G-value explained, and why it matters more than U-value here

G-value, also written as g-value or solar factor, is the proportion of solar energy hitting the outside of the glass that ends up as heat inside the room. It is a decimal between 0 and 1. A G-value of 0.60 means 60 per cent of the sun’s energy comes through. A G-value of 0.28 means 28 per cent does, and the rest is reflected away or absorbed by the glass and shed back outwards.

It is not one single mechanism. The figure bundles together two things: the solar radiation that passes straight through the glass, and the share of the radiation the glass absorbs and then re-emits inwards. That second part is why a dark tinted pane is a poor solar-control solution on its own. Tinted glass absorbs a lot of energy, gets genuinely hot, and then radiates a good chunk of that heat into the room anyway. It dims the view without solving the problem. A proper solar-control coating reflects the near-infrared before it ever gets into the glass.

The number you will see in North American literature is SHGC, solar heat gain coefficient. It measures the same physical quantity on a slightly different test basis, so the two are close but not identical. In the UK and across Europe, G-value calculated to BS EN 410 is the figure to ask for.

G-value is also not the same as light transmittance, and confusing the two costs people money. Light transmittance, usually written LT or TL, is the visible portion only, the part your eye responds to. A good solar-control unit is selective: it lets most of the visible light through while blocking the invisible near-infrared that carries the heat. You can have a rooflight that is bright and still cool. You do not have to accept a gloomy, brown-looking room to keep the temperature down. Divide light transmittance by G-value and you get the selectivity ratio. Anything above about 1.7 is doing real work. Ordinary clear double glazing sits at around 1.3.

The U-value is a rate of heat transfer: watts per square metre of glazing, per degree of temperature difference between inside and outside. Written W/m²K. Lower is better. It is driven by the number of panes, the gas in the cavity, the low-emissivity coatings, the spacer bar around the edge and the frame the glass sits in. Rooflights on the market run from about 1.6 W/m²K for a basic double-glazed unit down to around 0.8 for a good triple.

The U-value is worth caring about. It governs your winter heat loss, it decides whether the inside face of the glass runs cold enough to attract condensation, and it is the number Building Regulations police. Part L sets a maximum U-value for new and replacement rooflights in a dwelling, and any unit worth buying beats that limit comfortably. Every installation we carry out is notified to Building Control, and the glazing specification has to stand up on paper.

But here is the part that gets skipped. U-value works in both directions. On a hot day it also describes conducted heat coming inwards. The trouble is that the conducted component is tiny compared with the radiated component, and swapping one U-value for a better one barely touches it.

Run the arithmetic on a common size. A 1.2 by 1.2 metre rooflight is 1.44 square metres of glass. Suppose it is 30 degrees outside and you are trying to hold 20 inside, a 10 degree difference. At a U-value of 1.2, the conducted gain is 1.44 x 1.2 x 10, which is about 17 watts. Now put the sun on it. On a clear midsummer afternoon a roof surface facing the sun can be receiving something in the region of 700 watts per square metre. That is 1.44 x 700, roughly 1,000 watts arriving at the glass. With a G-value of 0.60, about 600 watts of that ends up in your room. With a G-value of 0.28, about 280 watts does.

Seventeen watts against six hundred. The U-value is a rounding error in that sum. Chasing a better U-value to settle an overheating room is like fitting thicker loft insulation to stop a radiator working. The radiator, in this case, is your own rooflight, and switching from 0.60 to 0.28 turns off more than half of it. The 320 watt difference between those two panes of glass is more heat than three adults sitting in the room would give off, and it arrives all afternoon, at ceiling height, in the hottest room in the house.

People reason about skylights using their experience of windows, and it leads them astray, because the geometry is completely different.

Southend sits at roughly 51.5 degrees north. At solar noon in late June the sun is about 62 degrees above the horizon. Think about what that means for a vertical south-facing window: the sunlight is arriving at a steep downward angle, striking the glass at around 62 degrees off perpendicular, so a large share of it glances off and only a fraction of the beam energy per square metre actually enters. That is before you count the eaves overhang above it, the reveal at the sides, the neighbour’s hedge and the curtains.

Now take the same sun and point it at a flat rooflight, which faces straight up. The beam is now only about 28 degrees off perpendicular. In round terms, a flat rooflight collects roughly twice as much solar energy per square metre as a vertical south-facing window at midsummer noon. A pitched-roof window on a shallow south-facing slope sits somewhere between the two, and a 30 degree pitch facing south is very close to square-on to the midsummer noon sun, with the angle opening up again as the afternoon goes on.

Then the seasons invert the whole thing. In late December the midday sun over Essex is only about 15 degrees up. That is nearly perpendicular to a vertical south window, which is why a south-facing bay warms a room beautifully in winter, and it is a shallow glancing angle across a flat rooflight, which collects very little. So the rooflight gives you the least free heat in the month you most want it, and the most in the month you least want it. That is the seasonal problem in one sentence, and no amount of U-value alters it.

Two more things stack on top. A rooflight sees the entire sky dome, so it picks up diffuse radiation from the whole hemisphere even on a bright overcast day, where a wall window sees roughly half. And warm air stratifies. The heat arriving through a ceiling aperture is entering at the top of the room, exactly where hot air already wants to sit, so it lingers instead of mixing away.

Most English seaside settlements face north, east or west. Southend-on-Sea faces south, across the Thames Estuary. That single geographical fact reshapes what a good skylight specification looks like across this whole stretch of coast, and it is the reason we specify glass before we specify a unit.

A south aspect here comes with an open horizon. There is no ridge line, no woodland and very little tall development sitting between a Southend roof and the winter or summer sun’s arc. Along the cliff top and the seafront the drop to the water removes obstruction entirely. Add the estuary itself, a large flat reflective surface directly to the south, bouncing additional diffuse light upwards onto every south-facing roof plane along that stretch. Sunlight hours in this corner of Essex are among the higher figures for England, and the roofs here are collecting through more of them.

Then look at what is actually being glazed. The rear extension on a 1930s semi, of which south Essex has an enormous stock, is typically a flat or shallow-pitched roof over a knocked-through kitchen and dining space, and the whole point of it is glass. A roof lantern or a run of flat-roof skylights over a room like that, with a south-facing garden, is a solar collector with a table under it. Loft conversions in the Victorian and Edwardian terraces along the cliff, and in the villa stock, often put bedroom windows into a south-facing slope with no shading at all above them.

None of this is a reason not to fit skylights. It is a reason to specify the glass for the aspect. We survey the roof, work out which planes face where, and pick the G-value to match. North-facing planes get clear glass and all the light we can give them. South-facing planes get solar-control glazing. On a house with both, the two units will not have the same glass in them, and anyone quoting a single specification across a whole roof has not looked at the compass.

These are typical published figures for common rooflight glazing build-ups. Exact values vary by manufacturer, by coating and by whether the number is quoted for the centre of the pane or the whole unit, so treat them as the shape of the market rather than a price list. What matters is the pattern.

Read down the U-value column and the range is narrow: 1.6 to 0.8, a factor of two. Read down the G-value column and it runs 0.62 to 0.26, a factor of nearly two and a half, on a quantity that is thirty times larger in absolute terms on a sunny day. That is the whole argument in one table.

Look at the body-tinted line as well. Its G-value is only moderately better than clear glass, but its light transmittance has collapsed to 0.44. It takes away almost as much of your daylight as it does of your heat. Selectivity ratio of 1.05, against 2.1 for the high-selectivity solar-control unit on the row above. That is the difference between a room that feels cool and a room that feels dim.

One more thing the table does not show: safety glass. Anything overhead needs laminated glass on the inner pane under Part K, and toughened outer panes are standard on rooflights. Laminated interlayers block essentially all UV, which is why solar-control rooflights also stop your flooring and furniture fading, though that is a side benefit rather than the reason to buy.

Lower is not automatically better. G-value is a trade, and the right side of the trade depends on which way the roof plane points and what happens in the room.

Go low, around 0.25 to 0.35

South-facing and west-facing planes. Large glazed areas relative to the room, which usually means a lantern or a run of units over a single-storey extension. Kitchens, where you already have an oven and a hob adding heat. Rooms with hard floors and plasterboard rather than exposed masonry, because there is little thermal mass to soak up a peak. Home offices with screens, where glare on a monitor ruins an afternoon. Any south-facing loft bedroom, because bedrooms have to shed heat overnight and a room that hits 28 degrees at six in the evening will still be uncomfortable at midnight.

Stay high, around 0.5 to 0.6

North-facing planes, where there is no direct beam to control and every bit of daylight is worth having. Rooms used mainly in winter. Landings, hallways and bathrooms where you want brightness and the occupancy is short. Rooms that are genuinely cold and would welcome the free heat.

The middle ground

East-facing planes usually sit around 0.35 to 0.45. Morning sun is strong but the room has all day to recover, and the peak arrives when most houses are empty or want the warmth. West is the one people underestimate: the low late-afternoon sun strikes a west-facing pitched roof at a favourable angle just as the fabric of the house has finished absorbing heat all day, so west often deserves the same treatment as south.

Whatever the glass does, ventilation still earns its place. An opening rooflight at the top of a room clears stratified hot air far faster than any window at head height, which is why we fit electric opening units on high ceilings where nobody is going to reach a pole twice a day. Glass controls what arrives. Opening controls what leaves. You want both.

You can settle this in one conversation with whoever is quoting. Here is the short version of what to say.

1. “Which way does this roof plane face?” If the answer is vague, that is your answer about the rest of the specification. Orientation comes before product choice, not after it.
2. “What is the G-value of the glass you are proposing?” A number should come back. If you get “it is energy efficient” or “it is A-rated”, press again. Energy ratings for windows in this country are weighted towards winter performance and do not describe summer behaviour at all.
3. “Is that U-value centre-pane or whole-unit?” Centre-pane figures ignore the spacer bar and the frame, and they always look better. Whole-unit values are the honest comparison and the ones Building Control cares about.
4. “What is the light transmittance?” Take it alongside the G-value. You are looking for a low G with an LT still up around 0.6, not a low G bought by making the glass dark.
5. “Can I see the glass data sheet?” Every manufacturer publishes one, with values calculated to BS EN 410 and BS EN 673. Asking for it costs nothing and tells you a lot about who you are dealing with.
6. “Who notifies Building Control?” New and enlarged rooflights are notifiable. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the job.

One regulatory point worth knowing. Part L, the thermal part, sets limits on U-values. It does not set a G-value limit for a rooflight in an existing home. Part O, which does address summer overheating, applies to new residential buildings rather than to an extension on a house you already own. So for most of the work we do, nothing in the regulations obliges anyone to think about solar gain at all. That is precisely why so many installers do not, and why a roof full of compliant glazing can still leave you with a room nobody wants to sit in between two and six on a July afternoon.

We have been installing skylights and rooflights across south Essex for over fifteen years, everything backed by a ten-year workmanship guarantee, and the orientation survey is where every job starts. If you want the numbers for your own roof before you commit to anything, ask us for a specification with the G-value written on it, alongside the U-value, for each roof plane separately.