Damp proofing

Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Dampness problems are among the most frequent problems encountered in residences. 

Rising moisture in the wall leads to various types of moisture damage:

dark or cloudy color changes and light edges, which mark the extent of the moisture penetration,

in outdoor areas, peeling wall paint, crystallization of salts and long-term flaking and peeling wall plaster,

indoors, mold or mildew infestation.

As humidity increases, thermal conductivity improves (and thermal insulation capacity deteriorates). The resulting reduction in the interior wall temperature can lead to condensation on the wall surface in winter and a further increase in moisture penetration.

Damp proofing is defined by the American Society for Testing and Materials (ASTM) as a material that resists the passage of water with no hydrostatic pressure. Waterproof is defined by the ASTM as a treatment that resists the passage of water under pressure. Generally, damp proofing keeps exterior moisture from entering a building; vapor barriers, a separate category, keep interior moisture from getting into walls. Moisture resistance is not necessarily absolute; it is usually stated in terms of acceptable limits based on engineering tolerances and a specific test method.

In the base masonry of new buildings, DIN requires at least one horizontal barrier at least 30 cm above ground level; in new buildings with a basement, a further 5 cm above the finished basement floor and a third below the basement ceiling, provided this is at the same level as the surrounding soil. In older buildings, there are often no barrier layers at all or they are no longer sufficiently waterproof. Subsequently installed horizontal barriers are then usually essential to achieve masonry drying out.

Methods

Damp proofing is accomplished several ways including:

A damp-proof course (DPC) is a barrier through the structure designed to prevent moisture rising by capillary action such as through a phenomenon known as rising damp. Rising damp is the effect of water rising from the ground into property. The damp proof course may be horizontal or vertical. A DPC layer is usually laid below all masonry walls, regardless if the wall is a load bearing wall or a partition wall.

A damp-proof membrane (DPM) is a membrane material applied to prevent moisture transmission. A common example is polyethylene sheeting laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. A DPM may be used for the DPC.

Integral damp proofing in concrete involves adding materials to the concrete mix to make the concrete itself impermeable.

Surface suppressant coating with thin water proof materials such as epoxy resin for resistance to non-pressurized moisture such as rain water or a coating of cement sprayed on such as shotcrete which can resist water under pressure.

Cavity wall construction, such as rainscreen construction, is where the interior walls are separated from the exterior walls by a cavity.

Pressure grouting cracks and joints in masonry materials.

Materials

Materials widely used for damp proofing include:

Flexible materials like butyl rubber, hot bitumen (asphalt), plastic sheets, bituminous felts, sheets of lead, copper, etc.

Semi-rigid materials like mastic asphalt

Rigid materials, like impervious brick, stone, slate, cement mortar, or cement concrete painted with bitumen, etc.

Stones

Mortar with waterproofing compounds

Coarse sand layers under floors

Continuous plastic sheets under floors

Masonry construction

A DPC is a durable, impermeable material such as slate, felt paper, metal, plastic or special engineered bricks bedded into the mortar between two courses of bricks or blocks. It can often be seen as a thin line in the mortar near ground level. To create a continuous barrier, pieces of DPC or DPM may be sealed together. In addition, the DPC may be sealed to the DPM around the outside edges of the ground floor, completely sealing the inside of the building from the damp ground around it.

In a masonry cavity wall, there is usually a DPC in both the outer and inner wall. In the outer wall it is normally 150 millimetres (5.9 in) to 200 mm (7.9 in) above ground level (the height of 2-3 brick courses). This allows rain to form puddles and splash up off the ground, without saturating the wall above DPC level. The wall below the DPC may become saturated in rainy weather. The DPC in the inner wall is usually below floor level, (under a suspended timber floor structure), or, with a solid concrete floor, it is usually found immediately above the floor slab so that it can be linked to the DPM under the floor slab. This enables installation of skirting boards above floor level without fear of puncturing it. Alternatively, instead of fitting separate inner and outer DPCs, it is common in commercial housebuilding to use a one-piece length of rigid plastic (with an angled section) that fits neatly across the cavity and slots into both walls (a cavity tray). This method requires weep vents to enable water to drain from the cavity, otherwise dampness could rise from above the DPC.

Execution for new buildings

Sanded bitumen sheets or robust foils are commonly used as horizontal barriers in new wall construction. These are usually embedded in mortar on both sides to prevent damage from the bricks. Care must be taken to ensure sufficient overlap between the sheets at the joints. For buildings with basements, two or three horizontal barriers are often installed: The first is approximately 5 cm, or one brick layer, above the floor slab. This distance from the floor slab prevents water accumulating on the floor slab during construction or due to a subsequent accident from penetrating the entire wall.

The top barrier layer is located approximately 40 cm above the ground level. This prevents moisture penetrating the wall in the splash zone of the building's base from moving further upwards. If a floor is located near the basement masonry or in the splash zone of the building's base, an additional barrier layer is installed approximately 5 cm below the ceiling.

Old buildings

In the past, a horizontal barrier layer made of lead sheets, glass plates, tar paint, tar paper, thicker cement joints, dense natural stone, hard-fired clinker bricks, continuous slate panels, and later also roofing felt and bitumen sheets were used to protect against rising damp in masonry. Some of these materials decomposed over time under the influence of salts or acids.

Even when using dense bricks, a certain amount of moisture could rise through the mortar in the wall joints.

Subsequent horizontal barrier

Depending on the accessibility and the structure of the wall, various methods for the subsequent installation of a horizontal barrier are possible.

For mechanical horizontal sealing, a barrier layer made of wall paper, stainless steel sheet, plastic panels or foils is inserted into the wall cross-section to prevent the water from rising.

For chemical horizontal waterproofing, suitable substances are injected into the masonry to reduce the spread of moisture.

Electrophysical dehumidification uses electroosmosis to prevent the effects of moisture in masonry. Its effectiveness is controversial.

Concrete walls and floors

Concrete normally allows moisture to pass through so a vertical vapor barrier is needed. Barriers may be a coating or membrane applied to the exterior of the concrete. The coating may be asphalt, asphalt emulsion, a thinned asphalt called cutback asphalt, or an elastomer. Membranes are rubberized asphalt or EPDM rubber. Rubberized products perform better because concrete sometimes develops cracks and the barrier does not crack with the concrete.

Wall sawing process

In the wall sawing method, the masonry joints are cut open in one-meter sections using sword, circular, or wire saws to allow the insertion of PE fiberglass or stainless steel panels. The panels are then wedged together across the entire wall cross-section, the joint is filled with mortar, and any remaining cavities are filled with expansive mortar. Such mechanical barriers are reliable and long-lasting. If the floor slab is also to be subsequently sealed, it is advisable to construct a continuous tray by extending the sealing membranes in the floor up to the sealing level in the wall and connecting them to the sealing panels in the wall, which project slightly inwards. 

Wall replacement procedure

The bricks of one or two levels are removed from the masonry over a length of up to one meter. After a foil or bitumen sheet is inserted into the resulting gap, the bricks are reinserted. To prevent settlement cracks, the joint mortar should be compacted or expansive mortar should be used. After the joint mortar has hardened, another meter of the old masonry is removed. This process is quite simple to perform, but comparatively time-consuming. The load-bearing capacity of the wall should be assessed beforehand by a specialist to determine the possible length of the section to be removed and to avoid cracks caused by subsidence of the masonry. 

Driving in sheets

If the masonry mortar does not consist of pure cement mortar, stainless steel plates can be driven directly into a bed joint. Without having to remove the mortar, a sheet of corrugated iron approximately 1.5 millimeters thick is driven in. Since this requires a continuous mortar joint, this method cannot be used on natural stone walls, which are usually made of stones of different heights. If work is carried out from the interior, it is difficult to drive the sheets into the outer corners of the masonry. Settlement hardly occurs because the plates are driven in without opening the mortar joint. The joint mortar is displaced or compacted upwards and downwards. The vibrations of the blows, usually generated using a special device, can cause settlement and cracks in previously damaged masonry. To accelerate the driving in, steel plates with a wedge shape on one side are also used. 

Core drilling method

In this process, core holes with a diameter of eight to ten millimeters are drilled in an overlapping sequence and then filled with a waterproof mortar. Because the holes are positioned so that they overlap, a continuous barrier layer can be created.

Injection procedure

To prevent capillary moisture transport in the masonry, a sealing layer is injected into the masonry. The size, location, and spacing of the drill holes are determined by the test certificates for the respective materials. Currently, test certificates are issued in Germany by the WTA (German Institute for Applied Materials), which also determines suitability based on the degree of moisture penetration.

The injected material is intended to permanently block the capillaries in the masonry, reduce their diameter, or make the capillary walls water-repellent through a hydrophobic "coating." This prevents capillary water transport, allowing the overlying masonry to dry.

Chemical horizontal barriers can be installed using pressureless or low-pressure methods with packers. For thin-viscosity substances, it is common practice to drill inclined bores that reach at least one bed joint. Gel-like materials are usually inserted into horizontal bores in the bed joint.

Natural stone masonry

Irregular natural stone masonry cannot be reliably sealed with thin-liquid injection compounds. Often, the masonry joints are not evenly spaced. Even natural stones that have been hewn into ashlars on the visible side are generally very irregularly shaped on the side facing the interior of the wall. Typically, the gaps left between the stones during masonry are unevenly filled with broken rock and mortar. Large quantities of the liquid injection compound can collect in the remaining cavities and seep downwards without forming a continuous barrier layer.

To remedy this problem, the holes are used to first inject injection mortar (also called filling, grouting or grouting mortar) into the interior of the wall. Before the injection compound has fully set, the sealant is then injected. This prevents the sealant from seeping downwards. Instead, it penetrates the injection mortar and is distributed evenly in all three dimensions. Since the sealant is usually many times more expensive than the grouting mortar, this method is more economical than injecting with sealants with a gel-like consistency. The latter are now offered by various companies to limit the seepage of the sealant.

Injectables

Also known as silicification, the process used alkali silicates. These can be applied without or with low pressure. Their effectiveness is limited, and the application often had to be repeated every three to five years. Silicification products are alkaline and carry the hazard symbol "Corrosive." Therefore, handling them requires the necessary caution and the use of appropriate protective clothing.

The chemical horizontal barriers available on the market today are mostly based on the use of silanes, siloxanes or paraffin.

If heated liquid paraffin is used as an injection material, it clogs pores. There are also paraffin oils with dissolved plastics. Silicone microemulsions, on the other hand, have a hydrophobic effect.

Colored injection materials are also available so that the distribution of the material can be checked using control holes.

The injection, mode of action, and application limitations of the process and the various injection materials are described in the WTA leaflet 4-4-04/D "Masonry Injections Against Capillary Moisture" published by the Scientific-Technical Association for Building Preservation and Monument Preservation. Among other things, the leaflet points out that not every injection material is universally applicable; rather, the injection of the various injection materials must be planned and applied specifically for each building material in order to achieve success, depending on the degree of moisture penetration, the capillarity of the building material, and the thickness, etc. According to the WTA leaflet, the manufacturer of the injection material can obtain a WTA certificate from a testing center. If the injection material passes the test conditions, they subsequently receive a so-called WTA effectiveness test certificate, which specifies the degree of moisture penetration of the injected building material for which it passed the test.

Procedure for high moisture levels

The decisive factor for the effectiveness of a pressureless borehole injection is the degree of moisture penetration of the masonry and the resulting quantity of injection material. However, if a building material pore is, for example, more than 95 percent filled with capillary water, there is insufficient residual volume to absorb the injection material. Injection into such a moist building material is therefore ineffective without preparatory measures, i.e., the pores must first be freed of water. This can be achieved through so-called pre-drying, which precedes the actual injection. Electrically operated heating rods are inserted into the borehole channels and heat the masonry to a temperature of approximately 110 °C. During this heating process, the water present in the building material pores evaporates in the area of the later injection level. This means that during the subsequent injection, the entire pore volume is available for pressureless absorption of the injection material. Pre-drying is necessary, for example, so that the building material can absorb paraffin oil.

Electroosmosis process

The electroosmosis process is not a horizontal barrier in the strict sense, but is based on the physical principle of electroosmosis, according to the information provided by the suppliers. In practice, however, there is no process that has proven itself over the long term.

Remedial damp proofing

Until the 20th century, masonry buildings in Europe and North America were generally constructed from highly permeable materials such as stone and lime-based mortars and renders covered with soft water-based paints which all allowed any damp to diffuse into the air without damage. The later application of impermeable materials which prevent the natural dispersion of damp, such as tile, linoleum, cement and gypsum-based materials and synthetic paints is thought by some to be the most significant cause of damp problems in older buildings.

There are many solutions for dealing with dampness in existing buildings, the choice of which will largely be determined by the types of dampness that are affecting the building, e.g., rising damp, hygroscopic damp, condensation, penetrating damp, etc.

In older buildings, damp stains on internal walls are usually due to external factors such as:

Leaking rainwater gutters

Misdirected rainwater downpipes

Insufficient external drainage

Poor drip details to cills and other protrusions

Bridging of the damp proof course

Health and safety

Some DPC materials may contain asbestos fibres. This was more commonly found in the older, grey sealants as well as flexible tar boards.

Sourced from Wikipedia