U-Values and Glazing Specification

What a U-value actually measures

A U-value is the rate at which heat passes through a building element, measured in watts per square metre per degree of temperature difference, written W/m²K. Lower is better. A solid insulated roof might be around 0.15. A modern double-glazed rooflight is somewhere near 1.2 to 1.4. A triple-glazed one can reach below 1.0.

So even a good rooflight loses heat several times faster than the roof it sits in. That is not an argument against rooflights, it is an argument for specifying them properly and for not fitting more glass than the room actually needs. Daylight is worth a great deal, and a room nobody uses because it is dark is a worse outcome than a slightly higher heat loss.

The important thing to understand is that a U-value is a property of an assembly, not of glass. The figure that matters is the whole-unit U-value, covering glass, frame, spacer and the junctions between them. The frame is always worse than the glass, so a small unit with a lot of frame relative to glass performs worse than a large one with the same specification.

Centre-pane against whole-unit, and why the difference is used against you

This is the single most common piece of misdirection in glazing specifications, and it is not always deliberate.

Centre-pane U-value describes the glass alone, in the middle, away from the edges and the frame. It is always the flattering number. Whole-unit U-value, sometimes written U_w, describes the entire rooflight as installed, and it is the one Building Regulations are written against.

A unit advertised at 1.0 on the centre pane may be 1.4 as a whole unit. Both figures are true and only one of them is relevant. If a quotation states a U-value without saying which it is, ask. A supplier who cannot tell you the whole-unit figure for the exact size you are ordering does not know what they are selling, because the figure changes with size.

What the Building Regulations require

Part L of the Building Regulations sets the standard, and rooflights have their own line in it because they are not windows and not roof.

For a replacement rooflight in an existing dwelling, the current standard requires a whole-unit U-value at or below 2.2 W/m²K, assessed in a vertical position for reasons of test methodology rather than common sense. For a new opening in an extension, the requirement is tighter and the whole extension has to satisfy an overall calculation, so the rooflight is one input among several.

Anything a mainstream manufacturer sells today clears the replacement threshold comfortably. Compliance is rarely the interesting question. The interesting question is how far past the minimum it is worth going, and that depends on the room.

Double or triple, and when the extra pane earns its place

Triple glazing adds a pane, a cavity, weight and cost. Whether it is worth it depends on three things.

The first is the room’s use. A bedroom or a home office occupied for hours at a time justifies it. A landing or a bathroom rarely does.

The second is orientation. A north-facing slope receives very little direct sun and loses heat steadily, so the extra pane works for you all winter. A south-facing slope has a different problem, and it is not heat loss.

The third is weight and structure. Triple glazing is appreciably heavier, and on shallow Victorian rafters or an old hand-cut roof that weight is a structural question rather than a preference. It is one of the things a survey settles.

There is also a comfort argument that the numbers understate. A cold pane of glass overhead radiates coolness downward and produces a draught as air falls off it. In a room where you sit still, an improved inner pane temperature is felt long before it shows on a bill.

G-value, the number nobody mentions

The g-value describes how much of the sun’s energy passes through the glass, expressed between 0 and 1. A g-value of 0.6 means sixty per cent of solar energy gets in.

On a rooflight this matters far more than it does on a window, because a roof slope faces the sky and, in summer, the sun is high. A vertical south window is shaded by its own reveal at midday. A rooflight in a shallow pitch is aimed almost straight at the summer sun.

So the specification splits by orientation. On a north-facing slope, or in a room that is genuinely cold, a higher g-value is desirable and free solar gain is a benefit. On a south or west-facing slope, particularly a shallow one, a solar control coating with a g-value in the low 0.3s is the difference between a pleasant room and one nobody enters after two o’clock in July.

Solar control glass carries a slight tint and reduces daylight along with heat, which is a genuine trade-off rather than a free improvement. It is still usually the right answer on a hot slope, and it is far cheaper than the blinds and the discomfort that follow from not fitting it.

Light transmittance

Written as LT and expressed as a percentage, this is how much visible light gets through. Clear double glazing sits around 75 to 80 per cent. Every additional pane and every coating takes some away, so triple glazing with a solar control coating might be nearer 50.

The number matters most on small units and on north slopes, where there was not much light to begin with. On a generous rooflight in a bright position, a lower transmittance is barely noticeable. On a single small unit lighting an internal landing, it is the difference between the light working and the light being a gesture.

Acoustic performance

Rated in decibels of reduction, and worth specifying in two situations: under a flight path or beside a genuinely busy road, and on any rooflight where rain noise will be heard from a bed.

Rain on glass is louder than people expect, and it is the most common complaint about an otherwise perfect installation. An asymmetric build-up, with panes of different thickness, disrupts the resonance and helps noticeably. Laminated glass helps more. Neither costs much when specified at order and both are impossible to add afterwards.

Safety glass, and where it is not optional

Overhead glazing has its own rules and they are not a matter of preference.

The inner pane of any rooflight above a place people occupy should be laminated, so that if it breaks it holds together in its interlayer rather than falling into the room. The outer pane is usually toughened, so that impact from above breaks it into blunt granules.

Anywhere accessible for cleaning or maintenance from above, or in a commercial setting, non-fragility becomes a separate assessment with its own standard. On a domestic roof window this rarely arises. On a flat-roof unit somebody might walk near, it does.

Coatings, gas fills and spacers

Three details that quietly do a lot of work.

A low-emissivity coating is a microscopically thin metallic layer on one of the cavity faces. It reflects long-wave heat back into the room and it is the main reason a modern sealed unit outperforms an old one. It is invisible and it is standard on anything worth buying.

Argon in the cavity conducts less heat than air and improves the U-value at almost no cost. It is standard. Krypton performs better in a narrow cavity and costs considerably more, and it is worth asking about only in a slim conservation-grade unit where the cavity has to be thin for appearance.

The spacer is the strip separating the panes at their edge. A traditional aluminium spacer conducts heat straight around the cavity and produces a cold edge where condensation forms first. A warm-edge spacer, in a composite or plastic material, improves the whole-unit figure and moves the coldest point away from where you notice it. On a rooflight, where the edge is often the first place moisture shows, it is worth insisting on.

Conservation units and the compromise they carry

A conservation-grade rooflight sits flush in the plane of the roof, is finished dark, and usually carries a central glazing bar. The design constraints are visual, and they cost performance.

The frame is slimmer, so there is less room for insulation within it. The cavity is often narrower, so the U-value is worse than an equivalent standard unit. Some are single glazed by conservation officer requirement, which is thermally poor and occasionally unavoidable on a listed building.

Where a designation forces a compromise, the sensible response is to recover the performance elsewhere in the build-up: better insulation in the surrounding roof, a properly detailed and insulated reveal, and a continuous vapour control layer. The unit is one element among several and it is the one with the least room to move.

Matching the specification to the room

Orientation and use between them settle most of it.

The habit worth breaking is ordering identical units for a whole roof because they will look consistent from inside. Two slopes facing opposite directions have opposite problems, and matching them means one of the two is specified wrongly.

How a good specification gets undone on site

A rooflight with an excellent whole-unit U-value can still produce a cold, streaming reveal, and when it does the glass is usually blameless.

The junction between the unit and the roof build-up is a thermal bridge unless somebody designs it not to be. Insulation has to be carried tight to the frame, without gaps and without being compressed into uselessness, and the vapour control layer has to be continuous and sealed to the frame rather than stopped short of it. Where either is missed, the reveal becomes the coldest surface in the room and moisture finds it first. The owner sees water and reasonably concludes the rooflight leaks.

This is why the installation detail and the glazing specification are one subject rather than two. Buying the better unit and fitting it into an unsealed opening spends money on the smaller half of the problem.

What to ask for in writing

A specification worth having states the manufacturer and model, the exact size, the whole-unit U-value for that size, the g-value, the light transmittance, the glass make-up pane by pane including which are laminated and which toughened, the cavity gas, the spacer type, and any acoustic rating.

That is nine lines. Any manufacturer publishes all of them and any competent installer can transcribe them. A quotation that offers one number and a brand name has told you almost nothing, and the gap between what you assumed and what arrives on the lorry is entirely at your risk.