VELUX glazing options and solar control

A VELUX product number is not one thing. It is a string that tells you the opening type, the frame material, the size and, at the end, the glazing. A centre-pivot timber window is a GGL, the white polyurethane version is a GGU, top-hung units are GPL and GPU, and the electric and solar-powered models carry the INTEGRA name. All of those can take more than one glazing unit, and the glazing is where the summer behaviour is decided.

The glazing itself is identified by a pane code that sits at the tail of the product number. Current double-glazed units generally fall in the region of the –0068 and –0070 codes, with the higher of the two carrying a laminated inner pane, and triple-glazed and coated variants running above that. VELUX has revised this numbering more than once over the years, so an old brochure or a code copied from a neighbour’s window may not describe what is on sale now. The safe habit is to read the code off the current data sheet for the exact unit being quoted, then look up what that code contains: how many panes, what gas is in the cavity, which coatings are on which surface, and whether the inner pane is laminated or toughened.

Two windows sitting side by side on the same roof, same size, same GGL frame, can have completely different pane codes and therefore completely different solar factors. That is not a fault. On a roof with planes facing more than one direction it is exactly what you want. It only becomes a problem when nobody checks, and a single glazing code gets copied across a whole order because it was easier than surveying the roof.

VELUX groups its glazing into tiers rather than selling raw glass specifications, which makes the choice easier at the counter and harder to interrogate. The table below gives typical published figures for the shape of that range. Treat them as industry-general guidance rather than a fixed price list: coatings change, and a figure quoted for the centre of the pane always looks better than the same unit measured whole, frame included.

Read across the tiers and the U-value moves within a fairly tight band, from about 1.4 down to 1.0. That column is the one Building Regulations care about, because Part L polices winter heat loss, and any current VELUX unit clears the limit for a replacement window in a dwelling without much drama. The solar factor column is the one that moves your comfort in July, and it swings much harder: from 0.53 on a plain double down to 0.30 on a coated triple. That is close to a halving of the heat that gets through, on the number nobody is legally obliged to check.

Notice the heat-protection double in the middle row. It holds the U-value of an ordinary double but drops the solar factor to around 0.35, and it does that while keeping light transmittance up at 0.66. That is the selective coating doing its job: turning back the near-infrared that carries the heat while letting the visible daylight through. A room under that pane stays bright. It does not go dim and brown the way it would under old-fashioned tinted glass, which knocks out daylight and heat in roughly equal measure and leaves you switching a lamp on at noon.

A solar-control coating on the glass is real and it works, but it has a ceiling. The coating sits on one of the internal glass surfaces and reflects a portion of the incoming solar energy back outwards before the pane absorbs it. Drop the solar factor from 0.53 to 0.35 and, on a square metre of glass taking a strong midsummer beam, you are turning back a meaningful slice of what would otherwise land in the room. On a modest window that is the difference between warm and uncomfortable. It is a genuine improvement and, for a north or east plane, it is usually all the shading the room will ever need.

The limit shows up on the big south-facing units. A coated pane still lets a third or more of the sun through, and a third of a very large number is still a large number. On a run of rooflights or a wide top-hung window over a garden-facing room, the glass can be doing everything it reasonably can and the room can still climb through the afternoon, because the total glazed area is collecting more energy than any coating percentage can hold back. At that point the honest answer is not a better pane. It is stopping more of the sun before it reaches the glass at all. That is the job the glazing cannot do from the inside, and it is where VELUX’s external shading comes in.

The most effective solar-control product in the VELUX range is not a pane of glass. It is a mesh awning blind that fits on the outside of the window. It sits in the airflow above the glass, intercepts the sun before it touches the pane, and lets the heat radiate away into the open air instead of into your ceiling. VELUX publishes that its external awning blinds cut the solar heat coming through by up to around three-quarters, a figure no internal glazing coating gets near, precisely because it acts first in the sequence rather than last.

The blind is a semi-transparent mesh, so it shades without blacking the room out. You keep a softened view and a good deal of daylight while the heat load drops away. VELUX offers the awning in a manual version you crank down by hand, an electric version, and a solar-powered version with its own small panel and battery that needs no wiring, which matters on a loft window that would otherwise mean chasing a cable across finished plaster. On the electric and solar units the blind can retract automatically, so it shades through the hot part of the day and rolls back up when the sun moves off, taking the guesswork out of it.

The order of operations is the point worth holding onto. On a south-facing roof plane the best result comes from stacking the two defences rather than choosing between them: a coated heat-protection pane to handle the baseline load, and an external awning blind to take the peak off on the worst afternoons. Glass alone leaves the peak too high on a large unit. A blind alone over a plain standard pane leaves you with more heat than you needed on every ordinary day. Together they cover both.

Internal blinds sell in far greater numbers than external ones, and it is worth being clear about what they do. A VELUX blackout or roller blind fitted on the inside of the window is a fine thing for sleep, glare on a screen and privacy. What it is not is solar control in any serious sense. By the time the sun reaches an internal blind it has already passed through the glass and released its energy into the room. The blind can soak some of that up and re-radiate it, and a light-coloured blackout blind will cut the felt heat a little, but the energy is already indoors. VELUX’s own figures make the gap plain: an internal blind trims the heat by a modest fraction, where the external awning turns back the majority of it.

This trips people up because the internal blind is the visible, everyday product and the external one is not. A south-facing loft bedroom fitted with a smart-looking blackout blind can still be too warm to sleep in on a July night, and the owner is often surprised, because it feels as though the window is covered. It is covered. The heat just got in before the cover did. If overheating is the actual problem, the money goes on the outside of the glass, not the inside.

Overhead glazing carries a safety requirement, and VELUX builds this into the glazing tiers as much as into the comfort ones. Anything installed above head height should have a laminated inner pane, which holds together if it ever breaks rather than dropping glass into the room below. That is the reasoning behind the Standard Plus tier: it takes the ordinary double-glazed unit and swaps the inner pane for a laminated one, which is why it is the sensible baseline for any window with people, a bed or a kitchen worktop underneath it. Toughened glass, which is standard on the outer pane, breaks into blunt granules and resists impact from above, so a typical VELUX safety build-up pairs a toughened outer with a laminated inner. In a new or enlarged rooflight this is the kind of specification Part K expects to see.

Laminated glass brings a second benefit almost for free. The plastic interlayer that holds it together also blocks close to all of the ultraviolet in daylight, so a laminated or solar-control unit protects flooring, furniture and artwork from the fading that plain glass allows. It is a side benefit rather than a reason to buy, but it is real, and it is another point in favour of the coated and laminated tiers on a bright south-facing room where the sun tracks across the floor all afternoon.

For bathrooms and en-suites, VELUX offers obscure glazing that passes daylight while blurring the view, which matters where a roof window sits low on a side-facing slope and privacy from a neighbouring property is in question. That is a specification decision made at the survey, alongside orientation, rather than an afterthought once the window is in.

Southend faces south across the Thames Estuary, which is an unusual thing for an English seaside settlement and the reason glazing choice is not a formality here. A south aspect on this stretch of coast comes with an open horizon, long sunshine hours by English standards, and a wide reflective surface of water to the south throwing extra diffuse light up onto every south-facing roof plane. A VELUX window on a south slope in this city is collecting through more of the day than the same window would inland or on a north coast, and it is doing it with very little natural shading above it.

Then look at what the roofs here actually carry. The 1930s semi, which south Essex has in enormous numbers, tends to gain a rear extension or a loft conversion over a garden-facing room, and that is exactly the south-facing, lightly shaded aperture where a plain standard pane will overheat a room. Victorian and Edwardian terraces along the cliff and through the older estates convert their lofts into bedrooms, often on south-facing slopes, and a bedroom has to shed heat overnight to be usable. On any of these, the plane that faces the garden and the estuary is the one that wants a heat-protection pane, and probably an external awning blind on top of it if the glazed area is large.

The flip side matters too. A north-facing plane on the same house should not get the same glass. There is no direct beam to hold back, and every bit of daylight is worth having, so a clear standard or a clear triple is the right call there, keeping the room bright and warm in winter. Fitting solar-control glass to a north slope throws away daylight you needed and pays for a coating that has nothing to reflect. This is why the compass comes before the catalogue. A house with roof planes facing more than one way should end up with more than one glazing code on the order, and a quote that puts a single pane across the whole roof has skipped the part that decides how the rooms will feel. VELUX supplies the range that makes plane-by-plane specification possible; using it properly is the point of a survey, and it runs through everything we do on VELUX roof window installation across the patch.

You can settle the glazing in one conversation, provided you ask about summer as well as winter. The short list below is what to put to whoever is quoting.

1. Which way does this roof plane face? Orientation decides the glazing, so it should come first. If the answer is vague, the rest of the specification is guesswork.
2. What is the solar factor, the g-value, of the pane you are proposing? A number should come back. An A energy rating does not answer this, because window energy ratings in this country are weighted towards winter and say nothing about how the room behaves in July.
3. Is the U-value the whole-window figure or centre-pane? Centre-pane numbers ignore the frame and always flatter the unit. Whole-window is the honest comparison and the one that goes to Building Control.
4. Is the inner pane laminated? For anything overhead with people, a bed or a worktop below, it should be. That is the Standard Plus specification rather than the plain standard one.
5. Does the south-facing unit need an external awning blind as well? On a large garden-facing area the pane alone will not hold the peak, and the external blind is the part that actually does. It is easier to plan the wiring, or choose the solar-powered version, before the window goes in rather than afterwards.
6. Who notifies Building Control? New and enlarged rooflights are notifiable under the Building Regulations. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the installation.

One regulatory note worth carrying with you. Part L sets a ceiling on U-values, so the winter side is looked after by the rules whether anyone thinks about it or not. There is no equivalent limit on the solar factor of a roof window in an existing home. Part O, which addresses summer overheating, applies to new residential buildings rather than to an extension or a loft on a house you already own. So for most VELUX work in south Essex, nothing in the regulations forces anyone to consider solar gain at all, which is precisely why so many quotes leave it off, and why a roof full of compliant, well-rated glazing can still hand you a room nobody wants to sit in on a bright afternoon.

Where the glass matters is on the roofs that face the sun, and this city has more of those than most. If you want the specification worked out plane by plane, with the solar factor written down next to the U-value and the right shading called out where the area needs it, the same logic applies whether the units go on a pitched roof slope or a dormer, and it starts from an orientation survey. We have been installing skylights and roof windows across south Essex for over fifteen years, every job carrying a ten-year workmanship guarantee, and the glazing choice is made against the compass rather than the catalogue. For the numbers on your own roof, including which planes want solar-control glazing and which want an automatic electric awning blind on top of it, ask us for a specification before you commit to anything.