Energy efficient rooflights for a loft conversion, kept warm and quiet

Heat rises, so the top room of the house loses it first. The same room gains heat fastest in summer, because the sun falls on a pitched roof for most of the day.

A kitchen extension has a flat roof, a lantern and a large volume of air to share the load. A loft bedroom is smaller and lower, and the rooflight sits close enough to the bed that you feel its surface temperature. On a January morning a single cold pane radiates chill onto your face. On a July afternoon an unshaded pane throws heat onto the floor. Both are problems of surface temperature, not of the air in the room, so a thermostat cannot fix them.

Three forces act on the glass.

• Conduction: heat passes through the glass, the spacer and the frame to the outside in winter.
• Radiation: the sun heats the pane and the room in summer, and the warm room radiates back out through clear glass at night.
• Air leakage: gaps at the frame and the seals let warm air out and draughts in.

An efficient loft rooflight deals with all three. It uses a sealed double or triple unit with coatings and a warm spacer, a frame that does not carry cold straight through, and a sash that seals against the frame without gaps. Our page on how heat escapes through a rooflight follows each path in turn.

The glass unit is where most of the performance is decided. A modern sealed unit is a small assembly of parts, and each part has a job.

Start from the outside. The outer pane faces rain, hail and sun, and it is often toughened so that it resists thermal stress and impact. Behind it sits the cavity, a sealed gap filled with argon rather than air, because argon conducts heat less readily. Around the edge of that cavity runs the spacer, and on a rooflight the choice of spacer matters more than on a wall window, because the glass is tilted and condensation forms first at the cold perimeter. A warm edge spacer made from a low-conducting material keeps the edge of the glass warmer and cuts the dark line of condensation that cheap units show in cold weather. The inner pane carries a low-emissivity coating on one of its faces, a microscopically thin metallic layer that reflects heat back into the room while letting most of the light through.

For a loft, the inner pane should be laminated. Glass over a place where people sit or sleep should be safety glazing as a matter of good practice, and laminated glass holds together if it breaks, so a fragment of toughened glass never falls onto a bed. A laminated inner pane also has a second benefit that we come to below.

Coatings and fills are covered in detail in our pieces on low-E coatings on rooflight glass and warm edge spacers and argon fill. Here it is enough to know that the four features arrive together in any unit worth fitting in a bedroom.

A third pane improves the heat figure of the glass, and it adds weight, thickness and a little daylight loss. Whether it earns its place in a loft depends on the room rather than on a rule.

Triple glazing suits a loft that is heated for long hours, such as a bedroom suite, a guest room used in winter or a home office where someone sits still for a working day. In those rooms the cold surface is noticed, and the extra pane makes the glass feel closer to the temperature of the wall. A double unit with a good coating and a warm spacer is a sensible choice for a room that sees less use, or where the roof is steep and the glass area small.

There are practical limits to check.

1. Weight. A triple unit is noticeably heavier, which affects how the sash hangs and whether the frame suits a manual or a motorised opener.
2. Thickness. A deeper unit needs a frame that can hold it, and it changes the depth of the reveal inside.
3. Light. Every extra pane and coating takes a little daylight, so the gain in warmth is traded against a slightly dimmer room.

We look at the unit as a whole, not at one pane, and we set it against the room's aspect and the size of the opening. The trade-offs are set out in triple glazed rooflights: when the third pane earns its place, and our answer on the weight of a triple glazed rooflight covers what the rafters have to carry.

A U-value measures how much heat passes through a material, and a lower number means less loss. On a rooflight the number on a brochure can mean two different things.

The figure for the centre of the glass describes the glazing alone, away from the edges. The figure for the whole unit includes the frame and the spacer, where heat escapes more easily. The whole-unit figure is always the less flattering of the two, and it is the one that describes how the finished rooflight behaves in your roof. When two quotes list different numbers, check first that both are quoting the same kind. The difference is explained in whole-unit versus centre-pane U-values.

Approved Document L sets limiting values for new and replacement glazing, and a loft conversion brings new thermal elements into the house, so the rooflights have to meet the limit that applies on the day of the work. We check the current figure and the route that fits the job at the survey, and we install to current Building Regulations. Where the work is notifiable, we handle the Building Control notification. The rules themselves are summarised in Part L and rooflights: what the energy rules ask for.

On a small loft window the frame can take up a fifth or more of the total area. A frame that conducts cold straight through will undo a good glass unit.

Aluminium is strong and slim, which suits a roof lantern but needs a thermal break, a strip of low-conducting material between the inner and outer metal, to perform well. Timber and timber with a polyurethane coating conduct far less and feel warm to the touch, which is why roof windows for lofts are so often made with a timber core and a weatherproof outer skin. Polyurethane frames add a high level of insulation and resist moisture, which matters where a loft bathroom sends steam up to the slope.

What matters in practice is the detail around the sash.

• A continuous seal that closes evenly on every side of the opening.
• An insulating collar or surround that fills the gap between frame and rafter.
• Thermal breaks in any metal, so cold has no direct path to the inner face.

More on the metal frames in thermally broken rooflight frames. For the wider choice of roof windows that suit a pitched loft, see our hub on roof window installation.

A rooflight can be a first-rate unit and still lose heat around its edges. The junction with the roof insulation is where most lofts go wrong.

Loft rafters are shallow. To reach a good standard of insulation, the build-up usually stacks rigid boards between and across the rafters, and the rooflight has to sit in that build-up without squeezing it. If the frame is fixed on the rafters with no insulated collar, the insulation stops short of the glass on every side, and a band of cold forms a cold bridge around the opening. You see the result as a line of condensation or a dark stain on the plasterboard splay in winter.

The answer is to treat the opening as part of the thermal envelope rather than a hole in it. The insulation is run tight to the frame, the gap is sealed, and a vapour control layer is dressed onto the frame so that warm, moist room air cannot reach cold timber. The reveal inside is then splayed, which reflects light into the room and leaves a wider, warmer surface to finish. Our pages on insulating around a rooflight and on cold bridging around a rooflight describe the layers.

Ventilation belongs here too. Part F asks for background and purge ventilation in a habitable room, and a loft rooflight that opens, or has a trickle vent in the head of the frame, can supply both. A sealed, efficient room with no way to change the air will hold moisture. We specify the opening type along with the glass so the room keeps its warmth and still breathes.

A loft that is comfortable in February can be unusable in July. The glass has to manage the sun as well as the cold.

A south or west facing slope receives strong sun through the afternoon, and the heat it lets in has nowhere to go but up to the ridge. Solar control glass reduces that gain with a coating that reflects a portion of the sun's energy, and its strength is described by a g-value. The lower the g-value, the less heat passes. The coating leaves the pane a little less bright and sometimes very slightly tinted, which is why it is a choice for the slopes that need it rather than for every unit. A north-facing rooflight seldom needs it, and giving one to a room that never overheats simply costs some light.

Think about the rooflight and the blind as a single system. Glass cuts the gain, an external blind or awning stops the sun before it reaches the pane, and an internal blind handles glare and privacy. The outer barrier is the most effective of the three. Our guide to stopping a rooflight room from overheating weighs them, and the page on solar control glass for rooflights explains the coatings.

A blind is often treated as a finishing touch. In a loft it changes how the room behaves in both seasons.

In winter, a honeycomb or cellular blind traps a layer of still air against the glass and slows the night-time loss. It works best when it runs in side channels that hold the fabric close to the frame. In summer, the same blind in a reflective finish keeps some sun out. Blackout fabric helps a bedroom, where early light in June arrives before anyone wants it. The blind has to fit the rooflight it covers, so it is chosen together with the glass, while the plasterers are still to come.

An electric rooflight and a matching powered blind suit a loft where the window sits out of reach above a bed. If you are weighing that route, our hub on electric and opening rooflight installation describes the options. Whether a thermal blind makes a measurable difference is answered directly in does a thermal blind help keep a rooflight room warm.

More glass means more light and more heat exchange. Every loft has a balance point, and finding it is part of the design.

A sloping ceiling is lit unevenly. A single large window at the eaves lights the floor but leaves the ridge dim, while a window high in the slope throws light deeper into the room. Two smaller units side by side, or one above the other on a steep pitch, often light a loft better than one big unit, and they let you open one for air while the other stays shaded. Stagger them with the furniture in mind: a bed under the glass in a bedroom, a desk beside it in an office.

The position of the glass changes the energy picture. A bright room that needs no lights by day saves electricity, and some of the gain in a cool season is useful. Our note on whether a rooflight can help warm a room in winter goes through when the sun is an asset. On the other hand, too much glass in a west slope will need stronger solar control. We set the area against the orientation at the survey and explain the trade-off on the quote.

The layers that keep a loft warm are the same layers that keep it quiet. That overlap is why the quiet spec we write for every quote covers both.

A loft sits close to the roof surface, which is the first thing rain, hail and aircraft meet. Rain on a thin single pane can be loud enough to wake a sleeper. Around Crawley, a loft bedroom can also hear departures and arrivals from Gatwick, and rooms near the A23, the M23 or the Brighton main line hear traffic and trains. A laminated inner pane, a wider or unequal pane thickness, a good cavity and well-fitted seals all soften that. None of it needs a separate product. The laminated inner pane you chose for safety and the warm edge you chose for condensation already do part of the work.

What changes is that we listen at the survey and write down what the room hears. The glazing build, the seals and the upstand or collar are then chosen for that. We describe the result by specification, never by a test figure, and the note goes into the written spec that comes with the quote. Read how the quiet spec works for the format, and one glazing build for warmth and quiet for the reasoning behind the overlap.

A loft rooflight goes in from both sides of the roof, so the order of work protects the house.

The roof is opened only as far as the day can be closed back up. Tiles or slates are lifted around the marked area, a rafter or two is trimmed and doubled where needed, and the frame is fixed and flashed into the roofing. The insulation is then fitted tight to the frame, the vapour control layer is sealed, and the splay is boarded for the plasterer. We cover the floor and stairs and clear the dust at the end of each day. Noise during the day is unavoidable, but the roof is weathertight by the evening.

The work carries a 10-year workmanship guarantee, described on our guarantee. If a wider plan is involved, such as a dormer or a full conversion, the rooflights fit into the builder's programme and the roof structure is agreed first.

Will a better rooflight lower my heating bill?

It helps, because the glass stops losing heat faster than the walls around it. The size of the saving depends on how large the glass is and how the rest of the loft is insulated, so treat it as one part of the envelope. Will energy efficient rooflights lower my heating bills goes through it.

Does a loft rooflight affect my EPC?

A better glazing specification can contribute to the rating, though it is one input among several, including insulation, heating and ventilation. Our answer on whether an energy efficient rooflight improves an EPC explains the link.

Can I upgrade the old roof windows in my loft?

Often the unit can be replaced with a modern one in the same opening, sometimes with a new collar to improve the insulated edge. Can an old rooflight be upgraded to be more energy efficient covers the choices.

What does a loft rooflight cost?

The price depends on the size, the glazing build, the roof type and the access. Our rooflight and skylight costs page gives a general guide, and a written quote follows the survey.

To talk through your loft, start from the energy efficient rooflights hub or get a quote.