DPC Training Manual

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Evaporating Condensation \| Porosity Capillarity \| Surface Tension & Capillary Effects in Buildings \| Properties of Porous Walling Materials
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	HYGROMETERS


	There is a number of these simple instruments, which are used to measure the RH of the air. The simplest to use is the wet and dry bulb hygrometer.

	WET & DRY HYGROMETERS (top)


	This merely consists of an ordinary dry thermometer reading in C (or F) and another alongside it which has its own bulb wrapped in a wick dipping into a small reservoir of water. When set up and steady the two thermometers are read and the difference noted. A table (Table FIG.2) is consulted and the difference and the dry temperature are used to find the RH% direct, which are the figures in the body of the table. If the difference is small it is because conditions are not conducive to evaporation, i.e.; the air is more humid than usual or colder. This is evident on studying the table. In warmer and drier air the difference is larger.

	CONDENSATION: AIR & SURFACE TEMPERATURE (top)


	We have notices that condensation occurs when the temperature falls below the dew point. In a room there may be a number of different temperatures, but in general the air temperature as an average is probably taken in the middle of the room at mid-height. Over a radiator it will be warmer, and also near the ceiling. Near the floor it is usually cooler.
	Supposing an average air temperature in a room is 20°C when it is cold outside, then it is certain that the surface temperature of the external wall will be lower, and that of the glass of the window still lower, especially at 0°C are 7°C for the inside surface and 6°C for the outside surface of the glass.
	If the relative humidity in the middle of the room was 50% then the dew point would be about 8.5°C. So dew would deposit on the window unless there was a radiator underneath or a fair draught. In nights in winter, this condensation frequently streams down window panes of bedrooms where there is only single glazing, and is seen as pools of water in the morning, particularly if there is little ventilation. This applies more to rooms that are slept in than to empty rooms, as the persons in the room give out their quota of moisture.
	Condensation will also stream down walls and glazed doors of kitchens, and down bathroom walls that have little heating. An extractor fan over the gas stove in a kitchen helps considerably to reduce condensation as well as to remove cooking smells. But all this condensation is seen and, though it may cause some slow change, it can be lived with. It is to be expected and it is familiar. But what is not expected is that condensation should occur unseen inside a wall construction and not make itself evident till rather late.
	Condensation can occur interstitially, i.e., inside a modern construction, and architects and designers should be alive to the possibility and take precautions before the event in certain cases. Before instancing such cases there is the matter of the “cold bridge” to discuss.

	COLD BRIDGES (top)


	These are areas of construction where the U value is considerably higher than the rest of the wall, if it is a wall. An example is solid concrete lintels, even if faced with brick outside. Condensation is likely to take place on them at times. Since the U value would be over 1 W/m² °C they are officially not allowed for new house building. Beside concrete lintels is it possible for concrete to “bridge” from inside to outside of a wall in the form of columns in some constructions.

	INTERSTITIAL CONDENSATION (top)


	In some modern constructions condensation can take place within the fabric of the wall and not be seen on the surface for a time, and meanwhile do damage. The moisture may be harmful to the fabric and it will certainly reduce the value of the insulation. The moisture can even cause dry-rot if wood or wallboard is present.
	Interstitial condensation occurs because the dew point at a certain layer of the construction is above the actual temperature. Such a condition tends to occur toward the back of the insulation where the construction and insulation is pervious to water vapour, i.e., allows vapour to travel through freely from one side to the other. Two instances will be quoted.
1	A construction of solid concrete for wall, with a good thickness of efficient insulation, e.g., 25mm of expanded polystyrene, with plaster or plasterboard face. Condensation occurs at the back of the polystyrene. Its temperature is below dew point.
2	If an existing one-brick thick house wall (solid) is to have insulation improved by battening out and fixing fibreboard to the battens it may cause the same effect, and the water in the space may cause danger of dry-rot to the battens or wallboard.
	A vapour “barrier” is needed. It must be put on the warm side of the insulation. It is fundamental to appreciate that the moisture did not have its origin as liquid water or water vapour from outside but that the origin was water in the air of the room and that the vapour travels from inside to outside of the building. Remember that the table of vapour pressures shows that the pressure is greater on the inside than on the outside of the building when it is cold outside.
	SEE FIG. 3 & 4.
	FIG. 3

	FIG. 4

	EVAPORATING CONDENSATION (top)


Q.	What conditions are conducive to drying or evaporating condensation, or its non-formation?
A.	Several
1	Obviously heat – raising the temperature.
2	A drier air – but this cannot necessarily be arranged. We usually have to take what Nature provides.
3	Draught or wind or mechanical ventilation. This is usually most effective (think of the washing on the line).
	Exposure of more surface to the air, of course, is another inducement, but this applies more to drying laundry as one cannot spread out the damp fabric of a building.
	Incidentally, one of the main reasons for increased frequency of condensation troubles in modern living as compared to years gone by, when the old open fire was the chief source of heating, is the reduced ventilation of modern houses with central heating where there is such an urgent need to conserve heat and fuel. The old open-fire ventilated the house well but only warmed a limited area at a very low efficiency of the use of fuel.

	POROSITY CAPILLARITY (top)


	The force of attraction between like molecules of one substance is called COHESION. Every molecule attracts every other molecule near to it with this cohesive force. If the material is solid and is fractured with a clean break and the two halves are fitted together, the pieces cannot usually be fitted so closely together that the molecules are close enough to regain their cohesive force at the fracture without a thin layer of adhesive between. This would be a liquid, which completely fills the gap and adheres to both surfaces. So adhesion is a force of attraction of unlike molecules or surfaces. A liquid can “wet” or truly touch a solid surface over most of its area so that molecular forces, which we call adhesion, take over. ADHESION, then is an attraction of unlike molecules.
	Consider glass and water. The force holding the glass molecules together or the water molecules together, we call cohesion, but the sticking of water to glass we call adhesion or surface tension between water and glass.
	FIG 5. Shows liquid molecules (crudely represented by little spheres for simplicity) sticking together by tensile forces in all directions (it should be three-dimensional, or course) except at the surface, where there are no upward forces. This gives the surface the resemblance to a stretched skin – horizontal forces are present in all horizontal directions, and downward forces. The forces in the surface are called SURFACE TENSION.
	If water is observed touching clean glass it turns upward in a curve called a MENISCUS. It is evident that the adhesion of the top most water molecules to the glass molecule, B to A, must be at least as great and presumably greater than the cohesion or surface tension force in the water molecules themselves, B and C. This means that two surfaces of glass close together will create a double curve of water between, forming nearly a semi-circle and according to the nearness of the glass walls, water will rise bodily in the gap (owing to the larger adhesive force) until the upward pull is balanced by the weight of liquid drawn up. This drawing up or along a channel is called CAPILLARY ACTION.
	FIG. 5

	SURFACE TENSION & CAPILLARY EFFECTS IN BUILDINGS (top)


	Water is a buildings worst enemy. This is a categoric statement, but it is true enough. One of the main aims of good construction is to prevent entry of water, and that often by capillarity, in some part of the building. In the study of construction you will continually come across devices to combat capillarity of water or service tension. Here is a list, which is not exhaustive.
1	Damp-proof courses.
2	Cavity Walls
3	Cavity walls, if the cavity is filled, should be filled with a foam which has no through channels – the pores in the foam are unconnected.
4	Good overlap to slates and tiles, or felt, according to pitch.
5	Bellied tiles to roof.
6	Lead flashings, overlapping soakers.
7	Rolls of drips in metal roof coverings.
8	Overhanging eaves to throw water that may escape gutters, clear of walls.
9	Throats under cills.
10	Water bars.
11	Water stops.
12	Anti-capillary grooves to casements.

	PROPERTIES OF POROUS WALLING MATERIALS (top)


Q.	What physical properties can one infer of such a material if:
a)	it has low porosity?
b)	it has high porosity?
	First it would be as well to consider a brick, and state approximate figures. For a dense brick the porosity may be 7 to 10%. For a light porous brick it may be 35 to 40%.
	FIG. 6

	In some cases, statements may be positively made; in others only a likelihood can be stated. Here are some:
a	A brick with low porosity will be strong: with high porosity it will be weak.
b	A brick with a low porosity will not absorb much water: with high porosity it is most likely to absorb quickly, and a lot of water.
c	A brick with low porosity will not be permeable or it will have very low permeability (e.g., Blue Staffordshire engineering bricks are sometimes used for damp-proof course). A brick with high porosity is most likely to be permeable.
d	A brick with low porosity will be frost resistant. A brick with high porosity may well succumb to frost damage. No certain prediction can be made about frost resistance of bricks of medium and high porosity, except from experience in practice with the bricks in exposed conditions.
	Here it should be borne in mind that the most important property of a brick is not its strength (in most circumstances) but its durability or weather resistance. Most bricks are never stressed to anywhere near capacity, so are stronger in this sense than they need be. But few could be said to be more durable than they need be, considering that they may have to stand up to frost over hundreds of winters. Therefore, in cold climates frost resistance is the main criterion of quality of exposed bricks, as frost is a bricks worst enemy.

	PERMEABILITY (top)


	A simple demonstration of extreme permeability would be to take a clinker block, lay it down supported on two bricks, and let a slow stream of water from a tap play on to the middle. It will probably only wet a patch 75mm across and be seen in a few seconds dripping through quite fast underneath.
	FIG. 7

	HEARTWOOD & SAPWOOD (top)


	HEARTWOOD forms the main content of a mature tree. SAPWOOD forms the outermost few years’ growth. Heartwood is regarded as neither growing or rotting, but where growth has ceased and is inactive. Sapwood is still more “sappy” and has not fully finished its growth. Sapwood is traditionally thought of as inferior, largely because of its greater liability to subsequent dry-rot attack or even wet-rot when in use. Owing to the high cost of timber it is not now rejected for good work as it may have been at one time, but greater precautions have to be taken against pests. SEE FIG. 6.

	MOISTURE CONTENT (top)


	The moisture content of a living tree on the one hand and a piece of “dry” joinery on the other differ enormously. Moisture content (MC) is traditionally tested by oven-drying a small sample slip. Hence it is best defined on this basis:

	Loss weight on oven drying
	MC = _________________________________ x 100%
	Oven-dry weight

	Timber dried too far will take up moisture according to its situation and atmosphere, and expand in doing so. Over dried floorboards laid as such have been known to expand and burst at the edges. This is unusual, as most floorboards, one finds, shrink and leave gaps through not being dry enough when laid. A timber adopts a moisture content in time according to its environment, known as its EQUILIBRIUM MOISTURE CONTENT.
	SEE FIG. 7.

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