How do you dehumidify efficiently at construction sites?

Dehumidification is often required to dry out excess water content in materials in connection with construction work.

In case of drying out a newly constructed building, you should keep your 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 only have a limited amount of time to get the job done.

Download our “Selection guide for mobile dehumidifiers” for professional guidance on how to apply mobile dehumidifiers at construction sites and how you choose the right one for your project.

Drying out buildings

Historically, 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 determine how much water to remove and the amount of time you have to do it.

This can be a difficult task. Sometimes, you can estimate the amount of water in the building materials using tables. Please note that in relation 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)

MATERIAL	AT START OF PROJECT	WATER CHEMICALLY BOUND	DESIRED CONDITION AT 50% RH	WATER 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 capacity required to dry out excess water from a newly constructed building in 30 days. The building is 2.4m high, 7m wide and 16m long. The walls and ceiling are made from pre-dried wood. The floor, however, needs to be dried out as it is made from 10cm 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, 10cm

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 process is stable and gentle. If time is not a crucial factor, the optimum dehumidification process is achieved by maintaining 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. It creates a risk of surface drying and cavitation that only dries surfaces, leaving a lot of humidity inside the wall behind the dry surface. 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. Moreover, remember to avoid pre-dried materials absorbing water because of open windows.

If the air change inside the room is not controlled, fluctuating ambient temperature and humidity conditions make it hard to control the drying process. In the winter, the cold outside air normally contains 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 room 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 such as painting or concrete mixing. Set up your dehumidifiers where they do most good.

You can download the complete Selection Guide for Mobile Dehumidifiers here: Dantherm Selection Guide