Maybe we want a clear colourless glass that will keep heat in or out when we need it to and provide perfect quantities of daylight.
Or perhaps a coloured or reflective glass which can create impact and help the building take its place in a landscape.
Or a heavily shaded façade which can minimise energy input to condition the internal space.
To pass all that on for inclusion in a specification, it often comes back to translating it into figures.
U-value (or R-value) to describe the levels of insulation offered at the glazed façade.
g-value (solar heat gain coefficient) sometimes expressed as a shading coefficient (SC) to limit the sun’s heating impact through the glazing.
Light transmission (LT) to bring all that natural daylight into our space.
Light reflectivity (LR) to limit or highlight the mirror finish affecting street views.
Colour rendition to reflect the aesthetic intent.
These figures do tell us something about the glass performance, what they can’t do is describe the outcome for the building in terms of comfort.
The problem is that the same solar heat gain coefficient and U-value can be reached by several different physical methods.
Reflection, where some of the heat simply bounces off and does not enter our building.
Absorption, where the heat is taken into the glass itself and raises its temperature.
Transmission, where the remainder of the energy passes through the glass and enters the space.
A simple thermal model allows us to test what happens when we use the same U and g values produced in different ways through:
Type A – ‘High performance’ glazing
Type B – Reflective glazing
Type C – Tinted glazing
The indoor air temperatures were controlled to 22°C. Then a scenario was run for each of the three different types of glass, all with U-value of 2.822 W/m²K and g-value of 0.387.
The first thing to note from the simulation output is that modelling simply for the energy outcome suggests that it matters very little which solution you use in this case (approximately ± 15 in 1500 kWh/annum), given the same internal temperatures and heat gains.
Energy modelling, when it doesn’t take account of the outcomes for occupants, can result in buildings which end up being operated very differently from their design intent. However, a building performance engineer, taking into account the impact on the indoor environment might view the consequences of the glazing choice in a completely different way.
Thermal comfort is a measure of how occupants experience their thermal environment and so we can use this to see if there are differences due to our three different glass types.
We’re not just looking at thermal comfort either. Daylight is often a contributory factor in the choice of glazing for a building. In this case the glazing types have Light Transmittance figures varying from 66% for Type A; 78% for Type B; to only 31% for Type C.
So when you’re considering painting by numbers with your next glazing specification it might be worth pondering that for your building to really work it has to do a lot of things.
Aesthetic suitability, Cost, and Availability
Structural Strength and Resistance
Thermal Comfort, Daylight and Acoustics
Energy in Operation through minimised heating, cooling and lighting loads
Unfortunately Rembrandt didn’t paint with numbers, and neither can we: however, engineering artistry and creativity can realise the vision.
