DOUBLE GLAZING DEBUNKED, PART THREE

Last week’s post ended with the assertion that ‘when it comes to insulation, the best window is no window at all.’ I didn’t mean to suggest that we should forego windows entirely in our houses - of course, habitable rooms need windows, and they are a regulatory requirement. But this is a false binary. What I meant is that windows can and should be consciously sized so as to achieve the best trade-off between light and heat. I put this in italics because the question ‘How much of the wall needs to be window?’ should always be asked, but more often than not isn’t; and the calculation itself is almost never made. Instead, ‘go big and go double glazed’ is the default mantra. Here I would like to demonstrate, via a calculated example, the effect window size can have on the overall rate of heat transfer of the wall it sits in, and the room it serves.

For our example, imagine a room that’s 3m square (room area 9m2) with a 2.4m ceiling. Only one wall is an external wall (with a total area of 3m x 2.4m = 7.2m²), and it contains one window. Assume that the outside temperature is 5°C and the indoor temperature 25°C, for a difference of 20°C.

Next let’s establish a single-paned and double-paned window option for the room. I looked on the WERS website and chose a manufacturer (Capral) at random, then took the worst-performing of each of their aluminium framed fixed single and double-glazed windows: 6mm clear single glazed with a U-value of 6.3, and 6mm clear/12mm air gap/6mm clear double glazed with a U-value of 3.4. Remember that the R-value is the reciprocal of the U-value, so to obtain the R-value simply divide the U-value into 1. So for the single glazed window, 1/6.3 = R0.16; for the double glazed window, 1/3.4 = R0.29.

Given these values, the single glazed window is transferring heat at the rate of 20°C/R0.16 = 125W/ m²; the double glazed window, 20°C/R0.29 = 69W/m².

Let’s also assume that our windows are 1.5 metre wide by 1.5 metres high, i.e. 2.25m² in area. So the total heat transfer of the single glazed window is 125W/m² x 2.25m² = 281.25W, and that of the double glazed window is 69W/ m² 2.25m² = 155.25W.

What area would we need to reduce the single-glazed window to in order to reduce its total heat transfer to that of the double glazed window, i.e. 155.25W? The answer is obtained by 155.25W/125W/m² = 1.24m², for example a window roughly 0.9m x 1.4m. For a 9m² room, this window clears the minimum natural lighting required by the Building Code of Australia, being 10% of the room area, or in this case 0.9m².

In our example using the windows given, it can be seen that reducing a window’s size by around 45% has the same effect as double glazing it. A shortcut way of calculating this equivalence is to simply take the difference between the two U values (6.3 - 3.4 = 2.9) and dividing the single glazed U-value (3.4) into this (2.9/3.4 x 100 = around 45%).

Note that this example hasn’t taken into account the effect of the increase in area of the wall that accompanies the reduction in window size, because for any reasonably-well insulated wall, the effect is negligible in comparison to the effect of the change in window area. But the calculation is worth doing anyway, if only to demonstrate just how terrible the insulative performance of even double glazed windows are when compared to even a moderately insulated wall!

For a 2.25m² double glazed window, there is 7.2m² - 2.25m² = 4.95m² of wall area. Assume a wall with an R value of 4.0, which transfers heat at a rate of 20°C/R4.0 = 5W/ m². The total heat transfer of the wall is 5W/ m² x 4.95m² = 24.75m². Add to this the 155.25W total heat transfer of the window, and we obtain a figure of 180W for the wall and double glazed window together. For the single-glazed example, we have 7.2m² - 1.24m² = 5.96m² of wall area, for 5W/ m² x 5.96m² = 29.80m². Add to this the 155.25W total heat transfer of the window, and we obtain a figure of 185.05W for the wall and glazed window together.

In conclusion, I hope that this and the previous two posts in this series have been persuasive in making the case that double-glazing shouldn’t necessarily be an automatic choice, and that its advantages should be weighed against other considerations such as cost, lifespan, and a more realistic appraisal of the need for natural light; also, I hope I have demonstrated that single-glazing is by no means obsolete but is very much still a viable option in many, and perhaps even most, cases.

DOUBLE GLAZING DEBUNKED, PART TWO

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.

DOUBLE GLAZING DEBUNKED, PART ONE

Insulated glazing unit (IGU) is the industry name for any glazing product that consists of two or more panes of glass separated by a metal or polymer spacer, with the whole assembly forming a thin sealed chamber that contains an insulating layer of air or other gas (typically argon). Insulated glazing was first patented as far back as the 1860s, and IGUs have been commercially available since the 1940s. Though triple, quadruple and even sextuple glazing is available for use in colder climates, double glazing is by far the most common type of IGU seen in Australia, where it has steadily gained market share to the point that it is now arguably seen as the standard choice (at least outside the tropics) in new houses, particularly since achieving first a five-star, then a six-star energy efficiency rating became mandatory in most states in the 2000s. IGU’s themselves are not mandated in the building code, but they are one of the easiest ways to ‘tick the boxes’ in the formal and largely meaningless exercises known as thermal energy assessments (which is a whole other subject in itself). Indeed, double glazing has become somewhat emblematic of ‘green’, ‘eco’ or ‘sustainable’ architecture - feel-good, nebulous and largely sham concepts that generally indicate the uncritical application of energy-intensive, high-tech solutions to perceived ‘problems’ in building design and construction.

But does double glazing work? Well, that depends what you mean by ‘work’. IGUs perform as advertised out of the box, but will they work for the lifespan of your house? Almost certainly not. Lifespans (and warrantees) given for IGUs range from around 10 to 25 years; the failure mode is almost always the failure of the seal, and an IGU is only an IGU as long as the seal retains its integrity. If you look closely at the strip of metal or plastic separating the panes of glass in an IGU, you will see two rows of tiny holes. Under these holes is a layer of desiccant. Once the seal fails, moist air enters the gap, the desiccant eventually becomes saturated, and all you have at that point is two expensive and very closely spaced single-glazed windows prone to internal condensation. If being ‘green’ is your concern, bear in mind that the whole IGU must now be replaced, with all the additional embodied energy that implies.

A sectioned timber-framed IGU showing the desiccant layer (white) under a perforated metal strip

Older, low-tech alternative to IGUs exist that provide much of the insulative benefits of IGUs without the limited lifespan. One very old solution is the use of external storm shutters, but these have the disadvantage of not being able to be used during the day. A more modern solution, common in cold climates from the early 20th century until the advent of IGUs, is just to use two single-glazed openable units in a single frame, separated by ten centimetres or so. While the large gap does mean that there will be some convection of air which will reduce the insulative performance, it also allows the internal faces of the panes to be easily cleaned, and the fact that the cavity is not sealed means that there is no seal to fail - the inevitable fate of all IGU’s in the end.

But perhaps the most fundamental ‘solution’ to this ‘problem’ of heat transfer across windows doesn’t require the application of technology at all, either high or low. Rather it simply requires a change of attitude, which is perhaps why it is almost never mentioned. It requires us to go right back to basics and challenge one of the assumptions that underlies the adoption and perceived necessity of double glazing in the first place: the idea that larger windows are always better and more desirable than smaller.

In next week’s post, we will demonstrate how this ‘no tech’ approach works, by first reviewing the physics of heat transfer and looking at how the insulative properties of materials and building elements are measured and calculated, and then applying this knowledge via a practical example to highlight the influence of window size on heat loss from a room or building.