In last week’s post, I made the case that insulated glazing units (double glazed windows being their most common form) are neither green nor even particularly effective.
In next week’s post, I hope to back up these claims with a concrete example; first, however, a short digression is required here into how the insulative properties of building materials and elements are measured.
The basic measure of a material’s ability to transfer heat from one side of itself to the other is called its thermal conductivity, defined as the rate of heat flow through one unit thickness of a material subject to a temperature gradient. The unit of thermal conductivity is W/ m⋅K, watts per metres kelvin, or W/ m⋅°C, watts per metres Celsius. For example, the thermal conductivity of concrete is given as around 1.30 W/ m⋅°C. From this basic figure, the heat transfer coefficient of a particular material for any particular thickness can be calculated by dividing the thickness of the material (in metres) by its thermal conductivity, then multiplying this figure by the temperature differential across the material. In practical terms, this means that a 0.2m thick solid concrete wall with a temperature gradient of 20°C (e.g. the temperature on one side of the wall is 10°C and the temperature on the other is 30°C) transfers heat from one side of itself to the other at the rate of (0.2m / 1.30W/ m⋅°C) x 20°C = 3.08 W/ m2⋅°C.
Since most building elements today are not monolithic but composites of cladding, timber, insulation, plasterboard, and so on, the insulative performance of a of a complete building assembly like a wall, floor, or roof is determined by adding together the individual heat transfer coefficients of each material, plus coefficients of surface thermal resistance at the external and internal air boundaries; this figure represents the thermal resistivity of the assembly, also known as the R-value (°C⋅m2/W). The overall heat transfer coefficient, or U-value (W/m2°⋅C), is simply the reciprocal of the R-value, i.e. it can be obtained by dividing the R-value into 1, just as the R-value can be obtained by dividing the U-value into 1. Thus the higher the R value, the better the insulative properties of the element; the lower the U-value, the better the insulative properties of the element.
The R-value tells you how many watts (joules per second) of heat you can expect to transfer across one square metre of a given building element for any given temperature difference across the element. For example, say you have a simple one-room cubic building, without openings, whose walls, floor and roof all have an R-value of 4.0. The outside temperature is 5°C and the inside temperature is 25°C. That means you have . Rearranging the equation 20°C⋅m2/W = R4.0 to 20°C⋅m2/R4.0 = W gives us a value of 20/4 = 5W per m2. Meaning we are losing 5 watts of heat from the inside to the outside for each square metre of wall/floor/roof. Suppose the building is 5m long by 5m wide by 3m high, giving a total surface area of 110m2. 5w/m2 x 110m2 = 550 Watts, meaning that to maintain the 25°C temperature in the room you would need to run a 550w heater.
Whereas the insulative ability of solid building elements such as walls and floors is usually indicated by an R-value, that of windows, in contrast, is given by a U-value. The reason R-value is not used for windows is that while R-values for well-insulated walls might be as high as 8 or more, the typical window, at least historically, has an R-value of less than one, and these numbers are unwieldy for use in calculations. In any case, that different values are needed for measuring the insulative performance walls and windows should serve to remind us of the fact that even the best double glazed window is a poorer insulator than a minimally insulated stud wall. In other words, when it comes to insulation, the best window is no window at all. A bold statement, perhaps, but one I will support with a calculated example next week.