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close this bookDesign Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)
close this folderVII. Detail design
View the documentA. General
View the documentB. Solar access, shading and window protection
View the documentC. Control of conductive heat flow
View the documentD. Evaluation of internal heat loads
View the documentE. Cross-ventilation and air flow
View the documentF. Glass-mass relationship
View the documentG. Air infiltration

F. Glass-mass relationship

The need for thermal mass (heat-storage materials) inside a building is very climate-dependent. Heavy buildings of high thermal mass are consistently more comfortable during hot weather in hot-arid and cool-temperate climates, while in hot-humid climates there is little benefit. In cool-temperate climates the thermal mass acts as a cold-weather heat store for free-running buildings thus improving overall comfort and reducing the need for auxiliary heating, except on overcast or very cold days. In intermittently heated buildings, however, it tends to increase the heat needed to maintain the chosen conditions.

It has been found from research that there is a direct relationship between the area of north-facing glass and the area of thermal mass. Thermal mass is usually introduced into a design as a concrete slab floor on the ground or as internal masonry walls. A suspended concrete floor tends to act in much the same way as internal walls because it is not attached to the ground. Research has also shown that the behaviour of concrete slab floors is not affected by hard finishes such as ceramic tile, vinyl tile or slate. Carpet and cork tiles tend to insulate the slab from the interior space and so reduce its effectiveness. The particular benefit of a concrete slab on the ground is that the slab and ground work together thermally to provide a much larger "cool. store for summer. In winter, however, there is a continuous flow of heat into the ground. This is far less than the loss of heat from a light-weight timber floor because the ground under a core-slab is not as cold as the ambient winter conditions. Carpet on a concrete floor tends to make the associated room warmer in both winter and summer. The designer must assess the climate and the occupants, needs to determine whether one season is more important than the other.

Figures 99 and 100 illustrate the relative behaviour of a simple house plan located in Sydney. The graphs assume either a concrete slab floor with tile finish (figure 99) or concrete slab floor with wall-to-wall carpet finish. Three construction forms are shown for each: brick veneer, masonry core (internal dividing walls only, in masonry) or full masonry. Each is correctly oriented with major glass areas in a northerly direction. The graphas how the three house constructions with three different north-facing glass areas plotted against winter heating load in Gj and summer discomfort. Research undertaken by the GMI Council and by CSIRO for the 5-star design rating system has shown that the acceptable limit to summer discomfort is 180. Graphs for hot- humid climates such as those in Queensland and the Northern Territory are not available at this time. Graphs for other centres and capital cities in Australia are given in the annexes.

Hard-tiled concrete-slab floors and carpet on concrete-slab floors are shown separately to simplify the picture. An insulated-timber-floor option has been added to the graph for Hobart in the annexes to illustrate minimum thermal mass. This was not included in the other locations because it fell a long way outside the acceptable summer discomfort range.

In the "masonry core" design, the area of internal masonry walls is equal to the floor area. The mass surface area to north-facing glass area ratio for the three steps, always from left to right, on the graph, are 10:1 for minimum windows; 3:1 common design midrange (typical of many of today's project homes), and 2:1 for large glass areas (all of the northern fae in glass).