How to Beat Condensation

Air always contains a certain amount of water vapour, which is a true gas. Water vapour gets into the air as liquid water vaporizes. This occurs as air takes up heat by conduction, convection or radiation from its surroundings.

If, however, the heat is removed from the vapour, it condenses-that is, reverts to its liquid state.

Air can only retain a limited quantity of water vapour in its gaseous state, and this quantity depends on the temperature of the air-the warmer it gets, the more vapour it can hold. As soon as the maximum is reached, the excess water vapour turns back into liquid water, usually on cooler surfaces such as walls, windows and ceilings.

The air is said to be ‘saturated’ when it cannot hold any further water vapour, and the temperature at which this happens is the ‘dew point’.

Condensation, therefore, results from a loss of heat by vapour, causing its temperature to fall below dew point. Its effect is simply to concentrate and make visible a quantity of dispersed and hitherto invisible water, present in the air in the form of a gas.

A simple example is the bedroom window which, on cold, damp, winter mornings, may be running with water. This happens because human breath emits a considerable quantity of moisture, so condensation forms on the window, a cold surface. This can only be cured by ventilation-opening a window or using an extractor fan.

The presence of condensation depends on three factors.

First, the amount of water vapour released into the air inside the home.

Second, the temperature of the air, of walls, ceilings and windows, because on these depends the amount of water vapour that can remain in the air.

Third, ventilation because it enables moisture-laden air to be carried away and replaced with drier air.

Many of the problems of excessive condensation are not the fault of the people in a building, but due to faulty planning and design.

Certain kinds of plaster and building materials cannot allow water vapour to pass through them, and create a build-up of vapour inside the home. The lack of effective wall or roof insulation keeps these surfaces cold, this cools the adjacent air, so condensation results. Adequate ventilation is essential to reduce the level of condensation.

Intermittent heating, as happens with some forms of thermostatically controlled central heating, can also cause condensation. Inside surfaces have no real chance to warm up, and moisture vapour, released in the warmed air, condenses on these cold surfaces.

The weather may also influence condensation. If there is a sudden rise in temperature, combined with dampness, streaming condensation on walls and windows may occur as they are slower to warm up than the surrounding air. Spasmodic or irregular heating may not warm a house fabric sufficiently, in cold conditions, to ‘lift’ wall surface temperatures.

Reducing condensation

Condensation can be avoided by supplying a constant level of heat, introducing ventilation and by carrying out a thorough programme of insulation in the home.

A combination of warm, inner surfaces including walls and ceilings and warm air should eradicate condensation as a persistent nuisance.

The intermittent, concentrated outbreaks that occur during activities such as cooking and running a bath, usually require some form of extra ventilation.

The easiest method is to open a window, but an extractor fan is more efficient and will quickly remove the saturated air without creating draught.

In an average-sized kitchen or bathroom, a 150mm domestic fan will probably be sufficient, but in a large room, two correctly positioned units, with the capacity to deal with the air changes required, may be more efficient.

All extractor fans should be installed as high as possible in a wall or window and as near sources of-steam as possible.

Louvred windows, a series of adjustable glass slats, aid controlled ventilation. Operated similarly to Venetian blinds, they can be opened to the extent necessary to provide sufficient ventilation, while the slats can be angled to prevent cold draughts. A simple device that will often cope with ‘minor’ condensation problems is the plastic window grill, which is activated by air pressure.

Condensation can often be reduced by following a few basic rules: When running a bath, or carrying out the weekly wash, keep the door closed so that steam does not disperse to other parts of the home.

Always try and minimize the ‘escape’ of any steam by enclosing hoses to washing machines. Trap the hose under the lid and always keep the lid in place while the machine is working.

To help prevent the formation of condensation when running a bath, first run a small amount of cold water before drawing off the hot.

To overcome condensation problems altogether, if practical to do so, fix a hose to the hot tap so that the water is fed under the layer of cold.

Surface damage

Condensation can, of course, cause both structural and surface damage, as well as being very unsightly. It often creates the conditions in which various damaging and unsightly moulds can grow.

It is possible to increase the resistance of the structure to heat loss, so that any heat generated is used effectively, fuel bills are reduced and temperatures within the home are kept steady. This greatly reduces the likelihood of condensation.


Insulation here means the resistance to the passage of heat from the inside of the house to the outside atmosphere. The most common ways of achieving this are roof insulation, cavity-wall insulation, double-glazing and floor lining.

There are some vital points concerning roof insulation. Many people overlook the possibility that condensation may occur in the roof area after efficient roof insulation has been completed. Because of the decrease in air temperatures in an insulated loft, and the fact that water vapour will be moving up from the house into the loft area through ceilings, ventilation is necessary in the loft area. This should equal one 900th of the roof area and should be in the form of cross ventilation.

While ventilation usually exists naturally, careful consideration should be given to this point. Sometimes, air outside the house is so heavily weighted with humidity that any additional water vapour entering the loft from outside will cause an acceptable level of humidity to be exceeded, resulting in condensation forming within the loft area.

There are two solutions: Either to fix the insulation material on to the rafters so that the loft area benefits from the heat from the house, or to fix a water-vapour barrier, such as polythene sheeting, between the joists, before laying the insulating material.

Cavity walls

The walls form about 85 per cent of the external vertical surface of a house, while 15 per cent consists of windows and door openings. Walls are vulnerable heat-loss areas.

Modern homes are built with cavity walls, consisting of either two brick leaves or a brick outer and building-block inner leaf. The cavity prevents water from reaching the inner walls, but in doing so, the free-moving air, circulating in the cavity, carries away the heat.

A cavity wall is a poor insulator. The thermal qualities can be improved by filling the cavity with a material such as mineral wool or urea foam, which is injected under pressure. These treatments are carried out by specialist firms. This type of insulation raises the ‘touch temperatures’ of the inner wall and helps to reduce condensation.

Solid walls

In houses with solid walls, the problem of overcoming condensation or the mould growth, often apparent in damp conditions, is more difficult.

A simple preventative method is to line the walls, before papering or finish décorating, with a polystyrene wallpaper. This layer, about 2mm thick, is supplied in rolls. Also suitable for this type of lining is aluminimum foil-backed paper. Both types raise the touch temperatures.

A more substantial inner skin consists of dry lining. This is in the form of cladding with insulating material such as preformed insulating board. This type of cladding is nailed on to battens fixed to the wall. To provide extra insulation, a layer of mineral wool quilting can be fixed behind the battens.

The board used should have its own built-in vapour barrier. This is necessary because condensation can occur in the middle of a ‘cold’ wall; and the water will then work its own way back to the warmer inner face and appear as a damp patch.

Wood cladding is a good natural insulant and provides a decorative surface. Its insulant properties can be further increased by using mineral quilting behind battens to fix this.


Floor insulation is most easily achieved by laying carpet with the appropriate underlay. However, where thermo-plastic floors are used, problems can occur since, in any room, the lowest temperature is at floor level. Therefore, if the floor itself is cold, condensation may result because a low temperature causes the water vapour in the air to condense on its surface.

The only solution may be to relay the floor, using a material, such as cork, or foam-backed sheet flooring, both of which have higher insulant properties. These will be warmer underfoot and raise the surface temperature.

10. November 2011 by admin
Categories: Featured, Handyman Tips | Tags: , , , , , , , , , | Comments Off on How to Beat Condensation


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