Environmental control of damp

Physical damp-proof courses have been compulsory in all buildings constructed since around 1875. In houses built from 1880 to around 1920 this would have been a horizontal layer of slates overlapping to form a continuous barrier at the base of the wall, usually at the same height of the air bricks Around 1930 slate was superseded by bitumen impregnated hessian which has now given way to the bitumen free polymeric damp-proof courses.
Slate and hessian/bitumen damp-proof courses are still fairly effective in controlling rising dampness from the ground and there should never be any need any replacement damp-proof courses (chemical, physical, electro-osmosis or siphon tubes) as most cases of 'rising damp' can be attributed to low-level penetrating dampness where water is sitting at the base of a wall or where the original damp-proof course has been bridged by raised ground levels. Both of these situations create a 'moisture reservoir' allowing water ingress at the base of the wall which then migrates through the brickwork by capillary action and this then appears as rising damp internally.
Instead of installing a chemical damp-proof course it is usually easier and cheaper to tackle the problem externally by reducing ground levels or installing drainage channels to prevent bridging and repairing any drainage and guttering defects which may be causing ponding externally. Once these works have been carried out then the walls will dry out gradually and there is usually no need to remove plaster internally so as well as saving money by not having to have damp-proofing work done then all the mess and disruption of plastering and re-decorating internally is also avoided.
Even if there is any slight passage of moisture through the old damp course it rarely causes any problems internally as the rate of evaporation from the wall above the damp course is usually greater than the uptake of moisture below the damp course.
Inadequate sub-floor ventilation is often a cause of dampness in walls as this tends to cause a build up of moisture in the floor void which can condense on the exposed masonry and timbers. Much of the condensate will be above the damp course and will then migrate upwards to appear as 'rising damp' internally. The easiest way to deal with this is to install extra air bricks at the base of the walls and ensure that there is an adequate through draught of air to remove moisture from the void, this will also help to reduce the moisture level of floor timbers and reduce the risk of decay and infestation.
Most of the damp situations that we encounter during our surveys are usually remedied without the need for any form of damp-proofing works. If the control of damp can be addressed by balancing the ingress of moisture and the evaporation rate of dampness through moisture reservoirs and moisture sinks ( see diagrams ) then this should be sufficient to prevent any internal damage to plaster, skirtings and sub-floor timbers.

Moisture reservoirs   Moisure sinks

Before considering the concept of moisture reservoirs and moisture sinks the external fabric of the property should be inspected and any obvious defects repaired. This may seem obvious but we get numerous calls to remedy rising or penetrating dampness and the source of the moisture is often a leaking downpipe or cistern overflow.
Internal pipe leaks are also often a cause of dampness that can be misinterpreted as rising damp and many of our clients have been recommended to have damp-proofing works where the cause of damp was from a leaking shower unit and was easily rectified with a tube of silicon mastic.

Moisture sources

The most common moisture reservoirs that create rising damp are undrained ground and raised ground levels bridging existing damp-proof courses. This allows low-level moisture ingress which then migrates through the wall by capillary action and can appear as 'rising damp' internally affecting plaster, skirtings and other timbers abutting the damp walls. Reducing ground levels and installing drainage or ventilating channels at the base of the affected walls will remove the source of moisture and create a moisture sink by allowing evaporation from the exposed wall.
Usually no plastering is required as the walls should dry out gradually but if the plaster needs to be renewed we find that the best option is to use a combined damp-proofing and insulating system with a damp-proof membrane such as Newlath 500 or Delta Plaster-Lath fitted to the damp walls which is then over-boarded with Larfarge Thermacheck plasterboard or Walltransform Insulating Plaster. As well as providing a damp-free internal finish ready for decorating, the U-value ( on a standard 9 inch/225mm thickness wall ) will be reduced in accordance with Building Regulations (Part L Conservation of Fuel and Power ) to around 0.3 which results in less heat loss through walls and lower energy bills and will provide full payback within 15 years.
Another major moisture sink is sub-floor ventilation. If existing air bricks are cleaned out and additional sub-floor vents installed then the improved ventilation under the floor will provide lower humidity levels and help to purge excess moisture from the sub-floor void. The lack of effective ventilation often causes water vapour to condense on the exposed masonry in the floor void which can then travel up the wall by capillary action and appear as 'rising damp' The increased air flow prevents this from happening and also reduces the moisture content of floor timbers such as joists, wall-plates and floorboards etc thereby reducing the risk of fungal decay and insect infestation, providing less justification for the use of chemical treatments to prevent woodworm or wet and dry rot.
Installing extra air bricks also has the benefit of reducing damp in the solid walls above them and the effect is similar to that described by G & I Massari in their book 'Damp Buildings, Old and New'. They showed how the water absorbed by a porous material is slowed by reducing the area of the base in contact with the moisture source. Three bricks were inserted into a water source. One with the whole cross section of 14cm intact, another with a 5cm section cut out to leave 9cm in contact with the water and a third brick with 9cm cut out leaving 5cm available to water. The different rates of absorption in each brick can be seen in the figure below with the rising wet front reaching the top of the brick with times ranging from 5 hours for the full brick to 33 hours for the brick with 9cm removed. Full saturation of the full brick as a result of capillary absorption and air diffusion would take approximately two years.


Moisture bricks

The experiment demonstrated that reducing the inflow area greatly extends the time it takes for water to rise to the top of the brick: by a factor of 7 in the brick shown on the right, where the area of the header face in contact with the water is reduced by around two-thirds.
The perforated profile of air bricks acts in the same way as the reduced section bricks and helps to slow down the upward passage of moisture in walls and when extra air bricks are installed in conjunction with ground reduction/drainage channels then these are usually sufficient along with the original physical damp-proof course to prevent any rising dampness occurring internally.
The Massari effect is also beneficial to chemical damp-proofing and the siphon tube system where the drilling of holes at regular intervals reduces the surface area available to moisture and therefore reduces the capillary effect of any rising damp and we have on occasion been instructed to install siphon systems internally behind skirtings to gain some of these advantages without having the unsightly holes on the outside walls.
If you need any more information about chemical-free, environmentally friendly damp control then please call us on 020 8226 3101 or e-mail enquiries@dampbuster.co.uk

tel: 020 8226 3101 email: enquiries@dampbuster.co.uk