Energy Efficient Rooflights in Bristol Warm in winter, cool in summer, and still able to open

The limits that apply to existing homes

For work on an existing dwelling in England, Part L 2021 sets out the limits in Table 4.2. The ceiling for a flat-roof or low-pitch rooflight is 2.2 W/m²K. A roof window lies in the pitch of the roof and is meant to open, so it is treated as a window and must reach Window Energy Rating Band B, or a U-value of 1.4 W/m²K. Two products, two yardsticks, and the opening one gets the tougher test. Products from reputable makers meet these limits as standard, so the question for an owner is how far to go beyond them.

Why heat leaves upward

Warm air rises, so the top of a room is the warmest part, and a rooflight sits right there. The inside pane of a poor unit chills quickly in winter, and air next to it sinks, setting up a faint downdraught that people read as a cold spot. Sitting beneath a loft window in January, you can feel the effect on your head and shoulders long before any thermometer does. A better unit keeps the inner pane closer to room temperature, and that is much of what you are paying for.

The worry about this tends to arrive as a single question, and the short answer on whether skylights make a house colder goes through the trade in plain terms: a little more heat lost in winter, set against daylight that lets you leave lamps off, and sun that warms a room on a clear February morning.

What this means on the roof

On a Victorian terrace in Bedminster the roof slope may carry only modest insulation between the rafters, so a new roof window with a strong U-value can be a real step up beside the old single-glazed skylight it replaces. On a 1930s semi the loft room may have a cold slope above a bed, and the unit overhead sets the comfort of the whole space. We note the existing glass, the insulation round it and the room's use at the survey, then explain which level of performance is worth having in that spot and which is more than it needs.

The spacer that holds the panes apart

Round the edge of every sealed unit runs a spacer bar, which keeps the panes apart and holds the gas in. Older spacers were aluminium, a metal that carries heat very well, so the edge of the glass became the coldest strip in the window and the first place for a line of mist to appear. A warm edge spacer is made from a material that conducts far less, which keeps that strip nearer to room temperature. The short answer on the warm edge spacer explains what to look for on a quote. On cost it is among the cheapest upgrades there is, and we would not specify a unit without one.

Glass that does more than insulate

Other coatings sit on the same panes. A self-cleaning layer on the outer face breaks down dirt and lets rain wash it off, which helps on a unit you cannot reach. Acoustic laminated glass dulls the patter of rain, useful in a loft bedroom under a slate roof. Laminated inner panes also hold their pieces together if broken; The regulations do not demand laminated glass over a house, so on our quotes it appears as a suggestion with its price beside it.

Each of these features is optional, and each adds a little to the quote. The sensible approach is to rank them by the room. For a landing, a warm edge spacer, argon and a good low emissivity coating are plenty. For a bedroom, laminated glass may earn a place. For a sash that opens, the roof windows hub lists the sizes and styles that take these glazing choices.

Choosing by the direction the roof faces

The compass direction of the slope decides how much of this matters. A rooflight on a north-facing slope seldom sees direct sun and rarely needs solar control, so clear, bright glass suits it. East-facing glass catches the morning sun, which warms a bathroom gently. South and west slopes take the strong afternoon and evening sun, and a bedroom under one of those is where solar control glass repays its cost. Georgian and Edwardian terraces tend to have roofs facing front and back, so one slope of a loft may face the sun and the other not, and the two units can be specified differently.

Blinds, and opening the glass

Blinds fitted outside the glass stop the sun before it reaches the pane, and they work far better than blinds hung inside, which catch the heat after it has entered. Manufacturers make external awning blinds and roller shutters for roof windows, some of them solar powered. Inside blinds still give privacy and darkness for sleep. Combined with glass that reflects some of the heat, they make a sound pair.

Air movement completes the picture. A roof window that opens lets the hottest air at the top of the room leave and draws cooler air in at the eaves or through a lower window. A rooflight with a trickle vent or a tilt position can be left on the latch overnight, so a loft cools down while you sleep. If you want a unit that opens at the touch of a button and closes when it rains, electric opening rooflights do that, and our page on fresh air rooflights looks at how opening and efficiency fit together.

Planning for summer at the survey avoids an uncomfortable surprise later. We ask which way the slope faces, whether trees or the next roof shade it, and which room is below. That conversation settles whether a unit needs a tint, a blind or simply a sash that opens.

Better products make this less likely, though they cannot rewrite the physics. A warm edge spacer keeps the rim of the glass warmer. A thermally broken frame has an insulating strip between the inner and outer parts, which keeps the room face warmer than the outer one. A triple glazed unit keeps its inner pane warmer still. Each step pushes the point at which water appears further off, which means that on all but the dampest mornings you may see none. Our guide on how to stop rooflight condensation lists the causes in order and what to do about each.

Moving the moist air out

The best defence against condensation is air that moves. This is where an opening rooflight earns its place. Cracking a roof window open after a shower, or leaving it on the vent bar overnight, lets the moisture leave before it condenses. A roof window that opens can count towards purge ventilation under Part F, meaning the fast airing that shifts steam and cooking smells, so a bathroom unit does real work. An extractor fan still belongs in a bathroom, and a window that opens sits alongside it, rather than replacing it.

When mist appears inside the sealed unit

One kind of mist is a different problem. If a film or water drops appear between the two panes, where you cannot wipe them away, the sealed unit has given up and its gas has escaped. No amount of airing helps, and the unit is replaced. If only the sealed glass is faulty in a sound frame, fitting a new glass unit is possible and generally not notifiable work. Where the entire rooflight or roof window is tired, a whole new unit is notifiable, so that paperwork is on our list, not yours. To tell which kind you have, see misted and failed rooflights.

It helps to wipe the glass in the morning with a dry cloth and open a window for ten minutes. A damp cloth hides the pattern; a dry one shows whether the water sits on the room side or inside the unit, which is the first thing we check when we visit.

Each evening we leave the roof closed to the weather, with dust sheets down over floors and furniture while the work goes on. One team handles the job from the first fixing to the final plaster line, which makes it easier to be sure that nothing between the frame and the rafters has been missed.

The short answer on whether an energy efficient rooflight is worth it goes through the arithmetic: since a rooflight is a small part of the roof, the saving in heating bills is real but small, and anyone who promises it will repay the extra within a few winters is stretching the case.