#solar drying

The importance of drying in damp houses as a part of the daily health is essential.  In some cases, changes in the weather can affect drying but, for now, let’s just concentrate on drying.

Drying simply means removing dampness remaining on the absorbent  parts of the house structure and soil as a result of flooding, natural seepage or pipe leaks.  This is accomplished after one of two ways of removing the  physical liquid.  Physical removal of liquids may be as simple as opening a hole in the brickwork at the lowest point of the subfloor area that will allow the flowing liquid to drain due to gravity.  Or, it may involve using a pit or sump pump in closed or encased subfloor areas to mechanically draw that liquid out to a more suitable drain area. Drying then begins with either two ways slow positive input of dehumidified warm air in combination with a negative balanced airflow to keeping motional cross balance. Two is to swiftly draw air at quantities using negative pressure only using means such as a large fan from the lesser part of the building drawing warmer dryer air in from the  greater side of the building. Both methods creating the more commonly known method of evaporation.  Evaporation of liquid is usually enhanced through the use of heat and the movement of air over the area. In today's Australian climate, cross ventilation is rarely suitable due to boundary enclosures, fences, close buildings, differential wind direction and incorrect vent placing.

At first, drying by evaporation would seem very simple.  The evaporation of liquids, after all, is nothing spectacular.  It’s a process we see every day.  It rains, the sidewalk gets wet.  The rain stops and the sun comes out and the water on the sidewalk evaporates and is gone.  Job done, however take a deeper look, and think where all that water has dissipated to? It also reveals that there is more to evaporation than one might think.  The belief is that the  rate of evaporation depends on temperature only!  The higher the temperature, the faster evaporation takes place!  Well, actually, yes but in fact not completely correct!  The rate of evaporation is actually driven by the relative humidity to a greater degree and air movement than by temperature.  As the temperature of air is increased, it can absorb more liquid and, therefore, the relative humidity is decreased. The greater the air movement the greater the displacement of lower humid air.  Lower relative humidity promotes faster drying.  The following chart and graph which both show essentially the same data are very interesting.

As temperature is increased, the amount of water required to saturate a specific volume of air increases.

This graph shows that as the temperature of air increases, the amount of water required to saturate it increases dramatically. A few degrees of increase in temperature has an increasingly large effect on the saturation point. So, take for example a subfloor area at a temperature of 10C, with 100 RH, this would hold 7.8 grams of water vapour per Kilogram g/Kg. -  Now, introduce airflow continually at only 20C, and 50% RH - not much, but this will have the capability to hold twice the what is present specs (20C -15 g/Kg). Take it another step, continuously drawing this 20C air which holds double the g/Kg capacity through your subfloor area and then displaces it externally will not only change the air temps and humidity but will draw the dampness (moisture) out of surrounding structures (timber and brickwork). Doing this process on a daily basis and allowing stagnation at night allows building structures to become dryer, subfloors temperatures raise by a couple of degrees and humidity levels to drop dramatically.

Air at 100% humidity is saturated with water.  If a volume of air saturated with water is heated, the level of saturation is decreased and the air requires additional moisture to again become saturated. Air that is saturated with water is at a relative humidity of 100%.  Air that contains only 50% of the water required to be fully saturated is at a relative humidity of 50%.  Similarly, if the temperature of a volume of air that is saturated is reduced, water comes out of the air as a fog or water droplets.  The “dew point” is the temperature at which air becomes fully saturated.  In weather terms, this is when it rains. In thermal bridging of surfaces at a lower temperature, this is when you see condensate.

Relative humidity in percent is the total water required for a volume of air divided by the amount of water that would be required to be totally saturate that volume of air.  In drying, it is important to understand the role of both temperature and humidity and how they are related.  Next consider the complete implication that by drying and slightly raising the temperature of what is normally the coldest and wettest part of the house, would have on a daily basis.