Total solar energy transmittance in glazing

Total solar energy transmittance, usually written as the g-value, is the fraction of incident solar energy that a glazing unit admits into a room. It combines the radiation transmitted straight through the glass with the portion absorbed in the glass and then re-radiated inwards. It is determined to the relevant European standard and declared by the manufacturer for a specific unit build-up, so a g-value quoted for double glazing in general tells you nothing about the unit you are buying.

Of every input in an overheating model, this is the one with the largest effect on the answer. Change it and the result moves more than it does for almost any fabric change.

What the value measures

Picture solar radiation striking a sealed unit. Some passes directly through. Some is reflected away. Some is absorbed by the panes and the coating, and part of that absorbed heat then flows inward by conduction and re-radiation while the rest flows back outward.

The g-value is the sum of the direct transmission and the inward-flowing share of the absorbed energy, expressed as a decimal between 0 and 1. A unit with a g-value of 0.4 admits 40 per cent of the incident solar energy.

Two related terms cause confusion. Light transmittance describes the visible part of the spectrum only, and it is a different number from the g-value. Shading coefficient is an older measure expressing performance as a ratio against 3 mm clear glass, so a g-value converts to a shading coefficient by dividing by 0.87. Specifications that mix the two produce modelling errors.

Typical ranges for clear and solar control glass

Figures vary by manufacturer, coating and build-up, so take the number from the product data. The typical ranges below give the order of magnitude for a sealed unit.

The ratio of light transmittance to g-value, sometimes called the selectivity, is the figure to compare between products. A coating that cuts solar gain while keeping visible light is doing useful work; a tint that cuts both equally is simply making the room darker.

Effect on overheating results

In a dynamic overheating assessment the solar gain through glazing is often the dominant heat input in summer, particularly in a single-aspect flat with a large window on a sunny elevation. Lowering the g-value reduces that input directly and proportionally.

The sensitivity is worth understanding before any redesign. A reduction in g-value cuts gain across the whole cooling season, including hours when the window cannot be opened because of noise or fall protection. That makes it more reliable than a ventilation measure, which depends on occupant behaviour and on external conditions.

It is also, in a modelling sense, a free parameter. Changing the g-value changes nothing about the layout, the massing or the facade proportions, which is why it is the first thing an engineer adjusts when a scheme fails. An Overheating Risk Assessment should state the g-value used for each orientation so a reviewer can see what the result depends on.

Trade-off with daylight transmittance

The cost of a low g-value is light. A unit at 0.25 with a light transmittance around 0.5 admits half the visible light of clear glazing, and in a deep single-aspect room that can push the internal daylight result below target.

So the two assessments have to be run on the same specification. A scheme that passes overheating with solar control glass and passes daylight with clear glass has not passed either, because it is two different buildings. Where both are tight, a Daylight and Sunlight Assessment and the overheating work should be iterated together rather than produced in sequence by different consultants.

And there is an appearance dimension. Strongly coated or tinted glass reads as reflective or green-grey from outside, and in a conservation area or on a heritage-sensitive frontage that becomes a design objection. Obtain a sample and a photograph of the product in situ before committing, because the elevation drawings will not show it.

Where external shading works better

Shading placed outside the glass stops solar energy before it enters, so it reduces gain without reducing the visible light transmitted by the glass itself. That is a better outcome than a low g-value where daylight is constrained.

• Horizontal projections, brise soleil or deep reveals work on southern elevations, where the sun is high when it matters.
• Vertical fins work on east and west elevations, where low morning and evening sun defeats horizontal shading.
• Balconies above a window act as shading and should be modelled as such rather than ignored.
• External blinds or shutters give the best performance and the worst maintenance position.
• Internal blinds help far less, because the energy has already entered and the blind becomes a warm radiant surface.

The orientation rule is the one most often got wrong. Horizontal shading on a west-facing window does very little, and a scheme that relies on it will fail the assessment for rooms on that elevation.

An illustrative sensitivity test

The useful exercise is a one-variable run. Hold the geometry, construction, gains, occupancy and window opening logic constant, then model the scheme with two or three g-values and report the exceedance hours for each.

Set it out as a short table: g-value, exceedance hours per room, and pass or fail against each criterion. That tells the design team the specification the scheme actually needs, rather than the one someone hoped would be enough.

This is a method rather than a result. The size of the improvement depends on orientation, glazed proportion, construction weight, ventilation and the weather file, so the numbers have to be calculated for your building and should be labelled illustrative where used as an example.

Run the test early. A sensitivity study at concept stage costs a day of modelling; the same question answered after the facade is tendered costs a redesign, with the planning consequences set out in our note on value engineering after permission.

Specifying it in a submission

Put the g-value in the window schedule, per orientation and per unit type, with the light transmittance beside it and the standard it was determined to.

Avoid the phrase solar control glazing without a number. It commits a contractor to nothing, and a condition requiring development in accordance with an energy or overheating report that contains no figure is unenforceable in practice and unhelpful to everyone.

Where a condition is likely, propose the wording: a maximum g-value and a minimum light transmittance by orientation, with a tolerance. That gives the contractor room to substitute a product while protecting the result the assessment relied on, and it keeps the Energy Statement and the overheating work describing the same building.

Browse the report types PlanScope can draft to see what a residential scheme needs.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.