One pane, two figures
Your Home gives the heat figure first: the solar heat gain coefficient for windows measures how readily heat from direct sunlight flows through a whole window, glass and frame together, and it is expressed as a number between 0 and 1. The light figure sits beside it: the amount of light that passes through the glazing is known as visible light transmittance, or visible transmittance, and a low figure can reduce heat gain from the sun, but if it is too low the room will be too dark inside.
What a coating is
Your Home describes low-e glass as glass with either a pyrolytic coating or a vacuum-deposited thin film metal coating. The two behave differently in one respect: pyrolytic coatings are durable and can be used for any glazing, while vacuum-deposited coatings are soft and are only used within insulated glass units.
Films, for glass already in place
For glass already in a window, Your Home describes window films. Applied to existing glass, some films can halve the overall solar heat gain coefficient of the window by absorbing or reflecting solar radiation, and the page says this can be particularly beneficial in hotter climates where cooling is the main concern, or on east and west elevations directly exposed to long periods of sunshine.
The same page sets the limits of the idea. Window films may also reduce visible light transmittance, and glass panes with applied films that are exposed to direct sun become hotter than untreated glass, so industry guidelines must be followed to avoid thermally induced cracking.
A side of the house, in the two systems that name one
In compass points, Your Home says some window films can be particularly beneficial on east and west elevations directly exposed to long periods of sunshine. The National Construction Code’s external glazing part works in orientation sectors instead: the sector for a wall or a glazing element is the sector that contains a line drawn perpendicular to the face of the element, in a figure based on True North with all angles measured clockwise from it. The code’s figure has eight sectors, which do not overlap at their boundaries, and the page notes that magnetic north, found by a magnetic compass, is not an acceptable approximation of True North.
That part of the code applies to a Class 1 building, and to a Class 10a building with a conditioned space.
The trade a side of a house makes
The code’s summer and winter clauses judge the same window differently by season. Higher P/H values are more beneficial in minimising summer solar heat gain, while lower values are more beneficial in allowing winter access, which the code defines as the availability of winter solar gains to offset conducted heat losses. And the balance is not asked of every climate: the winter clause does not apply in climate zone 1, where there is little or no need for heating at any time of the year, and the summer clause does not apply in climate zone 8, where there is little or no need for cooling.
The code asks for the assessment to be of the whole assembly, not the glass alone: by referring to glazing elements, it requires total system U-values and total system solar heat gain coefficients to be assessed for the combined effect of glass and frames. It is a designer’s calculation for a building, rather than a rule a household applies to one window.
Putting a figure on a window
What the U value measures, and the sealed gap that changes it, are the third guide’s ground: what the gap in double glazing does. If the window sits on land mapped as bushfire prone, none of these figures is the first question any more, which is the last guide: glass in a bushfire-prone area.
The figures for any particular window come from its own rating, as the third guide sets out. What this guide can do is name what each page says changes the number: the coating on the glass, a film applied to it, the frame around it, and the orientation of the window in the sun.