Environmental control of damp

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 fro decorating the U-value ( on a standard 9 inch/225mm thickness wall ) will be reduced in accordance with Building Regulations to around 0.3 which results in less heat loss through walls and lower energy bills.
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
Further reading:
Water transport in Brick, Stone and Concrete by Christopher Hall and William D Hoff
Capillary flow in Building Elements by Christopher Hall
Damp Buildings , Old and New by Giovanni and Ippolito Massari
Treatment of rising damp in historical buildings:
wall base ventilation Building and Environment, Volume 42, Issue 1, January 2007, Pages 424-435 M. Isabel M. Torres and Vasco Peixoto de Freitas
Moisture beneath suspended timber floors
Structural Survey, Volume 13, Number 3, 1995 , pp. 11-15 Harris D.J.
TAN 24- Environmental Control of Dry Rot’ written by John Palfreyman & Gordon Low, published by Historic Scotland.
Also recommended is the DVD by Mike Parrett, star of the BBC 2 series Raising the Roof, entitled Building Pathology which offers a holistic approach into the diagnosis of building failure together with effective remedies for common defects.
For more information or advice on environmental control of rising damp please call 0800 028 1903 or email: enquiries@ukdamp.co.uk

The cost of a damp survey can be found here and can be arranged by phoning 0800 028 1903 or click the enquiry button to the left of the screen.