Energy-efficient rooflights in Maidstone

Whole-unit figures and centre-pane figures

Data sheets often quote a centre-pane U-value, which describes the middle of the glass and leaves out the edge spacer and the frame. The whole-unit figure includes everything, so it is the one to compare. A slim aluminium lantern frame and a bulky uPVC one can carry identical glass and still return different whole-unit numbers. When we put a quote together we ask for the whole-unit figure for the exact size you are buying, because the frame takes up a bigger share of a small unit than of a large one.

Why a roof is a hard place to hold heat

Warm air rises, so the ceiling is where a heated house presses hardest on its envelope. A rooflight sits in that spot, and its inside face is colder than the plastered ceiling around it. Where the room is humid, such as a bathroom, kitchen or utility, water vapour condenses on the coldest surface it finds, and that is usually the glass or the frame. Better glazing warms that surface and helps the condensation clear, which is why an energy upgrade on a tired unit often cures damp patches around the reveals as well.

Heat loss is only half of the story. Good glazing is chosen to keep daylight and to keep it comfortable. A well-specified unit lets natural light through while limiting the winter loss and the summer gain, and we weigh all three when we choose the glass. Tinting that cuts glare too hard can leave a loft study dim, while clear glass over a south-facing landing can leave it too warm to enjoy.

Where energy performance matters most

Some rooms feel the difference more than others. A loft bedroom sits directly under the roof and meets both extremes. An en suite with a roof window collects steam. A garage conversion with a flat rooflight needs the kerb and the glass to match the insulation in the walls around it. A bungalow hall lit by a single glass rooflight can be the coldest spot in the house if the unit dates from an earlier decade. We look at the room first when we survey, then at the glazing already in the roof, and on a replacement we measure the old unit against what the Building Regulations now ask.

If you want to know whether skylights lose much heat in winter, our short answer on whether skylights lose heat goes through it, and the guide to rooflight U-values explained shows how the numbers are worked out.

The two limits

A new or replacement roof window must reach 1.4 W/m²K, which is the window limit, or achieve at least Window Energy Rating band B. A rooflight or roof lantern must reach 2.2 W/m²K. Both are whole-unit values, so the frame counts. A roof window with a good pane in a thin frame is judged on the sum of the two.

The gap between 1.4 and 2.2 can surprise people who compare the two on a quote. The two products are built and assessed differently, and the document sets a separate ceiling for each. A homeowner choosing between a lantern and a pitched-roof window for the same room should compare the whole-unit figure on each data sheet, then decide on light, headroom and the roof.

New openings and replacements

A brand-new opening in the roof counts as building work in its own right. It touches the structure, the energy performance and, in some rooms, the fire and escape arrangements. Replacing a rooflight in an existing opening also has to meet the energy limits for the new unit. Where the work is notifiable, we handle the Building Control notification, and either the local authority or a registered building control approver checks it and issues the paperwork. The Building Regulations guide for skylights sets out the full route, and Part L and rooflights explains the energy section line by line.

Planning, in one line

A roof window on a house usually falls under Class C permitted development if frame and glass stay within 150 mm of the original slope, a limit a flush roof window usually meets. Lanterns sit proud of a flat roof and normally need an application, unless the council tells you in writing that one is unnecessary. The skylight planning guide gives the other conditions, including the one for side slopes.

What it means when you pick a unit

Because the limits apply to the whole product as installed, a catalogue photograph tells you very little. What matters is the size, the frame and the glazing build-up. Manufacturers publish data for each size in a range, and the figure often changes from one size to the next. We use the sheet for the exact unit we are fitting and note the figure on the quote, so a roof window ordered from a catalogue code such as MK04 carries the number for that code and no other. Read the first pair of letters as the width and the digits as the height.

Double glazing as the base

Double glazing is the starting build-up for rooflights, roof windows and lanterns. Two panes trap a layer of air or gas that slows the heat passing through. On its own that is not enough to meet today's limits, so the panes are paired with a low-emissivity coating, a thin metallic layer on one glass surface that reflects heat back into the room while letting daylight through. Filling the gap with argon instead of air slows the flow further, because the gas conducts less heat. Whether a given unit reaches the Part L limit is shown by its whole-unit data, and we specify the build-up to suit the opening.

Edge spacers

The spacer bar keeps the two panes apart and seals the edge. A traditional aluminium spacer conducts heat around the rim of the glass and leaves the edge colder than the centre, which invites condensation along the bottom of the pane. A warm-edge spacer made from a less conductive material narrows that cold band. On a rooflight, where the lower edge of the glass sits at the coldest point, the spacer is worth asking about.

When triple glazing makes sense

A third pane improves the figure further. It also adds weight, and the weight bears on the frame, the kerb and the structure below, which on a large lantern means more engineering. We look at triple glazing where a room is used through the winter, such as a loft study or a garden room, and where the frame and the roof underneath can carry it. For a small roof window in a landing slope, a well-specified double unit can meet the limit.

Laminated and toughened panes

The outer pane of a rooflight is commonly toughened, and the inner pane laminated, with an interlayer that holds the glass together if it breaks. Laminated glass also resists being broken through from outside, which matters for security on a single-storey flat roof that can be reached from a garden wall. These layers add a little weight but carry the unit's safety behaviour, and the specification line names both panes so you can see what is being supplied.

Reading the build-up on a quote

A line such as 6.4 laminated inner, 16 mm argon gap and low-e outer pane tells you the thickness of each layer and what fills the space. Compare it with what you have now. A twenty-year-old sealed unit may have air in the gap and no coating, and swapping only the glass in an old frame can lift performance, though the frame then limits what the whole unit can reach. That is a matter for the survey, where we can see the frame.

Aluminium frames and thermal breaks

Aluminium frames suit lanterns and large flat rooflights because the metal is strong and the sections can stay slim. Metal also conducts heat well, so an unbroken aluminium frame would run cold on the inside. A thermal break solves this: an insulating strip fitted within the section separates the inside face from the outside face. A frame with a good break is warmer on the room side, which lowers the chance of condensation along the bars and improves the whole-unit figure.

uPVC frames

uPVC is a poorer conductor of heat than aluminium, and its sections hold chambers of trapped air. The sections are thicker, which takes some of the glass area, but a uPVC frame can perform well for energy. On a conservatory or orangery where the existing frames are white uPVC, a matching roof lantern can make visual sense, and we compare both materials by their whole-unit figures before recommending one.

The kerb or upstand

The frame does not sit on bare roof. On a flat roof it stands on a kerb, an upstand built up from the deck, and the insulation in that kerb joins the insulation in the roof. If the kerb is thin or has gaps in its insulation, heat leaks around the lantern even when the glass is excellent. This is why a replacement can disappoint when only the unit is changed: the cold ring around it stays. When we survey a flat roof we look at the kerb's build, the insulation under the deck and how the covering dresses up to the frame, and on a replacement we say whether the kerb can stay or needs rebuilding.

Pitched roofs and the lining round a roof window

On a pitched roof the equivalent detail is the flashing and the insulated lining that fills the gap between the window and the rafters. A roof window installed with the lining squeezed or missing allows warm air to touch cold timber, and the result is staining at the reveal. A splayed reveal inside also helps, since it angles the light further into the room as well as covering the thick edge of the insulation. Our page on roof windows covers pitched units in full, and the VELUX window page explains how one manufacturer's range is sized and specified.

Airtightness at the junction

Draughts around a rooflight waste heat just as a bad window seal does. The seal between frame and kerb, the tape and sealant lines, and the way the vapour barrier is lapped to the frame all affect air leakage. Good detailing here costs little in materials and is invisible once the plasterboard is up, so it relies on the fitter. Our 10-year workmanship guarantee, given in writing, covers the way the unit is fitted into the roof.

The solar factor in plain numbers

Alongside the U-value, a data sheet gives a g-value, the share of the sun's energy that passes through the glass as heat. A lower g-value means less solar gain. Solar control coatings lower it by reflecting part of the energy outwards, and some units add a tint that cuts it further. The cost is a slightly darker pane, so we choose the coating for the room as well as the roof.

Orientation and shade

A roof facing south or west receives strong low afternoon sun, and a loft bedroom beneath it can heat up quickly. A north-facing slope needs very little solar control, and clear low-emissivity glass lets all the softer light in. East-facing units get the morning sun when the room is usually cooler. A neighbouring tree or a chimney stack can change the picture, so we note the shading at survey and look at the roof at the time of day when the sun is highest on it.

Blinds, vents and the stack effect

Glass alone is only one layer of control. An external blind stops the sun at the pane, so the heat never enters the room, and an internal blind helps with glare but lets the heat pass the glass first. An opening vent at the top of a lantern or in a roof window lets hot air leave, because warm air collects at the highest point and a vent there draws cooler air in lower down. Our electric opening rooflights can take a rain sensor and a remote, so a vent can be left to open and close for you when you are out of the house.

Where overheating shows up first

Orangeries and garden rooms with a lantern in a south-facing flat roof, loft bedrooms with large glazed areas, and glass-roofed kitchens are the places to specify with care. A utility or hallway with a small rooflight is much less likely to give trouble. For the full discussion, our guide on whether rooflights overheat a room goes into the causes and the fixes.

Balancing light and heat

The aim is not the lowest g-value on the market. It is a pane that lets in enough daylight to leave the lights off, without turning the room into a greenhouse. For a deep kitchen under a flat roof, clear or lightly coated glass with a vent at the top often balances well. For a south-facing loft, a stronger coating and an external blind work together. We talk through the choice with you at the survey, with the roof, the aspect and the room's use in front of us.

Comparing the two materials

Polycarbonate sheet traps air within its channels and so insulates reasonably, but it transmits light diffusely, is easily scratched and can sound loud in rain. Glass is clearer and quieter, and with coatings and gas fill it can reach a lower U-value than multiwall sheet. A fair comparison uses the whole-unit U-value and solar factor for the specific product, which our guide to glass or polycarbonate for heat works through. We specify whichever meets the limit and suits the roof's structure.

A solid roof with a lantern

The biggest energy gain on an old conservatory often comes from changing how much of the roof is glass. A fully glazed roof loses heat in winter and gains it in summer across its whole area. Replace it with an insulated solid roof and keep a roof lantern at the centre, and the glazing shrinks to the part that delivers the light, while the rest of the ceiling insulates like the rest of the house. The frames and base need a structural check, and the work needs a Building Control notification.

The benefits of daylight

Good energy performance is only worth having if the room is pleasant. Natural light from above brightens the middle of a room that wall windows leave dim, and it changes through the day. A landing that was lit by a bulb at noon can read as a calm, bright space with a single roof window above the stairs. A bungalow hall with a rooflight feels less like a corridor. A kitchen with a lantern over the worktop reveals true colours and cuts the hours the lights are on.

Brightness has a measurable side, and a less measurable one. Daylight lets you see the weather and the time of day, and many people sleep, work and cook more comfortably in rooms that show both. What we can describe is that a unit chosen for energy as well as light, and fitted well, makes a loft study or a chalet bedroom usable all year, and keeps the heating bill from rising by more than it needs to.

Upgrades and what you can expect

Replacing a failed or tired unit with a modern one brings the glazing up to current limits. The room warms up faster, the pane stays clearer and the condensation lines often disappear. If the unit has misted, our misted rooflights page explains why the seal fails and what we replace. If a house stands in a conservation area, the conservation rooflight range keeps the look of a slim, flush unit with the glazing build-up of a modern one.

The price of an upgrade follows from the size, the glass, the frame, the state of the roof and how we reach it. We cover that on the costs page, and what it costs to replace a rooflight breaks down the lines on a replacement quote.

Measuring for the new unit

Every rooflight we fit is made to measure or chosen to fit the opening it goes into. On a pitched roof, the dimensions of the opening between the rafters limit the size, which is why catalogue sizes in a manufacturer's range often suit a replacement better than a custom one. On a flat roof the kerb sets the outside dimensions of a lantern or rooflight, and we measure it for square, level and height. A bespoke lantern is ordered after those measurements are final, so the frame matches the kerb to the millimetre.

Choosing the specification with you

At the survey we go through the glass build-up, the frame material and colour, the solar control choice and whether the unit should open. If the work is a replacement, we note which parts of the old structure can stay. If it is a new opening, we look at the structure, the energy limit that applies and the route through Building Control. Our roof lantern or VELUX window answer helps when you are weighing a flat roof against a pitched one, and roof lanterns and flat roof rooflights have their own pages.

Fitting day

Fitting follows the survey drawings. The old unit comes out, the opening and the kerb or lining are checked and corrected, the new unit goes in with its seals and flashings, and the reveals inside are finished. We handle the Building Control notification where the work is notifiable, and you receive the paperwork.

Where we work

Our office is in Maidstone, and the areas covered stretch about 30 road miles in every direction. They include Bearsted, Aylesford, Coxheath, Kings Hill, Tonbridge and Chatham, as well as the villages between them. Homes in this area range from Victorian terraces to new estates, and each roof is different.

To book, use the quote form for a free survey and a fixed written quote. WhatsApp works too, on 07880 284133, and a few pictures of the opening and the ceiling below help; the email address is info@orchardskylightsmaidstone.co.uk.