Need for Dehumidification

Why use dehumidifiers when you can just open the windows? So goes the conversation as the restorative Drying Contractor begins setting up his equipment. After all, the more equipment the higher the cost, right?

Let's examine what happens in a typical water loss where there are no sewage or mould problems:

WATER EXTRACTION

The extraction process removes the majority of the water but cannot completely dry the water-soaked material. The remainder of the drying process depends upon evaporation, which is the process of converting water from a liquid into a vapour. The layer of air at the surface of the wet materials is still saturated with moisture and it can't hold anymore water. In order for drying to occur, the moisture-laden air needs to be displaced with drier air. Then evaporation can continue.

EVAPORATION

High velocity air-movers are used to displace the wet surface air with warmer, drier air. The greater the airflow, the faster the rate of evaporation. Moisture will be drawn from the wet material and transferred to the air space. The result is drier material sends higher humidity in the air space. The air space will continue to accept the moisture load until the air is saturated. When the air is at 100% Relative Humidity (RH), drying stops. If the moisture is not removed from the air, then secondary damage can occur. Hygroscopic building materials such as drywall, wood, paper, some insulation and fabrics along with content items such as artwork, photos, musical instruments, documents, clothing and wood furniture can absorb enough of the moisture to cause damage. Most of these materials are designed for use in a normal relative humidity of 40% to 50% without damage or mould growth.

ACCELERATED DRYING

Drying can be accelerated and secondary damage can be prevented by keeping the air as dry as possible. This can be accomplished in one of two ways; an open-drying system or a closed-drying system. The open-drying system is the process of using outdoor air to reduce the indoor humidity. This can be done when the outside humidity is under 40% RH and the outside temperature is above 16°C. Using this system, the structure is ventilated and not dehumidified. While this system can work in some instances, it has a downside. The weather has to be consistently warm and dry. If the outdoor humidity rises, the drying process can actually be prolonged. The outdoor relative humidity is the lowest target RH for the interior. Once the interior humidity levels equal the outdoor humidity levels, drying begins to slow down. The open drying system does not work well in areas such as basements or office buildings, where there isn't adequate ventilation available. Security is also an issue. The closed drying system uses dehumidification in a closed indoor environment. Dehumidifiers are used to remove the water from the moisture-laden air. This system provides for a greater degree of control over the drying process. Indoor RH and material moisture content can be reduced faster, reducing the length of time that the equipment is required.

In either system, cost effectiveness is based upon the daily measuring of the humidity and the moisture content of hygroscopic materials. The results of the measuring should be recorded and kept as part of the file. The target goals should be below 40%RH at 22°C and under 20% moisture content (MC) in most structural materials. Below 20% MC, the materials generally will no longer support mould growth. Some structural materials, such as hardwood flooring require an MC of 8% to 12% in order to prevent or correct damage. Target goals will vary depending on the circumstances.

An open drying system may be appropriate in some situations, but dehumidifiers are easier to control. A closed drying system reduces the possibility of mould growth, and secondary damage to structural materials and contents resulting in reduced reconstruction costs.