Abbey Community College

Technological Subjects Support Site
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Damp and Condensation.

Moisture may enter a dwelling in any of the following ways:
Precipitation = Rain and Snow
Damp = Penetration of the building fabric by capillary action
Condensation = Water vapour in the air condensing on cold surfaces

1. Water introduced during construction. Several tones of water are used in block laying and plastering and often the walls remain damp until a period of warm weather.

2. Penetration through roofs and chimneys. Tiles and slate need to be laid at an adequate pitch and securely fixed to avoid penetration of driving rain. There should be a generous overhang at the eaves and the junction between chimneys, walls and the roof need to be damp proofed and sealed with lead flashing.

3. Penetration through walls. Penetration occurs most commonly through walls exposed to the prevailing wet wind or from leaking down pipes. Dense rendering can prevent moisture drying out more effectively than preventing its entry, and this tendency is accentuated in cracked rendering with moisture penetrating by capillary action. Modern building materials and methods such as cavity wall construction, vertical and horizontal DPC and DPM reduce the possibility of dampness. Blockwork and plaster are pervious materials and therefore in single leaf solid walls and older construction moisture can soak from the outside after prolonged rain. Rising Damp can be a problem in older houses where there is no DPM or in new buildings through defective DPM, bridging of DPC by a floor screed internally or mortar droppings on the wall ties.

4. Dampness may also result from leaks in plumbing system and condensation.


In older types of houses much of the moisture laden air from cooking, bath, clothes drying etc., could escape through draughty windows or doors, also these houses traditionally had more chimney flues than is common now. Nowadays, with increased cost of fuels and consideration for energy conservation, homes are now better sealed. This prevents the ventilation of moist air and gives rise to a sharp increase in problems caused by excessive condensation.

Factors that influence condensation include:

  • Moisture content of the air
  • Temperature of air
  • Temperature of the building fabric

Air holds water in the form of water vapour that is generally invisible.
The warmer the air is, the more water vapour it can hold. If warm, moist air is cooled it can carry less water in the vapour form and some of this changes to water form that is visible (fog, mist). [e.g. there is more steam generated when one runs a shower in a cold bathroom than a warm one].
Condensation forms on cold surfaces, usually windows and tiles, or if there is a cold bridge in the construction. If the temperature of the building fabric is kept high then condensation can be reduced.


Dampness caused by condensation can be remedied by considering heat and ventilation.
A ventilation system should provide

  • An adequate supply of fresh air for the comfort of the occupants
  • Rapid extraction or dilution of pollutants and moisture likely to produce condensation for example in the kitchen and bathroom

Ventilation can be achieved by providing:

  • Permanent opening e.g. wall ventilator, air brick
  • Open-able Windows
  • Mechanical Ventilator e.g. extractor fan

Ventilation includes background ventilation (a wall vent providing one air change per hour approx.) and rapid ventilation in a bathroom for example where a large amount of steam is generated and a window can be opened momentarily.

Room Ventilation opening
Bathroom 1/20 of floor area
Kitchen 1/10 of floor area

In situations where high levels are air change are required (3 -4 air changes per hour) the new air may need to be heated to maintain a comfortable temperature. This means more fuel is used and is expensive, some homeowners then reduce the amount of ventilation and therefore risk dampness.


Heating is equally important in controlling dampness. It should be remembered that warm air can hold more moisture that cold air and condensation forms on cold rather than warm surfaces.

Construction for Condensation

Even when all precautions are taken it is still common for condensation to occur, most likely places being bathrooms, kitchen and on any window. Occasional condensation is not a problem if it is anticipated and reparation made for it. The use of impermeable materials such as glazed ceramic wall and floor tiles in these areas prevents condensation from penetrating the building structure and causing damp.

Interstitial Condensation

Interstitial Condensation occurs within the structure or fabric of the building
Normally building materials are warm on the inside and get progressively colder as they approach the outside surface. Therefore while condensation may not occur visibly on the inside surface of the wall it may occur at some point inside the wall fabric (the vapour condenses at the dew point temperature inside the wall)
A vapour barrier has a high resistance to water vapour penetration. Some examples are gloss paints, special silicone paints, polythene sheet and foil backed plasterboards.
Interstitial Condensation can damage steelwork and can make insulation materials less effective.

Dew Point

The Dew Point is the temperature at which a fixed sample of air becomes saturated (condenses)
If moist air is cooled, at the dew point the air becomes saturated with water vapour.
When this saturated air comes in contact with a surface that is at or below this temperature then a thin film of liquid will form. This is known as dew or condensation.
Unit is °C or K

Relative Humidity

The Relative Humidity (RH) of a sample of air compares the actual amount of moisture in the air with maximum amount of moisture the air can hold at the that temperature.
The correct definition of Relative Humidity is:

Relative Humidity = Vapour pressure of sample X 100  
    S.V.P. of sample at same temperature      

Unit: Percent R.H. at a specified temperature. It is also common practice to describe humidity in terms of percentage saturation.

Cavity Insulation

In order to maintain a constant temperature within a building it is necessary to restrict heat loss, keeping heat inside a building for as long as possible to conserve energy and reduces heating costs.
Aerated lightweight concrete, aero-board, expanded polystyrene, fiberglass may be used in the cavity to prevent heat loss
Loose fill materials, expanded polystyrene granules, reflective materials e.g. aluminum foil may also be used in some situations.

Cold Bridge

Where materials of high thermal conductivity pass completely through a wall, floor or roof without insulation e.g. solid block wall, lintel.
Condensation and mould growth may occur.

Vapour Barrier

A layer of building material that has a high resistance to the passage of water vapour.
Needs to be installed when there is danger of interstitial condensation.
Must block water vapour before it meets an environment below the dew point temperature.
Must be installed on warm side of insulation layer.
Examples include: foils, liquid films, bituminous solutions, rubberized or siliconised paints, gloss paints.

Damp Proofing Old Buildings

One method involves sawing a slot in a mortar bed joint and inserting a damp-proof membrane. The membrane is normally in about 1m lengths and can be slate, bitumen-felt, copper, lead or polyethylene.
Another method is called electro-osmotic damp proofing. Damp rises in the wall from the soil by capillary action, the damp wall is negatively charged with respect to the soil. 25mm holes drilled from the outside with strip electrodes of high conductivity copper mortared into the drillings and looped into copper strips set into bed joints at damp-course level along the wall face. This layout removes the surface tension and thereby prevents the moisture rising.