A thermal assessment of air-spaces in limestone cavity structures

Abstract

If properly made use of, air proves to be one of the most effective means of thermal insulation available to man. Still air practically eliminates heat transfer by convection and, with a low thermal conductivity of 0.025 W/mK, heat transfer by conduction is minimal. The other mode of heat transfer, that is, radiation, is independent of the type of medium through which it passes, but is a function of the temperature difference across. The insulating property of air has been known to man for a long time. For centuries, the eskimos have been building their igloos in such a way that they retain heat inside, by entrapping air under their dome-like structure. Their effectiveness is emphasized by the fact that, with an outside temperature of -20°c, the temperature just under the dome inside a well-made igloo, may be as high as +20°C, that is, a temperature gradient of 40°c prevails. In 3urope, including Malta, air spaces in building structures for insulating purposes, have been in use for, at least, 200 years. In this century, especially, the cavity wall became increasingly popular, and was extensively used in small house building drive between the wars, even more so since 1945. Double glazing was invented by the early 19th century, and it was precisely on double glazed windows that the first recorded scientific research on the insulating value of air-spaces was carried out. This was in 1861, by Peclet, who estimated the heat loss through double glazed window glass panes, and showed theoretically that the maximum resistance to heat transfer for double glazing was obtained with an air-spacing of 20 mm. between the panes. At this distance, the conductance through the air was greater than the surface transfers. Modern researches, as well as published standard figures by international heating and ventilating engineering bodies like the Institute of Heating and Ventilating Engineers (I. H. V. E.) of the United Kingdom, and the American Society of Heating and Ventilating Engineers (A. S. H. V. E.), have proved Peclet to be right. Since the time of Peclet, especially during this century, many other various tests on different air-spaced structures have successively followed, and today, engineers and architects are able to refer to them, for optimal designing of air-spaced constructions of any kind: - walls, floors, ceilings, windows for building structures - metals, non-metals, insulating and building materials - slab/sheet test-pieces of any size, for laboratory experiments and simulations and for any possible condition of usage, considering different: - airgap thicknesses - inside and outside temperatures of test sections - cavity surfaces temperatures temperature gradients across the air-space - mean temperatures at cavity - materials with different properties e.g. emissivity, absorption, conductivity, conductance, density - widths and cross-sectional areas of test-sections - moisture contents and relative humidities of test-sections ventilation rates, if air-space '1 is not sealed - air-space orientations and directions of heat flows. Nevertheless, notwithstanding the fact that so many variables are present and have to be considered when conducting tests on air-spaces and analyzing results, certain properties are found to follow certain consistent universal trends, in all situations, irrespective of variations of other parameters. Examples of some such trends are: - Radiation heat loss is much greater than conduction and convection heat losses altogether, usually between two to four times as much. Conduction is inversely proportional to air-space thickness, thus negligible for thick air-spaces. For the same cavity thickness, air-space conductance increases with an increase in the temperature-gradient across the cavity. For cavity thicknesses over 80 mm., air-space conductance remains appreciably constant. - The dimensionless parameters Gr. (Grashof's number) and Ra (Rayleigh's number) respectively, determine the type of convectional currents present: Low values Middle values High values No convection Laminar convection Turbulant convection N. B. Actual ranges depend on different conditions of tests. - Convection is not proportional to radiation. - Decreasing the emissivities of the cavity bounding surfaces reduces radiation, hence limiting heat losses through the test-section. - The rate of heat transfer through the whole test-section is proportional to the mean temperature of the airspace. Increase in the moisture content of an airspace produces a corresponding increase in thermal conductivity. These, and many other relationships may be deduced, plotted, and a~alyzed from corresponding sets of experiments conducted with changes of variables. Air-cavities are not something of the past century or decades only. Even today, one finds that air-cavities form an integral part of many modern constructions and buildings. In many countries, including Malta, there exist many laws which govern the use and sizes of air-spaces. Naturally, these laws do not take into account just the thermal aspect of air-spaces for a building, but also consider the rainwater impermiability, structural strength, acoustics and ground area requirements. However, many do specify the limiting values of conductance C and overall transmittance U for design, for many different sorts of buildings e.g. schools, hospitals, houses, churches, and different orientations, wind exposures and ambient temperatures. Such laws were the direct resulting effect from the scientific studies on air-spaced structures carried out all over the world. In Malta, this is what the law has to say as regards to the use of air-cavities in stone-walled sections: "Every wall of any room exposed to the rain… and towards the middle of its thickness, there shall be… a cavity of three inches (7.62 cm.), crossed only for solidity of the wall." "In the external face of such wall, there shall be holes… for the passage of air into the said cavity. 11 - Code of Police Laws: Part IV, Chapter 13, Section 85b (i) and (ii) For this law, the main criteria upon which such an air-space thickness was decided, is that mud clogging inside the air cavity has to be avoided. Basically, this is done in order to safeguard against rainwater penetration through the store, which causes humidity on the inside wall. The thermal consideration of the situation may have been of secondary importance. It seems that no thermal assessments of cavity structures have ever been carried out in Malta before, and the only local source of reference in this respect has been to consult published foreign data on the subject. A point of interest is that until some years ago, many Maltese people used to build their houses with double-walls containing thick cavities of at least 6 inches (150 mm.) thickness. However, they used to fill up the void between the wall sections, with loose earth and gravel, since the idea of using air as an insulation was still new and not very popular. Nowadays in Malta, the idea of an air-cavity is more acceptable, and house cavity walls with air-gaps of thickness 25 mm. or 50mm. are most common to find, even though in reality, the law stipulates otherwise. The aim of this dissertation is to investigate the validity of the air-cavity as a thermal shield in limestone sections, under various conditions and at different situations.