<%@LANGUAGE="VBSCRIPT" CODEPAGE="CP_ACP"%> DPC Training Manual
You are here - Homepage > DPC & Timber Treatment > DPC Training Manual

 

 
o o o o o
 
 
TEMPERATURE RISE (top)  
     
  The rise of temperature of a surface, particularly a black surface, which may be as much as 70°C, should influence design. Black material like asphalt and bitumen would soften, and flat roofs of asphalt are best covered with a reflective material. The effect is seen in paintwork on wood. White colours stand up better to intense heat than black or dark ones, particularly on wood.
  
     

HUMIDITY (top)  
     
 

Iron or ironwork, steel tools, etc. should be protected from high humidity as well as actual condensation. Articles will rust in the course of time in any unheated roof space as well as in worse conditions in sheds and garages.

  
  The paint on external wood and ironwork should be watched and not allowed to deteriorate too far before repainting, even if the repainting is confined to selected areas on a temporary basis. This applies to cills in particular; otherwise one invites expensive renewal of the woodwork.  
     

SULPHUR POLLUTANTS (top)  
     
  The chief effect on buildings is that upon limestone, magnesium sandstone, marble or other forms of CaCO³. Limestones differ in their resistance to sulphur pollutants. If limestone must be chosen for a façade in a sulphur-polluted district the best type, known to stand reasonably well – such as Portland stone – should be chosen. Stones should not be mixed, e.g., sandstone and limestone together, or limestone and magnesium limestone. The question of stone “preservative” is a vexed one. Years ago stone preservatives were put on the market, but were not found to be effective. More is claimed of the more recent ones, but the time scale for stone preservation is long. Some of our finest old buildings are stone buildings and have lasted for many centuries. Modern stone preservatives have not been tested over very long periods.  
  Detailing of new work should be such as to allow rain to occur freely, without there being sheltered portions where soot, grime and calcium sulphate can only be allowed to accumulate. Stone cleaning, if carried out be a reputable firm, is a good thing. Only water and steam and a brush should be used, and no alkali, which has a subsequent harmful effect.  
     

METALS (top)  
     
  Apart from the obvious need for protection of ferrous metals, other metals such as copper, lead and aluminium do not in general need protection. Detailing however is important in connection with copper and bronze, that the green-coloured run-off does not mar other parts of the building.  
     

FROST (top)  
     
  In the case of bricks, use ones known to stand up well in frosty conditions, if the bricks are to be exposed to wet and frost. This applies particularly to freestanding and parapet walls and retaining walls. In the case of concrete, the main criterion of frost resistance is that the concrete is dense, well designed, with good graded aggregate and well executed. Vibrated concrete is in general better than non-vibrated.  
     

SULPHATE ATTACK (top)  
     
  Environmental conditions leading to sulphate attack, apart from concrete foundations in soil which we will here exclude, is largely confined to rain and driving rain on brickwork, plain or rendered. The sulphates are usually those in the bricks themselves. Bricks differ considerably in this respect, but most do contain some sulphates. To minimise the attack on exposed walls, e.g., self-standing walls, good design is important, to minimise the entry of water. First a good coping known to be frost resistant and sulphate free is required and immediately under it, a damp proof course, a little above ground. It is not always possible to choose bricks for the main part of the wall known to be fairly sulphate free, because most facing bricks do contain some sulphates. The wall must be kept as dry as possible and use of sulphate-resisting cement may be necessary. If in the right conditions the bricks show a lot of efflorescence; they contain a fair modicum of sulphates.  
  The trouble with rendering, which is after all very common, is that water gets into the wall behind via cracks or draws water up from the ground. This is a matter of expertise, which is more lacking in the U.K. than in Europe. Once water gets behind rendering it may stay there for a considerable time, causing sulphate attack, which pushes off the rendering, aided at times by frost.  
     

MOISTURE IN BUILDINGS (top)  
     
  Moisture in the air is water vapour, H² O in vapour form. Unfortunately it has a nasty habit of condensing to liquid water when it is not wanted so to do. We seem to be much more troubled with condensation in modern buildings and living conditions than our forefathers were, and there are good reasons to account for this.  
Condensation has the following ill effects:  
1
 It makes part of the building damp (with wet water).  
2
 Dampness encourages mould growth.  
3
 Dampness deteriorates wood, paper, clothing, plaster, paint etc.  
4
 Dampness reduces insulation – increases conductivity.  
  Sources of moisture, other than direct leaks from outside or leaky pipes, are:  
1
 Fuel such as oil in stoves where the fumes from combustion are let out into the room. All fuels contain hydrogen chemically combined which burns to form water vapour.  
2
 New buildings with fabric not yet dry.  
3
 Steam from kitchens, bathrooms and washrooms.  
4
 Moisture from persons (one perspires or respires 1.5 pints of water every night).  
  You can see this condensed on the windowpanes. If left, the water tends to get left behind the paint on the top of the bottom rails of the casements and gradually pushes off the paint.  
     

EVAPORATION OF LIQUIDS (top)  
     
  What exactly happens when a liquid evaporates? A simple explanation is that the molecules of a liquid are in a continuous state of motion, jostling each other – the higher the temperature, the more hectic the movement. The energy they possess enables some of them to burst out of the surface against the general action of gravity and career through the space above (air or no air – but much more readily of there is no air gap to oppose them). Once free they are “vapour”.  
  At any given temperature the air can contain a fixed maximum amount of water vapour (see table on FIG.2). This may be expressed in grams per metre cubed (g/m³) or as a pressure (SATURATED VAPOUR PRESSURE, SVP) in terms of a height of mercury that the vapour would exert in a barometer, in the same way as a barometer registers the total pressure of the air. Pressures are now expressed in N/m².  
  If the air becomes fully saturated then the figures in the table apply. It can be seen that the amount that can be contained without condensing rises sharply with temperature rise, or falls with fall of temperature.  
  Open air is seldom saturated, at least in a building. In a closed vessel with liquid water inside, given time, the space will be saturated or 100% humid. This state of affairs occurs outside on most nights of the year in the U.K. near ground level. As the saturation point of temperature (DEW POINT) is reached so the excess moisture begins to condense (forms dew). If below 0°C, hoarfrost forms.
  
     

DALTON'S LAW OF PARTIAL PRESSURE (top)  
     
  It may help one to understand vapour pressure if one considers a law enunciated by John Dalton many years ago. It says that in a mixture of gases in an enclosed space the pressure contributed by each is what would be exerted if each were separately confined in the space. Thus, to take a simplified case, consider the air in a sealed room at standard pressure (750mm of mercury). Consider it as 1/5 oxygen and 4/5 nitrogen. Now imagine the oxygen at one end of the room, occupying 1/5 suddenly removed. The oxygen would expand to five times its original volume and by Boyles Law its pressure would go down to 1/5, i.e., from 750 to 150mm. Daltons Law says that the 150mm out of the total of 750mm is, anyway, provided by the oxygen and 600mm by the nitrogen. If there are any other gases they contribute their bit also to the total.
Water vapour contributes its quota too, but not as much as shown in the table, which shows “saturated” vapour pressure (SVP), since the air is not normally saturated. If only 50% saturated then the pressure is one half of the table figure.
  
     
 
 
     

RELATIVE HUMIDITY (RH) (top)  
     
  Rather than quoting the actual humidity of the air in g/m² or in terms of pressure, it is more usual to quote the RELATIVE HUMIDITY or percentage saturation, which is the criterion that affects us and things to a greater extent than actual humidity. RH is “% saturation” and may be defined as:  
     
 
Actual amount of water vapour
present in the air
  
 
________________________ x 100%
  
     
 
Maximum amount of vapour that
can be held at the temperature
  
     
  We are used to a relative humidity of 50%, say 40% - 60% in a room. More that 60% induces peculiar descriptions of the air as “close”, “clammy”, “humid”, etc. It usually fells a degree or two warmer too, if the air is warm, as we cannot perspire so readily into a damper air. One may wonder how the average European would describe the air in the Amazon region which is stated to have 98% RH, quite often, in the daytime.  
  Knowledge of relative humidity is essential not only in meteorology and in washed-air ventilation but also in industrial occupations, storage, computer rooms, and in the operation of the timber drying kiln, where drying must commence in a very humid kiln atmosphere (over 80%).  
  In outside air, the humidity varies with weather and time of day or night. As stated, it rises to 100% on most nights of the year at ground level in the U.K. and dew starts to form. As the day progresses from early morning the air becomes warmer and so relatively less humid (unless there is a westerly unusually moist air stream arriving) and the morning dew evaporates.  
     

 

<PREVIOUS | 1 | 2 | 3 | 4 | NEXT>