Unless a house is entirely centrally heated and the heating kept on night and day so that the walls are never allowed to drop below the temperature of the room, you are bound to get condensation on ceilings; and this is aggravated in homes where a young married couple are both out at work.
Not wanting to waste fuel they switch off the heating in the morning and turn it on full on returning at night. What happens? The air becomes warm quicker than the walls. As it expands with heat it absorbs more moisture. Striking against the cold ceiling and walls it will shrink in volume and, in consequence, inconsiderately deposit surplus water in the form of a dew.
In a bathroom or kitchen, where there is a lot of steam from baths, cooking and washing, this condensation can be severe, particularly in new houses with relatively low ceilings. Not only is the fault a nuisance but it provides an ideal nursery for the germination of mould spores which can only be killed by applying a mould inhibitor.
Wiping surface water off a ceiling would be an endless job and ventilation can be improved only to a certain point. The alternative is to line the ceiling with some form of insulating and anti-condensation material, with which we will deal in a moment.
Meanwhile, let us see what could happen in a living-room where condensation is so slight that it is not readily discernible.
Air will absorb moisture from the breath of the occupants of the room, and its capacity to hold that moisture increases as it expands as the room gets warmer. Being lighter than cold air, this moisture-laden warm air rises, carrying dust with it — to be deposited on the ceiling. Now if the ceiling were at the same temperature all over, dust deposits would be evenly dispersed and the whole area would deepen in colour so gradually that the change would hardly be noticed. But it is not evenly deposited because the ceiling is suspended under wooden joists.
Wood is an insulator, warm to the touch; and so condensation will not be so great on the area immediately under the joists as it is on the area between the joists. That is why white lines some 50 mm (2 in) wide are often seen on a neglected ceiling. They mark the position of the hidden joists precisely. It is called pattern staining.
If you have no time for frequent washing, you can carry out an all-time remedy by lifting the floorboards in the room about and laying insulating material between the joists.
Suppose the joists were of steel, the reverse would occur; that is to say, dark lines over the joists — because steel is colder than the plaster in between them. You may often have wondered why nail heads exposed in the surface of wood, even though they are painted over, may eventually show as minute dark-coloured patches.
The remedy for these nail stains is simple; just punch the nails further in and even out surface temperature by stopping up the depression with cellulose filler. Steel is not used for joists in houses, thank goodness, but a large block of flats might incorporate steel girders.
Other Common Stains
You will have noticed how ceilings in public houses soon darken in colour.
That is due to rising tobacco smoke. It is not likely to occur in the home unless the inhabitants are abnormally heavy smokers and entertain a lot of friends who are also abnormally heavy smokers.
If an oil paint is applied direct over a tobacco-stained ceiling, the solvents in it will activate the tarry deposit which will ‘bleed’ through the coating. So wash off as much as you can and then use emulsion paint whose medium is water. If you wish to use an oil paint, seal in the stain with aluminium primer-sealer before painting.
Here is another use for aluminium primer-sealer. Perhaps you have been treating the ceiling joists above with a woodworm fluid and some has leached through the plaster. Leave the stain to disperse laterally for a few weeks and then touch over the patch with the sealer.