Some Physical Principles Affecting Housing.
By the commencement of the present century, a traditional type of domestic building-construction had been developed fairly generally throughout the Dominion. This construction had proved itself both economical and at the same time reasonably well suited to the climatic conditions of the country. Nearly 90% of the houses built between 1890 and 1910 conformed in the main to the following general specifications:—
Framework of timber, usually 4 in. × 2 in., lined externally with weatherboards, and internally with rough lining, scrim and wall-paper. Stud heights usually from 10 to 12 ft. Ceilings also of timber, either dressed below and painted, 01 left rough and covered with scrim and wallpaper. The timber was all thoroughly seasoned before use, and this, together with the good craftsmanship prevalent at that time, resulted in a tight wall-construction which left a comparatively dead-air space between outer and inner linings. The roof was generally of corrugated iron, fitting tightly, and leaving a dead-air space over the ceiling. To this was added wood sarking in a large proportion of houses. Windows were high and hung in sashes, with counterweights, so as to open by easily variable amounts at top and bottom. Most of the rooms were provided with an open fireplace and chimney.
As a result of all this, houses were warm, dry, and usually well ventilated, without excessive draught.
Within the last 20 years, radical departures from this traditional mode of domestic building-construction have developed. Weatherboards are to a very considerable extent superseded by veneers of brick or asbestos cement or concrete and the like; while interiors (both walls and ceilings) are being lined with such wall-board materials as fibrous plaster, pumice-cement board or wood-fibre board, or with lath and plaster. A wooden framework is still preserved in most types of construction; but the timber is green or partly dried and, either through indifferent workmanship or by intention, the wall cavities are usually very heavily ventilated. Roofing iron has been largely replaced by tiles, which are far from airtight and often not even reasonably watertight. Stud heights have been lowered to 9 or even 8 ft., and sash windows have given place to casements, with or without leadlights. With these, draughtless ventilation is more difficult, and, in fact, in a remarkably large proportion of homes, draughts are avoided by the simple but noxious habit of keeping all windows strictly closed in all but the calmest and warmest of weather. Chimneys have disappeared from all rooms except the living room—at least in smaller houses.
These modern houses are found in general to be much colder, much more damp and much less adequately ventilated than were those of the older type. The reasons for the deterioration is not far to seek, if we inquire into the physical features associated with the new materials and modes of construction.
In the first place, the thermal insulation provided by the traditional construction was reasonably good. Methods have been developed at the Dominion Physical Laboratory of measuring heat losses through a typical wall section of a house on the site. This is achieved by applying an open heated box to the interior of an external wall, and measuring the heat input to the box and the temperature drop across the wall section when steady conditions have been established.
The box itself is heavily insulated and pressed tightly against the wall interior by braces and jacks. A fan circulates the heated air within the enclosure. In order to ensure that all the heat passes out through the section perpendicularly to the surface, the whole room containing the box is heated to the same temperature as the box.
In this way, a typical traditional wall section was found to have a thermal transmittance value (U) or about 0.27, the units being B.Th.U. per sq. ft. per hour per °F. temperature difference across the section.
In the modern house, the wall linings are poor insulators, are usually comparatively thin, are backed by a draughty cavity instead of a sealed one, and often do not have the outer protection of as poor a thermal conductor as weather board timber. In consequence, U values have been found to be much higher. Here are some typical values:—
|Weatherboards and lath and plaster||0.37|
|Brick veneer and pumice-cement board||0.54|
|Brick veneer and lath and plaster||0.00|
Modern standards abroad call for U values of not more than 0.20; and; values even as low as 0.15 are strongly recommended in Great Britain. These low values mean much leas heat loss in cold weather, and much less fuel consumption to keep the interior warmed up to comfort levels. It will be realised therefore that the high values discovered in modern New Zealand dwellings convey a very pointed condemnation of our present-day building methods.
Turning next to the problem of ventilation, we have found that the rate of ventilation in modern domestic rooms, in reasonably calm weather and with windows closed, is usually less than one air change per hour.
This has been measured by releasing about 1% of hydrogen gas into the air of the room, and measuring the rate of replacement of the mixed air by fresh outside air, by an electrical method. The method depends essentially on the high thermal conductivity of hydrogen compared to air. We have found it convenient to employ an aeroplane fuel-air-ratio analyser, which estimates the proportion of carbon dioxide in the exhaust gases of the engine. The method gives results which agree with more direct but less convenient methods to within 2 or 3%.
As a result of these measurements, we have learned that the effect of a sash window open slightly top and bottom is much more effective than that of a casement window open a similar amount; and that in a room with on open chimney, ventilation rates are from two to three changes per hour without a fire, and more than double that amount when a fire is burning.
The most serious consequence of this combination of low ventilation late and high thermal loss is the development of very high humidities in occupied rooms. The moisture contributed to the air by the occupants builds up into high moisture contents which the ventilation is inadequate to remove, and which the low wall temperatures in the winter convert to high relative humidities. Rooms are damp, and unhealthy, and in over 50 per cent, of modern houses, mould develops on ceilings and walls. This mould rapidly disfigures very seriously interior finishings, and results in a demand for more frequent redecoration.
The fundamental remedy for this dampness and mould in houses must clearly lie in an increase in ventilation together with a decrease in thermal losses through walls, ceilings and floors. So far has the theory of this problem now advanced, and been confirmed by experimental methods, that it has been found possible to survey a room and calculate the extra insulation and ventilation necessary to remedy the mould and dampness, under given external conditions. Unfortunately the remedy is expensive, both in houses during erection and especially in such as are already erected. It is probable, therefore, that those responsible for the present chaotic state of housing comfort will hesitate as long as possible to apply these remedies in order to put the matter right.
A further important problem of domestic comfort is at present under investigation. The open hearth fire as a means of household heating has always been in favour, in spite of its recognised inefficiency. So long as fuel is cheap and abundant, little attention is paid to the wastage. Now, investigation has been prompted by scarcity and costliness, and it has been shown that little more than about 15 per cent, of the heat available in even the best of domestic coal goes to increase the comfort of the household. More efficient methods of utilising the heat without losing the homeliness of the open fire are under consideration. By using the convective heat in a heat exchanger and by suitable control of the air supply to the fire, a considerable increase in efficiency has been secured in Britain. It is not certain if these improvements can be applied without modification to our locally available fuels, and until experimental work has been carried out on these lines, no very satisfactory conclusions can be drawn. At the moment, progress is held up for lack of a specially-designed laboratory. In this it is hoped to be able to measure the total heat contributed to the room, by making the room behave as its own calorimeter. When funds are available for this project, it is confidently expected that much saving in fuel and a much more efficient method of domestic heating will be developed, in spite of the low quality of the fuel at present available for household use.
But even if an efficient method of room heating is introduced, our room will not warm up quickly if the walls consist of material of low insulating quality or of high thermal capacity. There is on record an outstanding example, of this. A room lined with plaster usually took about an hour and a-half to warm up to comfort levels in the winter mornings. When, however, the lining was supplemented with oak panelling, the same gas fire was able to warm the room adequately in half-an-hour. The panelling improved the insulation of the room, but more important, being constructed of a material of low thermal capacity, its temperature rose much more rapidly than did the plaster. In consequence, radiant heat was retained to the room more rapidly and the room reached a level of comfort much more quickly. The development of new types of wall-linings having these desirable characteristics is fraught with difficulty, because other factors must be kept to the fore. Of these, cheapness of manufacture and ease of application are of prime importance. There is a big field for a young physicist in this domain. But with the present shortage of qualified men interested in classical aspects of the science, progress must necessarily be slow and results delayed.