Energy efficient rooflights in Exeter

Heat leaves a room through a rooflight in three ways. It conducts straight through the glass and the frame, it escapes along the weakest line of the unit, which is the edge where glass meets spacer and frame, and it leaks around the outside of the window where the opening has been packed badly. A rooflight earns the label "efficient" only when all three are dealt with, not just the glass.

The measure everyone quotes is the U-value, which says how quickly heat passes through a material or a built-up unit. Lower is better. A U-value can describe the glass alone, measured at the middle of the pane, or the whole window including frame and edge. The whole-window figure is always the less flattering one, and it is the one that tells you what the room will feel like. When two quotes give two different figures, check which kind each one is before comparing them. Our guide to rooflight U-values explained sets out how the figures are produced and how to read a data sheet.

A rooflight also has a harder job than a wall window. It faces the sky, so it loses heat to the coldest thing around, and warm air rises straight into it. On a still, clear night a rooflight is often the coldest surface in the room, which is why the same glazing build performs worse in a roof pitch than it would upright in a wall. Roof window makers publish figures that allow for this, and that is a good reason to use their figures rather than a generic glass specification.

The last ingredient is the one most quotes leave out. The efficient glass sits in an opening, and the opening has to be insulated and sealed to match. We come back to it later in the chapter on the opening itself, because it is where a good unit is most often let down.

A double glazed sealed unit is the standard for new and replacement rooflights and it meets current regulations when it is specified properly. Triple glazing adds a third pane and a second gap, which lowers the U-value further. It also adds weight and thickness, and it does not always give the best return in a small opening.

The question is not which one is better in the abstract. It is which one suits your room, your roof and the frame the unit has to sit in. The trade-offs look like this.

Our guide to double or triple glazed rooflights takes that comparison further, room by room. If you are wondering about the extra load, the answer to is a triple glazed rooflight heavier explains what the added pane means for handling, fixing and the roof structure.

The coatings matter as much as the pane count. A low emissivity layer can be sited on different surfaces of the unit to favour heat retention or to limit solar gain, and the choice is not interchangeable. A coating that keeps winter heat in will let in more of the summer sun than one designed to reject it. Choosing between them is a decision about the room, and the chapter on sun in summer covers it.

Energy efficiency has a summer half. A south or west facing rooflight over a kitchen, a landing or a loft bedroom collects a great deal of sun between May and September, and a room that held its heat well in January can become hard to live in in July. Solar control glass is the answer, and it works by reflecting or absorbing a share of the sun's heat while still letting daylight through.

The measure to ask about is the g-value, sometimes called solar factor: the share of the sun's energy that passes through the glass as heat. A lower g-value means less heat gain and also a slightly less bright, sometimes slightly tinted, room. For a rooflight that faces north, or sits in a stairwell with deep shade, a high g-value is preferable, because the free winter sun is useful and the summer risk is small. For a lantern over a south-west facing extension it can be the other way round. We go through the options and what each does to colour and brightness in solar control glass for rooflights.

A blind inside the room stops the glare but not the heat, because the sun has already passed through the glass.

Solar control is not the only way to manage heat gain. Internal or external blinds, an opening vent that lets warm air out at the top and shading from a tree all help, and they can be combined with the glass. A blind inside the room stops the glare but not the heat, because the sun has already passed through the glass. An external blind stops the heat before it arrives.

Then there is the glass that cleans itself. A thin coating on the outer face breaks down organic dirt in daylight and lets rain wash it away more evenly. It does its best work on a sloped pane that rain can run over, and it is less use on a roof under overhanging trees or in a courtyard where the rain cannot reach. We weigh it up in is self-cleaning glass worth it on a rooflight, because on a roof you cannot easily reach, the case for it is strongest.

If rain on the glass is a worry over a bedroom, an acoustic laminated inner pane is one of the glazing options and it can sit within an efficient unit. It adds a little weight and cost, and it is chosen for the room, not for the energy figure.

The coatings can be combined in a single unit, but not always without a trade-off, so the final choice is written into the schedule with the maker's product name beside it.

This is the part where an efficient rooflight is most often undone. The glass can be excellent and the room still cold if the shaft between the ceiling and the glass is an uninsulated box of bare board, or if the gap between the frame and the roof is stuffed with whatever came to hand.

The shaft, or lining, runs from the window down to the ceiling, and its depth is the depth of the rafters or joists plus the insulation and the finishes. In a typical pitched roof it is a sloping box; in a flat roof it is often a deep vertical one. Either way it should be lined with insulated plasterboard or a board with an insulated backing, and the insulation must meet the frame insulation without a gap. The vapour control layer comes out from behind the lining and seals to the frame, so that moist room air cannot reach the cold side. That detail alone prevents most winter condensation staining round a new rooflight.

The lining shape does a second job. Splayed wider at the bottom, it lets the insulation be thicker at the frame without making the opening look like a tunnel, and it spreads daylight into the room. A straight-sided shaft through a deep roof loses a lot of the glass. The difference is easy to see, and the answer to can rooflights cause condensation shows how the lining and the seal together keep the plaster dry.

Finish is part of the efficiency picture in another way: a lining that is plastered or boarded and taped, sanded and left ready to paint has no gaps for air to move through. The edge is crisp where it meets the ceiling and the paint line is straight, and the room is left clean. That is what the written finish schedule describes, one line at a time. See our guarantee for what the workmanship covers.

The day on site follows the specification, and the order of work is what protects the energy performance. On a new opening, the covering is cut back, the rafters or joists are trimmed and doubled, and the frame goes in with its insulation collar or kerb. Flashings and the weathering follow, then the glass and the seals. Only then does the work move indoors to the lining.

The details that decide the thermal result are small. The insulation round the frame is packed tight and not stuffed. The vapour control layer is lapped and taped to the frame, not just tucked behind the board. The lining board meets the frame insulation without a void. We check each of these before the lining is closed, because after plastering they cannot be seen.

A temporary weatherproof cover goes over any open section each evening, so the room is dry overnight. Dust is managed with sheeting and a screened route, and the room is cleaned when the plaster and decoration are done. A new opening with a lining to form usually takes a few days on site, and a like-for-like replacement can be shorter.

Afterwards you receive the product details, the figure from the maker and the Building Control paperwork, so there is a record of what is in your roof.

Many people come to us not for a new opening but because an old rooflight is cold, drips with condensation or has misted. What you do then depends on what is already there.

If the frame is sound and only the sealed unit has failed, a replacement glass unit in the existing frame can be done without a notification. The gain is a clear pane and a better coating, but the frame and the opening stay as they were. If the frame is old, poorly sealed or a single glazed type, a whole replacement is the better investment: it brings the entire window up to current limits, and the Building Control notification follows. Our page on misted and failed rooflights explains how to tell which case you have.

A whole replacement is also the best time to fix what is round the window. The old flashings come off, the insulation round the frame is renewed, the vapour layer is relapped and the lining inside can be reformed and finished. That is where most of the comfort gain comes from, and it is why we price the lining and finish as a line of its own.

For a pitched roof, a like-for-like roof window swap is often quick, but we measure first, because makers change sizes and an old opening may not match any current product. For a flat roof, the kerb may be sound or may need rebuilding, and we say which at survey.

Condensation forms when moist air meets a cold surface, so the aim is to keep every surface in the opening warm.

No. It gives a lower U-value, but it is heavier, deeper and dearer, and in a small opening the gain is modest. It pays for itself mainly in large rooflights, exposed roofs and cold rooms. For a typical bathroom or landing, a well specified double glazed unit with a warm-edge spacer and a well sealed, insulated opening usually does more for the money.