should be sealed and made fully air tight and again foam is a good product to use for this.

There are two types of foam available, expanding foam and fixing foam - the latter does not swell up after application. Problems can arise when expanding foam is used and too much accidentally gets behind plasterboard as it can deform it and for this reason we generally used filling foam. This join in the loft is one where expanding foam can be used but only use it with great care.

[edit] Dealing with window reveals

These, as we have already explained, have a plethora of problems; thermal bridging, air infiltration, misplaced damp courses and often large holes are present leading into the roof void, cavity or even into the eaves box. Inappropriate materials and poor workmanship are all too common and in concert with dot and dab are difficult to deal with.

Injecting perfect amounts of foam through a set of holes all around the reveal might be the least disruptive solution to a difficult problem, but it will not be easy to check how effectively the rectification has worked. Removing the plasterboard reveals and lifting window boards will allow inspection of all the problems and is the best way forward round openings.

Finding trades people with the requisite skills is both difficult and time consuming. Nonetheless such investment would repay itself fairly quickly, comfort levels will suddenly improve and energy bills would go down which will make it all seem very worthwhile when looking back.

[edit] Air leakages behind the linings

Once wind or air can no longer find its way into the house it is far less of a problem. There will still be convection currents but their effect will be orders of magnitude smaller than those due to draughts. There is a bellows effect on windy days that will be pulling warm air out from behind the linings and in the next pressure change forcing some cold air back in.

Without foam in the cavity of the wall or wet plastering (or the hoped for foam between the lining and blocks) this process will continue but as long as air is not entering the home though far from ideal it is liveable.

[edit] Problems with internal walls

Where internal walls abut outside walls it is very likely that air can get from behind the plasterboard on the outside wall to behind the plasterboard on any masonry internal wall. From there it will have a general cooling effect and will try to escape into the building under the skirting, through sockets and switches and even from behind or over architraves, door linings even round pipes. The best place to seal these is at the point of abutment but this is by no means easy so it may be as well to seal all the possible exit routes.

Generally the floor boarding upstairs and the ceilings are fixed before the partitions are built, but if the walls are load bearing then there could be a gap leading into the loft. If they are solid walls, there will often be gaps at the edge of the ceilings allowing air to get from outside walls via internal walls and into the floor void and the places mentioned above.

[edit] Socket outlets

As we have pointed out already these are often a source of fairly severe draughts. The draughts can be stopped in several ways: seal the back box fully all around and seal the wire entry points with acrylic sealant; foam round the box and seal the wire entry points with sealant; fit a polythene or proprietary draught pouch behind the socket box and seal this to the lining; or use some other form of sealing. Sometimes plastic child safe covers are used to prevent draughts but these will not stop draughts coming out around the face plate itself and neither will they be fully airtight themselves.

[edit] Pipes and wires

These problems are not isolated to the types of homes we are discussing but are problems in all buildings. Mains gas, water, drains, electricity and communications wires are often in sleeves or ducts and these can bring in draughts both round the duct as well as between the services and the duct.

Cable entries to ceiling fittings can allow draughts to enter and these can be safely sealed with acrylic sealant; in new homes or on rewiring this should be done routinely now. Waste pipes where they pass through walls or floors can be draughty too and pipe casings especially soil pipe boxes may require special attention at both floor and ceiling levels.

[edit] Conclusions

There are far too many homes in the country suffering from the problems we have discussed and we cannot see any reason why we should not return to wet plastering for finishing walls, especially as, despite having repeatedly tried to fix the problem, it is still the norm to find a lot of draughtiness associated with dot and dabbed homes.

For these reasons we tend to call this system 'blob and dob' as it more closely expresses our negative feelings about it. It should be noted that neither the requirements for parging, which does nothing to stop one half of the problems, thermal bypass, nor the ones requiring the use of ribbons of adhesive have had any significant impact in the real world with too many people living in nothing more than plasterboard tents as a result.

These problems have been known about for far too long now and despite this we as a nation are losing vast amounts of energy simply because these measures have not been effective nor is there any likelihood of them being effective.

Thanks to “Andyman” from the green building forum for his input into this article.

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An interesting read, thanks for sharing. Almost all of what's written about here i have a current problem with.
Victorian house with new extension to the rear, builder used D&D with wet floor heating which becomes almost useless when cold air ruches from the skirting boards, sockets and makes the internal plasterered walls as cold as the outside walls!

Next time we ever have a build it'll be wet plastering all the way. Our current setup has me chasing my tail trying to plug gaps and foam fill where i can to minimise the thermal bypass.

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