KEEPING MOISTURE AWAY

Press, Volume CIV, Issue 30737, 29 April 1965, Page 17

KEEPING MOISTURE AWAY

Prevention By Use Of Building Materials

One of the chief functions of a house is to protect the occupants from penetration of moisture from the outside. Moisture may take the form of rain, hail or snow—or it may consist of soil moisture rising by capillary action through the materials of walls and foundations.

Preventing penetration of moisture may mean the use of building materials which are themselves highly impervious to moisture; the use of sufficient thickness of a building material of moderate moisture permeability 7 to reduce the moisture to insignificant levels; or the use of protective coatings.

\\ ell made and well laid brick and stone work (even brick veneer) seldom allows significant amounts of moisture to penetrate, unless entry occurs through cracking of the units or mortar joints.

Complaints of internal dampness in such structures are usually found to be due to internal condensation because the exterior walls become unduly chilled through a lack of adequate thermal insulation.

In concrete block construction, moisture may penetrate through the concrete or through the mortar joints. The more common cause or such penetration is through cracks in the joints, because the blocks were laid wet and then dried and shrunk.

Such penetration is usually caused by bad laying, which leads to poor bonding between the blocks.

Concrete walls need some kind of external treatment. The traditional treatment is to apply a plaster coat, about half an inch thick, consisting of a mixture of sand and cement with possibly also some lime.

In modern New Zealand homes, the most prevalent cause of internal dampness is not moisture pentration from the outside, but internal condensation.

Its most obvious sign is usually the appearance on ceilings and walls of mould growth either a grey-black or green discolouration. A recent New Zealand survey showed that more than 50 per cent of homes in this country are affected by this mould. In almost every case, this could be traced to internal condensation.

What is the cause of this condensation?

Various household activities, such as cooking and washing, are liable to disperse lots of steam throughout the house. But such activities are confined to short periods and, in any case, are so obvious that doors and windows are opened to allow the steam to escape.

What is not so generally recognised is the large amount of moisture contributed to the air within a house continuously, day and night, by the occupants themselves.

Most adults breathe out something like half a pint of water vapour in nine hours. It is this moisture, so readily overlooked, which is the chief contributor to the dampness which leads to mould in modern homes.

Now why should there be any difference between modern houses and the older traditional type in which mould hardly ever appears? Investigations have shown that there are at least three modifications introduced recently which contribute to some extent to the dampness associated with mould.

Wall structures and especially wall linings are today almost invariably poor insulators of heat The old all-timber

house, with its rough timber linings and scrim, resulted in a fairly warm temperature and the warmth was not readily lost to the outside through the walls. But fibrous plaster and other similar wall linings in use today allow much more of the internal warmth to escape, and the houses are therefore much colder.

Shortage of seasoned timber in recent years has led to the use of unseasoned or partly seasoned timber for

studs in houses. It was then discovered that this green timber was subject to attacks of “dry rot.” To combat this by ensuring that the timber dried out satisfactorily after erection, wall cavities have been allowed. There is some indication, however, that this ventilation has been overdone. Air changes of up to 60 times an hour have been measured. One change an hour would be adequate to save the studs from dry rot.

Natural ventilation of rooms was formerly ensured by providing each room with a chimney.

Even in Calm weather, the effect of a chimney is to replace air in a room completely two or three times each hour. In closed rooms without a fireplace, it may take as long as two hours for the air to be replaced once.

Nowadays only lounges are provided with chimneys, and bedrooms, which need abunant ventilation just as much, rely on the cracks around windows and crevices under doors.

Many people seem frightened to allow cold air into their rooms, especially at night. It is not sufficiently recognised that no dampness from outside can be more harmful than the dampness which accumulates in an occupied room if the windows are kept closed.

Here are some points to bear in mind when designing a house:

(1) A room with a large area of window space is difficult to keep free of mould, especially during periods of sunless weather. The window area, therefore should not be greater than 20 per cent of the floor area.

(2) Heat losses from the back of a chimney are considerable and can be made to contribute to the dryness of the rest of the house if the chimney is placed on an inside wall. With an outside chimney, this valuable back heat is lost, though some loss may be avoided by introducing thermal insulation behind the fireplace.

(3) Rooms with three outside walls are particularly difficult to keep free from mould, especially on an exposed site. The problem is much easier for a room with not more than 50 per cent of its wall space exposed to the weather.