Glass specification for a lantern over a south facing kitchen
A roof lantern over a south facing kitchen is the one position where the glass specification does more work than anything else on the order. Get it right and the room is bright in February and usable in July. Get it wrong and you have built a conservatory roof over your cooking.
This page sets out what the specification actually controls, in the order it matters, and what we would put on a Worthing or Goring kitchen extension facing the Channel.
Two numbers pull in opposite directions
Every sealed unit carries two performance figures, and on an overhead south facing installation they fight each other.
U-value: how fast heat leaves
The U-value measures heat loss through the unit, in watts per square metre per degree. Lower is better. It is driven by the low emissivity coating, the cavity width, the gas fill and the spacer at the edge. Building Regulations set a limiting standard that a replacement rooflight has to meet, and any current product from a reputable supplier clears it comfortably.
g-value: how much solar heat comes in
The g-value, sometimes called solar factor, measures how much of the energy from the sun passes through the glass. It runs from 0 to 1. A clear double glazed unit sits high, around 0.6 to 0.7, which means most of the solar energy arriving at the glass ends up in the room. On a north facing rooflight that is free winter heat. On a south facing lantern over a kitchen in June it is a problem.
Why you cannot simply have both
The coatings that block solar gain sit on the glass surface and they also reduce light transmission. Push the g-value down far enough and the room reads grey, which defeats the reason for putting a lantern there. The specification is a choice about where on that curve you want to sit, and the honest answer depends on the room, the shading and how the kitchen is used.
What a south facing kitchen actually experiences
Overhead glazing is not the same as a south facing window, and the difference is the reason this page exists.
A vertical window facing south takes the sun at a low angle in winter and a glancing angle in midsummer, because the sun is high overhead in June. A lantern does the opposite. In midsummer the sun is close to normal to the glass for hours either side of noon, so the glass is taking solar energy at its most efficient angle exactly when you least want it.
Add a kitchen, which generates its own heat from cooking and appliances, and a rear extension, which is frequently the least ventilated part of the house because it has one set of doors and no cross flow. That combination is what turns an ordinary summer afternoon into an unusable room.
The specification we would write
For a south facing kitchen lantern within the Worthing radius, the sensible starting point is a solar control coated outer pane over a low emissivity inner pane, argon filled, with a warm edge spacer.
| Specification | Typical g-value | Light through | Where it suits |
|---|---|---|---|
| Clear double glazed, low-e inner | 0.60 to 0.70 | High | North facing, or a shaded plot |
| Neutral solar control | 0.35 to 0.45 | Good | Most south facing kitchens |
| High performance solar control | 0.25 to 0.35 | Moderate, slight tint | Unshaded south or west, glazed on three sides |
| Triple glazed with solar control | 0.25 to 0.35 | Moderate | Where the U-value is being pushed hard as well |
The middle row is where most kitchens land. It takes roughly a third to a half of the solar energy out without the glass reading as tinted from inside, and it holds enough light transmission that the room still feels like it has a lantern in it.
Laminate, and why the inner pane is not optional
Any glass over your head should be laminated on the inner pane. Toughened glass fails into small blunt fragments, which is safer than annealed glass, but those fragments fall. A laminated inner pane holds together on the interlayer if it ever breaks, which is why it is the standard specification for overhead glazing.
Laminate brings two side effects that are worth having:
- The interlayer blocks essentially all ultraviolet, which is what fades a worktop, a floor or a dining table sitting under the lantern.
- It adds a small acoustic benefit, which matters more than you would expect under a heavy Channel downpour.
Specify toughened outer, laminated inner, and the argument is finished.
Self cleaning coatings and what they need
A self cleaning outer coating works by two mechanisms: it breaks down organic dirt in daylight, and it makes the surface sheet water rather than bead so the loosened dirt washes off. Both require rain to reach the glass and run off it.
On a lantern that works well, because a lantern is pitched by definition and the rain has somewhere to go. It is far less effective on a flat glass rooflight set at the minimum fall, where water sits and dries in place.
Within a mile or two of the sea there is a second consideration. Salt film builds on any glass facing the Channel, and self cleaning coatings do nothing about salt because salt is not organic. The coating still earns its place, it just is not a substitute for occasional cleaning on a coastal roof.
Ventilation is part of the glass decision
Solar control glass reduces the heat coming in. It does not remove the heat already in the room, and a kitchen makes plenty of its own.
An opening vent in the lantern
A lantern can be built with an opening vent, usually electric because nobody reaches an overhead unit by hand. Heat collects at the highest point of a room, so an opening at the ridge of the lantern removes it far more effectively than an open door at floor level.
Cross ventilation across the plan
If the lantern vent is the only opening, air has nowhere to come from. The combination that works is an opening at the top of the lantern and an opening low down on the opposite side of the room, so warm air leaves at the ridge and cooler air is drawn in behind it.
Overheating, and when it becomes a regulation
New dwellings are assessed for overheating under Part O, which controls glazed area and requires a means of removing excess heat. An extension to an existing house is normally assessed under the thermal elements and glazed area rules rather than Part O in full, but the physics does not care which document applies. Where we are notifying Building Control we will tell you which route your job sits on.
Orientation is not the whole story
South is the obvious case and west is frequently the worse one.
A south facing lantern takes its peak load around the middle of the day, when the house is often empty and the room is coolest to begin with. A west facing lantern takes it in the late afternoon and early evening, onto a room that has already been warming all day, at exactly the hour a kitchen is busiest. If your extension faces west, specify it as though it faced south.
Shading changes the picture too. A mature tree to the south of the extension, a neighbouring gable, or a first floor of the same house set back over the lantern all reduce the load, and are worth mentioning at survey because they can move the specification down a step.
What this looks like on a Sussex kitchen
On the coastal strip through Worthing, Goring and Ferring, most rear extensions are single storey, unshaded, and face south or south west onto a garden. That is the demanding case, and it is the one we specify for by default.
Further inland, at Findon, Steyning or Storrington, plots are frequently more wooded and the ground rises. A lantern shaded by mature planting for part of the day can sit comfortably on a neutral solar control unit and keep more of its light transmission.
On the Downs edge there is a third factor which has nothing to do with the sun. Exposure drives the fixings and the coating, and it is covered properly on the wind loading page. It does not change the glass specification, but it does change the frame around it.
The spacer at the edge of the unit
The edge of a sealed unit is where the two panes are held apart, and what holds them apart used to be aluminium. Aluminium is an excellent conductor, which is precisely the wrong property for a component bridging the warm inner pane to the cold outer one.
A warm edge spacer replaces it with a low conductivity material. It improves the whole unit U-value by a useful margin on a small pane, and more importantly it raises the temperature of the glass at the perimeter. That is where condensation forms first, in a band an inch or two in from the frame, and on an overhead unit that band is directly above whatever is beneath it.
Specify warm edge on any overhead glazing. It costs very little and it is not retrofittable.
Blinds, and where they belong
Blinds are not a substitute for glass specification, and the reason is physical rather than aesthetic.
Internal blinds
An internal blind stops light after the solar energy has already passed through the glass and entered the room. The heat is inside, and the blind is now a warm surface radiating into the space. It controls glare well. It controls heat poorly.
External shading
External shading stops the energy before it reaches the glass and is far more effective on heat. On a domestic lantern it is uncommon, because it is exposed to the weather, it needs cleaning and it changes how the roof looks from outside. On a genuinely difficult west facing case it is worth raising.
Which order to decide in
Specify the glass for heat and add a blind for glare and for privacy after dark. Doing it the other way round means paying twice for the same problem and only solving it once.
What the room will actually feel like
Numbers on a specification sheet are hard to translate into a room, so it is worth saying what the difference amounts to.
A clear double glazed lantern over a south facing kitchen will be noticeably warm from late morning on a clear day from about April, and uncomfortable at the peak of summer without ventilation. The same lantern with a neutral solar control unit is comfortable through most of the year and warm at the peak, which ventilation then handles. A high performance unit is comfortable at the peak and slightly darker in December, which is the trade being made.
Nobody notices good glass. They notice bad glass every July.
Replacing the glass in an existing lantern
Where a lantern is sound but the room overheats, the glass can sometimes be changed without replacing the frame. Whether that is possible depends on the unit.
- The frame has to be designed to be reglazed, with the beads or caps accessible from outside or from inside without dismantling the structure.
- The new unit has to match the thickness the rebate was built for, which limits how far the specification can move.
- The seals and gaskets are usually replaced at the same time, because a thirty year old gasket does not go back on.
Where the frame is not reglazable, or where the coating on the frame has already chalked, a whole unit replacement is the better value. We say which one you are looking at after we have seen it, not before.
The order to decide things in
- Establish the orientation and any real shading, at survey, on the actual roof.
- Fix the safety specification: toughened outer, laminated inner. This is not a variable.
- Choose the solar control step from the table above, against how the room is used.
- Confirm the U-value clears the limiting standard, which any current unit will.
- Decide the ventilation strategy, including whether the lantern carries an opening vent.
- Add self cleaning if the pitch and the position make it worth having.
Taken in that order the specification writes itself, and nothing on the list is a preference dressed up as a technical requirement.
What the survey covers on a lantern
We survey the roof, take the orientation and the shading from the site rather than from a plan, and write the glass specification into the quote so it is fixed before anything is ordered. We notify Building Control where the work is notifiable and the completion certificate comes to you.
The glass is the part of a lantern nobody sees and everybody feels.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.