Guide · Skylight & Rooflight Installation & Replacement Chelmsford
Upgrading the glass while the roof is open
The glass in a rooflight costs a fraction of what it costs to get at it. The scaffold, the strip-back, the day of labour and the making good are all bought and paid for the moment a replacement is booked, and the difference between a basic sealed unit and a well specified one is a line on an order form.
That is why the specification conversation belongs at the point of ordering a replacement rooflight rather than five years later when somebody notices the room is unusable in August. This page sets out what can actually be changed in the glass, what each change buys, and where the improvements stop paying for themselves.
Why the specification decision has a deadline
A sealed unit cannot be upgraded in place. The panes, the cavity, the gas, the spacer and every coating are assembled in a factory and sealed, and nothing about them can be altered once the unit is glazed into a frame.
So the whole of the glass decision happens before the order is placed, usually two to six weeks before anyone is on the roof. That is a short window and it arrives at the least convenient moment, when the conversation is still about dates and disruption. Getting it settled early is the difference between a considered choice and taking whatever the standard build happens to be.
What the labour has already paid for
It is worth being blunt about the economics, because they are unusual on this one item.
On a typical single unit replacement, the access, the covering works, the flashing, the fixing and the making good account for the larger share of the total. The glass upgrade rides on top of that with no additional labour at all, because a solar control unit is fitted in exactly the same movements as a plain one. Later, the same upgrade means paying for the whole job again to change one component. There is no other point in the life of that rooflight where the improvement is this cheap.
Reading the old unit before choosing the new one
The unit coming out carries useful evidence, and it is worth ten minutes at survey.
A data plate on the sash edge or the frame face gives the manufacturer, model and often the glazing code, which converts directly into what was originally specified. Where there is no plate, the spacer bar between the panes tells you a good deal: a bright aluminium bar with visible desiccant beads is older construction, a dark grey or black bar is warm edge and therefore relatively recent. The edge seal colour and condition indicates age too. From there the improvement available is a known quantity rather than a guess.
The build-up of a modern sealed unit, pane by pane
A current rooflight unit is not simply two sheets of glass, and knowing the order of the layers makes every other decision on this page readable.
From outside inwards: a toughened outer pane, usually four or six millimetres, which takes the thermal and impact load. Then a cavity of twelve to sixteen millimetres holding argon. Then a laminated inner pane, two sheets bonded with a plastic interlayer, which is what stops glass falling into the room if it ever breaks. Around the edge sits the spacer bar carrying desiccant, sealed with butyl on the inside and polysulphide or silicone outside. A low emissivity coating sits on one of the cavity faces. Everything discussed below is a change to one of those layers.
Which face the coating goes on, and why it is never the outside
Low emissivity coatings are microscopically thin metal oxide layers that reflect long wave heat back the way it came. Their position within the unit decides what they do.
The pane faces are numbered from outside in, one to four. A coating on face three, the cavity side of the inner pane, reflects room heat back into the room and is the standard choice for a cold climate. A coating on face two, the cavity side of the outer pane, intercepts solar energy before it enters the cavity and is what a solar control unit uses. Neither goes on an exposed face, because the coating is soft and would not survive a season of weather and cleaning. A unit can carry both, on faces two and three, and that is the specification a south facing flat rooflight in Essex usually wants.
Gas fills, and the difference between a claim and a certificate
Air conducts heat better than the heavy inert gases, so filling the cavity with argon reduces the U-value of the unit by roughly a quarter compared with the same build in air.
Argon is now close to universal in mainstream units and costs very little. Krypton performs better still and is normally reserved for narrow cavities where argon has too little depth to work, which happens in slim triple glazed builds. The point worth raising at order is that gas fill is a manufacturing claim, and the number that matters is the declared whole unit U-value on the paperwork rather than a description of the fill in a brochure.
Spacer bars, and the edge that used to be aluminium
The perimeter of a sealed unit is where the heat leaves fastest, and for decades that perimeter was an aluminium bar, which conducts heat about as well as anything in the building.
Warm edge spacers replace that with stainless steel, a composite or a structural foam, cutting the conduction at the edge substantially. On a rooflight this matters more than on a wall window, because the edge of the pane is where the reveal meets the glass and where condensation shows first on a cold night. The visible sign of the upgrade is a dark grey or black perimeter band rather than a bright metal one, which most people prefer to look at anyway.
Stepping up from double to triple, and what it costs elsewhere
Triple glazing adds a third pane and a second cavity, taking a good double glazed rooflight from somewhere around 1.2 W/m²K down towards 0.8.
| Build | Typical whole unit U-value | Weight per m² | Practical note |
|---|---|---|---|
| Single skin acrylic dome | 5.0 to 5.5 | Very light | Period unit, no coating |
| Early double glazed, air filled | Around 3.0 | Moderate | Aluminium spacer, no low emissivity layer |
| Current double, argon, low emissivity | 1.0 to 1.4 | Moderate | The usual replacement specification |
| Triple, argon, two coatings | 0.7 to 1.0 | Around half again heavier | Deeper frame, heavier lift, less light through |
The costs are not only financial. A triple unit is heavier, which affects manual operation on a hinged unit and the fixings into an older kerb. It is deeper, so the frame is deeper and the reveal detail changes. And each additional coated surface takes a little more light out. Triple glazing earns its keep on a north facing plane in a room heated hard through winter. On a south facing rooflight it is often the wrong end of the problem.
Summer heat, and the number that governs it
The g-value is the fraction of solar energy that gets through the glass, and on a rooflight it is the number that decides whether the room is pleasant in July.
A plain double glazed unit sits around 0.6 to 0.7, meaning around two thirds of the sun’s energy arrives in the room. A solar control unit runs between 0.25 and 0.4. That difference is not subtle in a roof plane, because a flat rooflight in June takes the sun almost perpendicular at the hottest part of the day, when a vertical window is receiving it at a glancing angle and shrugging most of it off. A rooflight is the most solar exposed piece of glass in a house by a wide margin.
Choosing a solar control level for the room, not the roof
Solar control is not a single product and specifying the strongest available is a common mistake.
The right level follows the room. A north facing pitched slope over a study takes a neutral unit and the room stays even and cool all day. An east facing loft bedroom takes mild control, because the gain arrives early and the room has all afternoon to shed it. A south or west facing flat rooflight over a kitchen with a hard floor and no cross ventilation takes the strongest control that still gives acceptable light, because that room has thermal mass storing the gain and no route for it to leave. Orientation, room use and ventilation together set the answer, which is why it is decided at survey.
A rooflight facing the whole sky collects more solar energy per square metre than any wall window in the house.
Daylight through a coated pane, and the colour it leaves
Every coating takes light as well as heat, and the two cannot be separated entirely, so the honest framing is what the light looks like rather than whether there is enough of it.
Light transmission for a plain double glazed unit is around 78 to 80 per cent. Good modern solar control glass holds 60 to 70 per cent while cutting the heat gain by half, which is a favourable trade. The older reflective products cut both equally and gave the room a green or bronze cast that was noticeable on a white ceiling. Current coatings are close to neutral and the useful question to ask a supplier is not the transmission figure alone but whether the glass is described as neutral, because that is a claim about how daylight will look in the room rather than a laboratory percentage.
Safety glass overhead, and what the build has to do
Glass in a roof is glass above people’s heads, and that changes what the panes have to be.
The outer pane is toughened, which means heat treated so that it holds edge stress and, if it does break, granulates rather than forming shards. It also resists thermal fracture, which is a real risk where a coated pane is partly shaded by a kerb or a chimney and heats unevenly. The inner pane is laminated, two sheets bonded to an interlayer, so that a breakage stays hanging in the frame instead of arriving in the room. Any replacement unit should be specified this way as a matter of course, and an older dome or an early roof window frequently was not.
Acoustic upgrades, and the asymmetry that does the work
Rain on a rooflight is louder than rain on tile, and on a flat unit over a quiet room it is described as charming for one winter and less so by the third.
Two specification choices reduce it. A laminated pane damps the drumming, because the plastic interlayer absorbs vibration a single sheet transmits straight through. And an asymmetric build, where the two panes differ in thickness, breaks up the resonance rather than reinforcing it, since the two sheets no longer share a natural frequency. A four millimetre outer with a six and a half millimetre laminated inner is meaningfully quieter than two four millimetre sheets. Neither change can be made after the unit is glazed.
Coatings on the outer face, and the ladder they save
Face one, the exposed outer surface, is where the maintenance coatings go, and their value depends entirely on how hard the glass is to reach.
A self cleaning coating is a titanium dioxide layer that breaks down organic dirt in daylight and lets rain sheet off rather than bead, carrying the loosened material away. It reduces the frequency of cleaning rather than removing the need. On a rooflight it works better than on a wall window because rain lands on it directly and in quantity. On a unit three metres above a kitchen floor with no safe access from outside, it is one of the more useful pounds spent. On a low garage rooflight reached from a step ladder, it is optional.
Privacy, and the slope that is overlooked
Terraces in Chelmsford and Colchester and the tighter interwar streets frequently put one roof directly in view of a neighbouring upper window, and a replacement is the moment to deal with it.
Obscured glass is available in the inner pane on most rooflight ranges, in patterns rated for the level of obscuration they give. It costs a little and it changes nothing about the thermal specification, because the obscuration is on the inner laminate rather than in the coating. On a bathroom rooflight it is usually the right call. On a kitchen it rarely is, because it takes the view of the sky away, and the sky is the point.
Getting the glass onto the paperwork
A specification agreed in conversation and left off the order is a specification that will arrive as whatever the standard build is.
The quotation should name the manufacturer and model, the glazing code or a written description of the build, the whole unit U-value with the measurement basis stated, the g-value, the light transmission, and any coating on the outer face. That is six short lines and it converts an intention into something contractual. It also makes two quotes comparable, which a single figure against the words “double glazed rooflight” never does. The energy performance page goes further into what those numbers mean in use.
Lead time, and the point where the upgrade stops being free
Standard glazing builds in standard sizes are usually held in stock or made within a fortnight. Anything outside that grid is manufactured to order.
Solar control, acoustic asymmetry, obscuration and self cleaning are all catalogue options on mainstream ranges and rarely add more than a week or two. A genuinely bespoke unit, in a size off the grid with a non-standard build, can run to four weeks or more. That is the only real cost of upgrading at replacement, and it is a planning cost rather than a money one. Deciding the glass at the same time as the dates keeps the whole thing inside one programme, and the costs page sets out how the ranges are built before a survey fixes them.
Keep reading
More on skylight & rooflight installation & replacement chelmsford
Sizing a rooflight for the room
Read the guide →Roof window or rooflight, and the difference
Read the guide →Flat roof against pitched roof rooflights
Read the guide →What happens on installation day
Read the guide →Cleaning and maintaining a rooflight
Read the guide →All about skylight & rooflight installation & replacement chelmsford →
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