Painting By Numbers – Why Specifying Glass Isn’t So Clear

Whitaker Place shaded facadeU-values and g-values tell us how a glass will perform, right? Surely it’s a bit like painting by numbers where you fill in the shape with the right colour and you get the desired result at the end?

The true outcome for the building is often driven by aesthetic initially, and is arrived at through discussions with the investor, the architect, and the officials in charge of resource consent.



A vision is produced for the façade…

  • Glazed facadeMaybe 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.

Glazing heat transfer

Taking a simplified approach, the heat from the sun, sky, or reflective surface, is handled at the glass through three basic routes.


  • 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.


Modelling Glazing Performance

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

Simple room modelA 10m x 5m room model was created with three large glazed windows in the North wall. The space was ‘zoned’ in the model to allow for simulation of different areas of the space, indicated by the various coloured areas in the second picture.

Simple room model with zonesThe 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 and Building Performance Engineering

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.

Comfort due to glazing typeAt risk of alienating those of you who shudder at a graph, this one shows the difference quite clearly.

Looking at the three zones immediately inside the windows, and looking at how many hours would be experienced as a ‘feels like’ temperature over 25°C, there is a large difference in performance.

People would experience comfort temperatures over 25°C for around twice as many hours with the tinted glazing as the reflective version with the ‘same’ U-value and g-value specification. Our ‘high performance’ glazing gives us a middle of the road effect in this case.

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.


Author: Ruth Williams – Principal, Buildings That Work InsideOut Ltd

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