How to dry basements, foundations and buildings

The process of drying a flood- or water-damaged property is fundamental to its permanent restoration. This section provides a high level view of the issues connected with the drying process.

Establishing a drying program

A drying program needs to be controlled and monitored to ensure an optimum rate of evaporation (i.e., the maximum safe rate of drying that allows wet materials and products to return to their former dimensions and colors). Whatever type of drying or dehumidification process is used it will depend on heat to aid evaporation, air changes to carry away moisture-laden air, and air movement to transfer wet surface or boundary moisture into collection systems. These inputs will be factored into the process in varying combinations depending on ambient temperature, room temperature, amounts of water in the air and absorbed by materials, and air speed. The collection system can be a simple refrigerant or desiccant dehumidifier, or convection drying methods.

Before drying operations can start, standing water in buildings must be allowed to subside or be pumped away.

There are a number of questions that need answers before establishing a drying program.

• Can the building be sealed?
• Is the building's heating or air conditioning system available to use?
• What is the condition of the property's electrical system?
• What types of material are being dried?
• How long were materials under water?
• What was the depth of flooding?
• What type of equipment will be used for the drying process?
• What are the outside weather conditions?
• What are the attitudes of, and levels of cooperation between, the insured and building occupants (e.g., tenants)?
• How quickly can the insurer, its representative or the property manager make a decision about drying out the property? For example, are there issues of insurance cover which need first to be resolved?
• What is the overall condition of the fabric of the property?
• Is the property at risk of re-flooding?
• Is there any trapped interstitial moisture (i.e. moisture trapped in insulated floors or walls)?

As a result of considering the answers to these questions there are a number of ways that a property can be dried. There are some general rules of thumb that may be used.

The various factors that contribute to moisture in the air and in materials, and how they interact, are explained in Understanding dampness [2]

Key factors in the drying process

To a great extent the efficiency and speed of a drying program will depend on the effectiveness of any operation to remove floodwater that remains in the property. If the water has not subsided of its own accord, other means will need to be found for removing it (e.g. pumping - the sort of service provided by local fire services). The more floodwater that can be removed at this stage, the less that has to removed by the drying processes. Pumping is, of course, a very fast method of removing large volumes of standing water from a building.

When as much standing water as possible has been taken away, a program of drying can start. The water remaining in the property should be confined to building components and materials - wood and concrete in floors, bricks and blocks in walls, for instance. Portable furniture should be removed; in fact anything that can hold water or delay the process of drying (e.g. carpets, and Welsh dressers hard against walls).

The drying process involves a number of criteria, principally:

• speed of air movement
• ambient and room air temperatures
• air moisture content (which is related to air temperature).

Other factors are also important such as the moisture content of materials, permeability of surfaces and the presence of moisture barriers.

Whatever type of dehumidification is used, it must be combined with a suitable number of 'air movers', and at a suitable temperature. All these factors are important in the evaporation process; they can be adjusted during the process to get the maximum drying capacity by a trained and skilled drying technician. There are different ways of deciding which type of equipment to use, and how many pieces of equipment to employ. These decisions and processes are relatively complex, often involving detailed calculations, and are best left to technical experts.

Importance of monitoring

Monitoring the drying process provides the technician with valuable information about the condition and state of an installed drying program, and allows for alteration and re-focusing of drying criteria. It will identify equipment malfunction but, most importantly, provide information on the effectiveness and speed of drying thereby allowing for manipulation of the process in terms of equipment. Monitoring records should be taken and retained for inspection.

During monitoring visits, equipment will need to be adjusted, or increased or reduced in number, to ensure that effectiveness and speed are maximized. In most circumstances the ideal drying conditions should be held at approximately 40 to 50% relative humidity with temperatures at 18 to 23 °C.

High temperatures

Higher temperatures can be used in certain circumstances in order to dry the property by evaporation but is made considerably more effective with ventilation. For effective drying, temperature management must normally be combined with other mechanisms such as control of vapor pressure.

Vulnerable materials

The ideal conditions for drying will depend on the nature of the property and on the equipment selected, and how they are used and monitored. It is to be expected that some vulnerable materials will warp or shrink if they are not dried in equilibrium to the local environment.

Common problems

The most important aspect to look at is the possibility of trapped moisture within the structure; this can be done when drying has commenced. The most likely areas for trapped moisture are within insulated floors and cavities in walls, but the problem can occur in other areas - in fact it can be any location where evaporation of excess moisture is restricted or prevented because of the lack of permeability of the materials surrounding the affected area.

Surface coatings and impermeable membranes can inhibit the release of moisture and slow the drying process.