Can a Rooflight Improve an EPC Rating?

What an EPC actually measures

An Energy Performance Certificate rates the whole dwelling, from walls and loft insulation to the boiler and the lighting. The assessor records what they can see and what the house's age suggests, and software turns that into a band and a score. A single rooflight is one line among many in that calculation.

So a rooflight can move the result, but it rarely changes the band on its own.

When a replacement can help

The best case is an old unit that loses heat badly. Think of a single-glazed rooflight, a plastic dome that has yellowed, or a sealed unit that has failed and fogged. Replacing it with a modern frame and a double or triple glazed unit with low-E coating and a warm-edge spacer cuts the heat that leaves through that opening. Our page on rooflight U-values explains the figure that describes this.

The bigger the opening and the worse the old glazing, the more there is to gain. A long run of old glass over a kitchen extension offers more than a small dome over a landing.

When it will not help much

If the existing rooflight is already reasonably modern, a swap changes little. And if the room is cold mostly through an uninsulated flat roof, the rooflight is not the weak point. Loft or roof insulation usually does more for a score than glazing in a single opening.

Adding a rooflight where there was solid, insulated roof works the other way. Glass always loses more heat than the insulated roof it replaces, so a new opening can slightly lower the thermal performance of the roof. You are buying daylight, not an energy saving, and that is a perfectly good reason to fit one.

What the assessor needs to see

An EPC does not update itself. Once the work is finished, a new certificate has to be issued by a qualified domestic energy assessor, who will ask what went in. Keep the paperwork together. At handover we give you the product details and the makers' warranties, plus the Building Control completion paperwork where the job was notifiable, which makes the assessor's visit simpler.

• The product name and the declared U-value of the unit.
• The completion certificate for the replacement, if there is one.
• Dated photographs of the hidden work, which we hand over with the rest.

Does the glass type change the answer?

Yes, within limits. Triple glazing loses less heat than double, and a solar control coating changes how much warmth comes in from the sun rather than how much leaves. For a score based on winter heat loss, the insulating qualities matter most. Our guide to double or triple glazed rooflights sets out the trade-offs.

Think about the whole house first

If a better band is the goal, a sensible order helps. Look at the biggest losses first: loft or roof insulation, draughty doors, an ageing boiler. Glazing in one opening comes after those. An assessor's recommendation report, which comes with every certificate, lists the improvements they think are worth doing and roughly in what order. Treat a rooflight swap as a comfort and daylight upgrade that may add a small point or two, and you will not be disappointed.

There is also the question of which rooms matter. A cold, glassy kitchen extension is where people feel the difference most, and that is often where the old rooflight sits.

Other reasons to upgrade

An EPC is a useful number, but it is not the only measure of a warm house. A better rooflight is also less likely to mist up inside, and it makes the room beneath it more comfortable to sit in on a cold morning. You may notice that long before any certificate changes. If the old unit weeps, our note on condensation on rooflights helps you work out why.

Rules for a replacement

A whole new rooflight, frame and glass together, counts as a controlled fitting, so the council or a registered building control approver has to be told. We arrange that. The glass alone does not need telling. The new product has to meet today's limits: 2.2 W/m²K for a rooflight, while a roof window is held to 1.4 W/m²K or Window Energy Rating Band B. A like-for-like swap on a house rarely needs planning permission, and the exceptions, such as flats and listed homes, are covered in the planning guide for skylights.

Selling or renting

An EPC counts mainly at a sale or a new letting, because the certificate is shown to buyers and tenants. If you are planning either, ask whether a new certificate would change anything you care about before you spend money on glazing. A buyer is often more impressed by a dry, bright, warm room than by a point on a score. Even so, a fresh rooflight that comes with its paperwork, photographs and warranties reads well, and it tells the next owner the work was done properly.

An existing certificate stays valid for its full term, so you are not obliged to replace it, but an assessor can issue a new one once the improvement is complete.

For the full picture of what we fit, see our energy efficient rooflights page.