Drying out buildings

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Excess water content in materials
Dehumidification is mainly applied to dry out excess water content in materials in connection with construction work or water damage.

In case of water damage the general rule is to apply dehumidification as soon as possible, but as the nature and extent of water damage varies considerably it is necessary to assess the right approach from situation to situation.

An all important parameter in case of water damage is how much time the water has had to penetrate the building structure, furnishings, etc. It is also essential to keep the air change as low as possible to avoid humid air from entering the room. The rules of thumb will give you some empiric data to go by as it is often almost impossible to calculate the absolute accurate dehumidification load needed in a water damage situation.

In case of drying out a newly constructed building you should also keep air change low, but the most important parameter to consider is the water content in the various materials used. Often you have to meet a deadline i.e. you have to consider a finite amount amount of time to do the job.

Drying out buildings

Dehumidification _en _0906_Side _09_Billede _0001Formerly construction work on an average building went on for 6-9 months and the building materials were usually dried out by natural ventilation by the time the building was finished. Today, however, construction work is very efficient and much faster. This means that dehumidification is required to remove the excess water in the various building materials before the building can be occupied.

When selecting a dehumidifier for drying out a building you need to consider how much water should be removed and how much time you have to do it.

This is actually quite a difficult task. In some cases it is possible to estimate the amount of water in the building materials from tables. Please note that in regards to drying out a newly constructed building it all comes down to the specific building materials used for walls, floors and roofs. The water content of various building materials differ so much that a simple rule of thumb is unworkable.

 

Water content of different building materials (kg/m3)

MaterialAt start of projectWater chemically bounddesired condition by 50% RHwater to be dehumidified
Wood 80  - 40 40
Tile, roof 10 - 10 0
Brick, wall 80 - 10 70
Lightweight concrete  100-200 - 20 80-180
Concrete K 15 II 180 42 38 100
Concrete K 25 II 180 57 46 77
Concrete K 40 II 180 71 51 58

 

In this example we want to calculate the dehumidification load required to dry out excess water from a newly constructed building in 30 days. The building is 2,4 m high, 7 m wide and 16 m long. The walls and ceiling are constructed from pre-dried wood. The floor, however, needs to be dried out as it is made from 10 cm thick concrete, K 40 II.

The data:

Period 30 days
Drying condition t = 20°C and 50% RH (average between starting humidity at 60% RH and ending at approx. 40% RH)
Volume of building 2,4 * 7 * 16 = 268,8 m3
Materials Concrete K 40 II, 10 cm

 

The calculation:
Concrete volume to be dehumidified:
V = 16 * 7 * 0.1 = 11.20 m3

Water content in concrete floor:
Q = 11.20 * 58 kg water/m3 = 649,6  kg water

We need to remove 649.6 L water in 30 days:
W = 649.6/30 = 21.65 L/24 hours

We need a dehumidification capacity of 21,65 L/24 hours.

Recommendation: CDT 40. Capacity: 0,70 litre/hour at 20°C/50% RH. One CDT 40 will remove 16,8 L/24 hours. This means that two CDT 40 units should do the job.

Note that the drying process is quickest in the beginning as the water content is very high when you start the process. As the RH-value decreases the overall dehumidification capacity will also decrease.

Guidelines for the drying process
When dehumidification is used to dry out buildings and materials the dehumidifier runs continuously. The relative humidity is gradually lowered allowing further evaporation from the damp materials in the room. The amount of evaporation depends on the temperature of the room, the materials and the humidity of the air.

One of the advantages of condense drying is that the drying proces is stable and gentle. If time is not of the essence the optimum dehumidification process is achieved by maintaining a stable condition of 20°C and approximately 40% RH in the room. This way you maintain a perfect balance between the dry air in the building and humid building materials, avoiding surface drying and cavitation as well as damage to pre-dried materials such as parquet floors.

Add heat if necessary, but keep in mind that forcing the drying process might be harmful. There is a risk of surface drying and cavitation, with only surfaces being dried, whereas a lot of humidity remains within cavities inside the wall. This prolongs the drying period as the humidity will not easily penetrate the dry surface. Surface drying also involves the risk of cracks appearing in the surfaces of walls, ceilings and floors.

It is important that the room/building is as sealed as possible. Also make sure that the building is well protected against rain and snow. You need to ventilate while painting inside the building, but remember to seal the room or building properly when it is empty. Also remember to avoid pre-dried materials absorbing water because of open windows.

If the air change inside the room is not controlled, then changing ambient temperatures and humidities make the process much more difficult to control. In the winter the cold outside air will normally contain a minimum of water and the humidity is not likely to increase much even if the air change is considerable. Energy consumption, however, will increase dramatically as you need to heat up the cold incoming air. In the summer the water content could be quite high and you will have to remove even more water from the building or locale if it is not sealed off adequately.

In most cases the humidity is concentrated in cellars and in areas where water is being used in the construction work going on i.e. painting, concrete mixing, etc. Set up your dehumidifiers at these positions where they can do most good.

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