The fundamental challenge when dealing with buildings of special architectural or historic interest is to maintain structural and aesthetic integrity, while remaining sympathetic in the product application and still achieving the desired effect. Toby Champion of Newton Waterproofing Systems reports.
Meeting stringent listed building requirements
When treating damp in historic and listed buildings, it is important that the products used meet with the stringent Listed Building Requirements. Many damp proofing products require surface preparation techniques that are unacceptable due to the detrimental effect on the structure, which in most cases is irreversible.
However, some products, such as damp proofing membranes, can still be sympathetically applied with little or no preparation, requiring only the very minimum of strategically placed fixings to be held in place. Once installed, the membrane achieves its design function by providing an impervious barrier between the damp surface and the new internal wall. Moisture and moisture vapour are prevented from affecting internal finishes, and, when supplemented by drainage, this becomes one of the most effective forms of waterproofing.
Problems with chemical damp proof courses
The walls of many historical buildings are constructed with an irregular mix of materials that could not be expected to accept or sustain contemporary injected chemical damp proof courses (DPC’s). Due to the wall thickness and the sparse population of physical mass, modern, injectable chemical DPC’s do not achieve an even distribution throughout the substrate.
Even when well installed, a chemical DPC should only be seen as a damp inhibitor and not as a final solution to rising damp, and as a result of the difficulties of installing such products, many historic buildings are resigned to the idea that their walls will be damp to a degree.
Damp proofing walls above ground – rising damp
However, if completely dry and untainted internal wall finishes are required, there are still some products that are ideal for providing a barrier between the damp construction and internal finish. The air gap created by the studs of cuspated damp proof membranes, for example, provides an equilibrium of moisture-laden air, which maintains the natural state of the structure, helping to prolong its longevity while protecting the internal finishes from the damaging effects of dampness. Internally applied damp proofing membranes should be installed to about 200mm above the highest evidence of rising dampness so as to allow the natural vapour drive to continue unimpeded through the walls above the treatment.
Damp proofing walls above ground – penetrating damp
Penetrating dampness should always be treated externally at the source where the water is coming through. Water in its liquid state can only pass through a wall if there are defects large enough to accommodate it, and it is these defects that need to be repaired.
Many old walls were designed to be dampened by wind-driven rain, with the expectation that they would dry out before the moisture moved to the internal wall surface. However, persistent rain sometimes means that the moisture does reach the internal wall finishes, and shows as penetrating damp.
Specially formulated externally applied treatments will penetrate into the wall materials, blocking the capillary network and preventing moisture from entering the wall. Breathability also means that residual moisture and internal humidity can exit through the wall, resulting in damp-free internal walls that are dryer and therefore warmer, and with a dramatically reduced probability of recurring internal condensation.
Treating areas below ground
Subterranean areas will always be subject to penetrating damp due to the earth that bears against the structure. Water courses within permeable soil channel the water like pipes, and saturated soil develops a head of hydrostatic pressure that surrounds the subterranean structure, leading to water ingress through gaps and joints in the building fabric. To create habitable or useable areas the structure therefore needs to be waterproofed.
From a historic and listed buildings perspective, cavity drainage systems are ideally suited for below ground areas. When installed by professionally qualified contractors, the systems depressurise and collect water that enters the structure, removing it either via gravity or a pumping system. Internal finishes are isolated from the water by the membrane, leaving a dry and habitable space for the occupier even in the most demanding situations and the most historic structures.