Conservatory U-values explained

A conservatory U-value is a number that says how fast heat passes through a part of the building. The lower it is, the better the part insulates. It is measured in watts per square metre per degree of temperature difference (W/m²K), and it applies to glass, frames, roofs and walls alike. A conservatory with a low U-value roof and glazing stays warmer in winter and cooler in summer.

What a conservatory U-value tells you

A U-value covers a whole part, not just one layer. A window’s U-value includes the glass, the spacer and the frame. A roof’s includes the glazing or tiles, the insulation and the bars that hold it up. However, suppliers sometimes quote a figure for the centre of the glass alone, which looks better than the figure for the whole window. When you compare products, ask which one they quote.

Window Energy Rating, or WER, is a related measure. It grades a window as a band, taking in the heat it loses and the sunlight it lets in. For replacement windows in England, the Approved Document L limit is a U-value of 1.4 W/m²K or a rating of band B or better.

Does a conservatory U-value have to meet the limit?

Not always. A conservatory that is exempt from Building Regulations isn’t held to those figures. Instead, it must be separated from the house by external-quality walls, doors or windows and have its own heating controls, which keeps its poor performance away from your energy bills. A conservatory that is not exempt, such as one with a solid roof, must meet the standards for new work. Our guide to planning and building a conservatory explains the exemption.

A replacement window or door in the house itself must meet the limit, and that includes the doors between the house and the conservatory if you replace them. A FENSA or CERTASS registered installer certifies that. See our guide to a FENSA certificate.

How conservatory roofs compare on U-value

Because the roof is the largest area of glass or panel, its U-value matters most.

• Polycarbonate performs worst of the three, because the sheets are thin.
• Glass with a low-emissivity coating and a warm edge spacer does better, and it adds solar control if you need it.
• A solid roof with insulation performs closest to the roof of the house, which is why it changes how the room feels. See solid and tiled conservatory roofs.

Price follows performance: a replacement polycarbonate roof typically costs £4,100, a glass roof £6,000 and a solid roof £10,000.

Questions to ask before you order

1. What is the U-value of the whole roof system and the whole window, not just the glass?
2. Is the figure measured and certified, or an estimate?
3. What is the glass specification: coatings, spacer, gas fill and number of panes?
4. Does the roof need a stronger frame, and who checks it?
5. Which certificate will I receive, and who applies for any building control approval?

Then a good installer will answer these in writing. If a quote only says “energy efficient glass”, ask them to say what that means.

What to do next

Read our guide to an energy efficient conservatory for how roof, glass, frame and doors work together. When you are ready, compare quotes and use our quote checker to line up the specifications. Our glass conservatory roof page shows typical prices for the glass option.