Structural Drying >> Ceiling Leak Under Bathroom

For constant boundary conditions, a homogeneous material, and a fully developed moisture profile, the signal velocity of this profile inside the material is constant over the square root of time. This is well known and frequently observed for water absorption, Ceiling Leak Under Bathroom but it applies similarly for the drying. The main difference is that the moisture profile does not develop as quickly. 

In fact it is the end of the first drying phase that is characterized by the completed development of a distinct moisture profile. Once this profile is there, Ceiling Leak Under Bathroom the end of the drying processshould only depend on the signal velocity and the height of thematerial sample. 

The duration of the second drying phasemust, therefore, increase quadratically with the sample height,provided that the drying was started at a moisture contentsufficiently high to allow a clear distinction between first andsecond drying phases.This was investigated by a simulation study on drying offour different materials: calcium silicate brick, ceramic brick, aerated autoclaved concrete, Ceiling Leak Under Bathroom and calcium silicate insulation.

The material properties were taken from Scheffler (2008). The drying was calculated one-dimensionally for different sampleheights with constant boundary conditions of 20°C (68°F) and50% RH. It was started at effective saturation. The results are presented in Figure 7. The left chart shows the total dryingduration versus sample height; Ceiling Leak Under Bathroom the right chart shows only thesecond drying phase duration versus sample height.

The results confirm the above reasoning. The duration of the second drying phase increases quadratically with the sample height. In other words, Ceiling Leak Under Bathroom the square root of the second drying phase duration plotted versus sample height gives a linear relation (see right-side chart of Figure 7).

We therefore propose the following relation (Equation 3) as the definition of the new drying coefficient for building Ceiling Leak Under Bathroom materials: (3)where= drying coefficient in = drying time of the second drying phase in = sample height in The second drying phase duration is the timebetween the deviation from the linear drying and the end of thedrying process (see also Figure 1). 

In other words, it is the timebetween the end of the first drying phase and the moment where the equilibrium moisture content is reached everywhere inside the sample. For measured data, Ceiling Leak Under Bathroom this depends on the frequency of measurement as well as the boundary temperature and relative humidity and might, therefore, be susceptibleto error. 

It is proposed to use calculation results of the dryingprocess normalized to standard conditions. From such data, Ceiling Leak Under Bathroom the second drying phase duration can be easily determined. For most materials, the first drying phase duration isnegligible in the overall drying time. This allows simplifyingthe found relationship to be true also for the total drying timeand not only for the second drying phase duration for which itis strictly correct. 

The left-side chart in Figure 7 shows thetotal drying time versus sample height. The linear relationshipholds for all materials except the calcium silicate insulation, Ceiling Leak Under Bathroom which has a very high porosity (90%) and a very long firstdrying phase. This new drying coefficient gives a measure for the timeit takes to one-dimensionally dry out one meter of material. 

Asdiscussed before, Ceiling Leak Under Bathroom for most building materials this is correct forthe total drying time even though it is strictly only valid for thesecond drying phase. Example values for drying coefficientsof typical building materials are given in Table 2.Discussion of the Proposed Drying CoefficientThe drying coefficient gives a measure of the timerequired to dry out 1 m of material. 

By that, it has a particularmeaning and is a practically useful material parameter. Thequestion is now, Ceiling Leak Under Bathroom however, whether this parameter contains additional information in comparison with the other two moisture transport parameters, i.e., vapor diffusion resistance andwater absorption coefficient. 

For that reason, these coefficients were related to each other for numerous building materials (drying coefficient of material A divided by dryingcoefficient of material B and so on, Ceiling Leak Under Bathroom and similarly for vapordiffusion resistance factors and water absorption coefficients).These related coefficients are plotted against each other inFigure 8 in order to visualize any possible correlation.

If the corresponding properties are correlated, the relatedvalues of different building materials should follow some kindof curve. Between drying coefficient and vapor diffusion resistance, Ceiling Leak Under Bathroom a positive linear correlation is observable, though witha lot of scatter. Between drying coefficient and water absorption coefficient, a negative linear correlation can be found,again with significant scatter.

A weak correlation is not a surprise, Ceiling Leak Under Bathroom since all these coefficients are the result of the material’s specific pore structureand pore connectivity. Some kind of distinct correlation between drying coefficient and vapor diffusion resistanceseems to be obvious, since vapor diffusion dominates thedrying process to quite some extent. 

However, the left-sidechart in Figure 8 indicates there is only a weak correlation.Therefore, Ceiling Leak Under Bathroom the drying coefficient cannot contain exactly thesame information as the vapor diffusion resistance. It mustcontain its own, drying-specific information as well. In orderto investigate this further, the sharp-front model according toHall and Hoff (2002) and Hens (2007) is applied as follows. 

The sharp front model distinguishes between two casesthat can be treated in a simplified way. The first case comprisesstages above critical moisture content. Water transport can be Ceiling Leak Under Bathroom described by liquid conduction. The second case comprisesstages below critical moisture content. Water transport isdescribed by vapor diffusion. 

Due to the high permeabilitiesfor liquid conduction, this treatment leads to a small gradientfor liquid transport and a high gradient for vapor transport.This sharp front has given the model its name.Applying this to the drying problem leads to the modelshown in Figure 9. Assuming the drying starts above criticalmoisture content, Ceiling Leak Under Bathroom the drying rate is determined by a constantvapor transfer at the material surface. 

This process—the firstdrying phase—continues until critical moisture content isreached at the material surface. Then the surface dries outentirely and a steep moisture front is established, Ceiling Leak Under Bathroom which movesslowly into the material. The drying rate is now determined bythe vapor diffusion from the moisture front to the materialsurface.

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