The main VELUX glazing options are double glazing with a laminated inner pane and a triple-glazed version, and for most Devon homes the decision is between those two. Double glazing is the sensible starting point for most rooms. Triple glazing earns its extra cost in rooms you sit in for long stretches, in cold lofts, or where you want the window to match the warmth of the walls around it. Other options sit on top of that choice: a noise-reducing pane, a heat-reducing coating, an obscured pane for a bathroom, and an outer coating that helps rain clean the glass.
This guide sets the options side by side, explains what each one changes in the room, and shows where the glazing decision meets the Building Regulations. It is about VELUX windows specifically. If you are weighing glass for a flat rooflight or a lantern instead, the principles carry over, and our guide to double or triple glazed rooflights covers that side.
What is inside a VELUX glass unit
A roof window pane is a sealed unit, built in layers. The outer pane faces the weather and is toughened, which means it is heat-treated to resist hail, wind-blown debris and the occasional branch. The inner pane faces the room. Between them sits a gap filled with an inert gas, usually argon, held apart by a spacer bar around the edge. On a triple unit there is a third pane and a second gap.
Each part matters for a different reason. The gas slows the passage of heat across the gap. A low-emissivity coating, a microscopically thin metallic layer on one of the inner glass faces, reflects heat back towards the room it came from. The spacer matters because it runs around the whole perimeter, and a metal spacer conducts heat and chills the glass edge. A warm-edge spacer is made of a less conductive material and keeps the edge of the inner pane nearer to room temperature, which is where condensation tends to start on a cold morning.
When you compare glazing, you are comparing combinations of those parts: the number of panes, the coating, the gas, the spacer and the type of inner glass. A salesperson who quotes only "double" or "triple" has told you very little. Ask what the inner pane is, what the coating does and what the spacer is made of.
Double or triple glazing
Double glazing has two panes and one gas-filled gap. Triple has three panes and two gaps. The third pane lowers heat loss through the glass, so the window feels less cold to be near and the glass is less likely to mist on the inside in a heated room. It also adds weight, and a heavier sash puts a little more load on the frame and hinges, which is allowed for in how the window is built.
| Point | Double glazed | Triple glazed |
|---|---|---|
| Panes and gaps | Two panes, one gap | Three panes, two gaps |
| Heat loss through the glass | Lower than a single pane by a wide margin | Lower again than double |
| Feel beside the window | Cool glass on frosty mornings | Glass closer to room temperature |
| Weight and thickness | Lighter, slimmer unit | Heavier, thicker unit |
| Cost | The lower-priced choice | A step up in price |
| Where it makes most sense | Landings, hallways, bathrooms, rooms used briefly | Bedrooms, studies, loft rooms, north-facing and exposed roofs |
The gain from the third pane is real but it is not dramatic in a well-insulated roof. The lining, the insulation around the frame and the airtightness of the junction all affect how warm the room feels, and a triple unit sitting in a poorly insulated lining will disappoint. We would rather you spend on the lining and the insulation collar and keep the double glazing than pay for triple glass in a cold reveal.
Where triple glazing is the better choice is a room where the window is large, close to your head or bed, and in use all winter. A loft bedroom in a Victorian villa in Torquay, or a study under the slates on the Dartmoor edge, is a good candidate. A downstairs hall in a 1930s semi in Heavitree that you walk through twice a day probably is not.
The laminated inner pane
Most VELUX windows in the standard ranges have an inner pane made of laminated glass. Laminated glass is two sheets bonded to a clear plastic interlayer. If it breaks, the pieces stay stuck to the interlayer instead of dropping into the room. Over a bed, a staircase or a kitchen worktop, that is the property you want.
It is worth being plain about the status of this. Part K of the Building Regulations covers glazing in critical locations such as doors and low-level panels, and it does not require laminated glass in a rooflight. A laminated inner pane overhead is good practice and standard industry guidance, and it is one reason we keep it on the specification even where it is not demanded. If a quote offers a toughened-only inner pane on a window above a room people use, ask why.
Laminated glass has a second, smaller benefit. The interlayer filters out much of the ultraviolet light that fades fabrics, timber floors and rugs. It does not stop fading, and it is not a substitute for blinds in a sunny room, but it takes a share of the effect away.
Noise-reducing glass
For windows in a loft room under a flight path, above a busy road or close to a school playground, there are panes built to cut the sound that comes through. The usual route is a thicker laminated pane, sometimes with a different thickness on each side of the gap so the two panes do not vibrate at the same pitch.
Rain on a roof window is louder than rain on a tiled roof, and the choice of glass changes that sound, too. If rain drumming on a loft room window bothers you, tell the surveyor at the start. It affects which pane is specified, and it is a conversation to have before the order goes in, not afterwards.
Noise-reducing glass costs more than the standard pane, and its effect is most noticeable on airborne sounds such as traffic, not on every noise in the room. It is a useful option for a few windows, not a default for all of them.
Heat-reducing coatings, tints and obscured glass
A south-facing roof window can turn a loft room into the warmest room in the house on a bright afternoon. There are coatings designed to reflect part of the sun's heat before it passes through the glass. They are applied to the outer pane or sit inside the unit, and they give the glass a slight tint when seen from outside. The trade-off is that a little less daylight comes in.
For a bedroom or study that gets strong sun in summer, a heat-reducing pane works alongside an external or internal blind. For a north-facing hall, it has no benefit and costs light you want. This is one of the choices that depends on the room, not on the window model, and your surveyor should ask which way the roof faces.
Obscured glass is a different option for a different job. It scatters the light so the glass is translucent, not clear. It suits bathrooms and en-suites, and it is the form of glass that planning rules ask for on side-facing rooflights below a certain height. Obscured glass still lets the daylight in: the room stays bright and nobody can see through it.
Coatings on the outer face
Some VELUX panes carry a coating on the outside of the outer pane that helps rain wash dirt and pollen from the surface. It does not clean the window by itself, and a roof you cannot reach with a ladder will still get dusty in a dry summer. What it does is stretch the time between cleans and make each clean easier.
It is worth thinking early about how the glass will be reached. A low-pitched roof window over a stair void is hard to clean from inside, and an outer coating plus a pole-mounted cleaning kit may be the only sensible way. We would point that out at survey, because the answer changes which window pivots or opens best for the room.
Which glazing for which room
The same window can be specified several ways. These pairings are the starting points we use when writing a quote, and we adjust them for the roof and the room.
- Loft bedroom
- Triple glazing and a laminated inner pane, with a blackout blind. A bedroom window is close to the bed and in use at night, when the glass is at its coldest.
- Kitchen or kitchen-diner
- Double glazing with a laminated inner pane is usually fine. Cooking steam is the issue, so a window that opens for purge ventilation matters more than a third pane.
- Bathroom
- Obscured glass in the pane, or clear glass with a blind. The opening matters here too, because a bathroom has steam to shift.
- Hall, landing or stairwell
- Double glazing with the laminated inner. The glass is a long way from where anyone sits, so the extra cost of triple glass is hard to justify.
- South-facing loft room or study
- Triple or double with a heat-reducing coating, plus a blind. Choose according to which problem is worse, winter chill or summer heat.
- Rooms under a flight path or beside a main road
- A noise-reducing pane on the windows facing the source.
Glazing and the Building Regulations
Whatever the glass, a replacement VELUX window has to meet the limiting U-values that the 2021 Volume 1 edition of Approved Document L sets for replacement windows and rooflights. U-value is the measure of heat lost through a material, and lower is better. The standard unit in a VELUX range is generally built to meet the figure for replacements, though you should confirm that on the specification for the exact model.
The notification rules turn on what you are replacing. Swapping the whole window, frame and glass, has to be notified to Building Control, since windows are controlled fittings and must be shown to meet Part L. Putting in only a new sealed glass unit, with the frame staying, needs no notice. When we replace a whole window, the notification is part of our job.
A new VELUX window in a roof that did not have one is building work, because the rafters are cut and trimmed. That also needs Building Control approval, and the planning position is usually permitted development for a house, within limits we set out on the VELUX window installation page. Check with the local planning authority if your house is in a conservation area, is listed or has an Article 4 direction.
Glass, condensation and the inside of the room
People often blame the glass when a roof window mists on the inside. Sometimes it is the glass, but often it is the lining. If the lining is cold, damp air from the room condenses on it, and the stain appears first along the sill. A triple unit set in an uninsulated reveal will still mist at the edges, because the glass is not where the cold is coming from.
That is why we treat glazing and finishing as one decision. The best glass in a thin, uninsulated lining gives you less than ordinary glass in a well-built one. The insulation collar around the frame, the vapour control layer lapped onto it, the lining insulated behind its plasterboard and the paint finish all decide how the unit performs. If you want to see how that is built, read how a rooflight is finished inside, and what a splayed lining is shows how the shape of the reveal changes the room.
The glass specification and the lining go on the same page of your quote. Our finish schedule records the lining and its finish for the room, so that you can read the glass and the finish together before you agree anything.
How to choose and what to ask
Start with how the room is used and which way the roof faces. Then decide between double and triple for the main windows, add a noise-reducing or heat-reducing pane only where there is a specific problem, and use obscured glass where privacy needs it. Do not pay for options on every window if the problem exists on one.
When you read a quote, check that it names:
- the window model and size, with the size written as a code, such as MK04 or CK02, or described in words if the quote does not use one;
- the number of panes and the type of inner pane;
- any added coating, such as heat-reducing, noise-reducing or an outer easy-clean coating;
- whether the glass is clear or obscured;
- the type of window, centre-pivot or top-hung, since this affects how the glass is reached and cleaned.
The sizes and codes themselves are explained in our guide to VELUX window sizes and codes. If you have a window in place and want to match it, can a VELUX window be replaced like for like covers what has to line up. And if the window is going into a new loft room, planning a loft room around VELUX windows deals with the room as a whole.
Everything above is set out in more detail on our VELUX window installation page. When you have a room in mind, ask for a quote and we will write the glazing and the finish into one specification.