Drying time
Background
After flood damage has occurred the cost of reinstatement will be influenced by the time elapsed before the building is returned to it’s normal background moisture contents. Some moisture will be normal within the structure of building materials and the longer that materials remain at higher moisture levels the greater the risk of structural deterioration and mould (fungal) growth, both on the surface and hidden within the structure.
Clearly drying after serious flooding will take a significant time and that in itself may have cost implications through loss of use and other disruption. Where insured repairs are undertaken there is a responsibility to minimise these costs.
The question of interest to us in managing the drying process is what steps are worthwhile and cost effective to use. To evaluate this we need some understanding of the process of drying.
It is worth noting here that drying relies upon evaporation from the surface of the material and any impervious covering, such as laminate flooring, will act as a vapour barrier and completely prevent that evaporation. Floor and wall coverings should be stripped as soon as is practical as trapped moisture will quickly lead to mould growth
Drying process
The commonly found building materials in which we are most interested are all porous and therefore able to absorb moisture deep into their structure. The longer the period of saturation where surface water is present, the deeper the moisture will penetrate. The rate of drying will depend on :-
- Temperature
- Relative humidity of air
- Air velocity
- Properties of the material
1st stage of drying
Initially drying of the surfaces of the building by evaporation will take place and this will be similar for both porous and non porous materials. This will normally be completed relatively quickly. The time will be a function of temperature, relative humidity of the air and air velocity across the surface. Without adequate use of fans to give air movement the less accessible surfaces will not dry as quickly, as you might expect. Accumulated moist air in these areas will slow the process and since the slowest drying area will determine the progress towards reinstatement it is essential to focus on those less accessible areas as early as possible.
Lowering relative humidity will be beneficial and ensuring that air dried by dehumidifiers reaches the least accessible areas needs to be a primary aim.
For the purposes of determining what additional equipment to use to further accelerate the drying, as a general rule increasing the ambient temperature by 10 deg K will double the evaporative rate and so halve the time needed for this 1st stage drying. A similar effect on the rate of evaporation can be achieved by four times the air velocity though, as noted above, this is clearly difficult to achieve in less accessible locations where some air movement to avoid stagnation will be a more practical aim.
2nd stage drying
When 1st stage drying is complete the surface of the floor and walls may appear dry. The vapour transport limits of the material will determine how fast the moisture trapped within the structure can progress to the surface where it can be evaporated off. These vapour transport limits differ for different materials.
If drying though dehumidification, heating and air movement, is stopped before the moisture content of the material is back to it’s background level – that which existed before flooding – then the surface will feel damp again after some time. New finishes will be compromised and relaying of impervious floor coverings would lead to further damp problems as damp migrates to the surface. For this reason adequate time for 2nd stage drying must be allowed before reinstatement.
Measurement of the progress in this 2nd stage drying can only be meaningful if they are made with probe type measuring instruments which can be inserted into holes drilled into the material.
Research (Moisture Transport in Porous Building Materials by L Pel, K Kopinga & H Brocken of Eindhoven University of Technology) has indicated that as drying progresses a ‘drying front’ moves deeper into the material and that under steady conditions this front moves at approximately constant velocity. The usefulness of this in a practical sense is that if, for example, a concrete floor with damp proof membrane under, so that drying takes place from one side only, is completely saturated and has a thickness of 150mm, then a drilling to 75mm showing a ‘dry’ reading will indicate that the drying process is at least half way towards being complete.
At 20 deg C and 65% relative humidity the following is a guide to the equilibrium moisture content of different materials. These are the levels which would indicate that drying is complete after flooding. Clearly there may be pre-existing reasons why the moisture levels may have been higher in some areas. Significantly higher levels would be a reason for concern.
| Wood in enclosed building | 8-12% or up to 16% in roof timbers |
| Concrete | < 2.2% |
| Cement mortar | <3% |
| Cement screed | <3% |
| Brick | <1% |
| Lime mortar | <2% |
| Anhydrite | <0.5% |
| Plaster | <0.8% |
Care and judgement should be used if utilising a resistance based moisture meter as hygroscopic salts (White surface discolouration) can show readings of 30-50%. This is not a cause for concern as salts readily absorb moisture from the air much like a desiccant on the surface but do not affect the vapour transport properties of the material.
In conclusion
Surface wetting can be achieved in much shortened times with the use of heat, dehumidification equipment and air movement. The 2nd stage of drying takes much longer and relies on the rate of vapour transport within the structure of the material. Moisture removal is much slower during this 2nd stage and monitoring is essential to determine when drying is complete.
Provided excessive ventilation of the area is avoided the relative humidity of the area can be maintained at the required level during 2nd stage drying with less equipment without extending the drying time.
