Triple glazing or HR++ in a roof lantern? The figures

Triple glazing or HR++? For most living rooms and open-plan kitchens with a south-facing roof lantern, HR++ double glazing is more than enough. Triple glazing pays off mainly above bedrooms, on a lantern facing north or west, and when you want to make the house as efficient as possible. Triple insulates more than twice as well, but does not always earn back its extra cost within ten years. Below are the figures, so you can weigh it up yourself.

What is the difference between HR++ and triple glazing?

HR++ glazing is insulated double glazing with an invisible metal coating and a noble gas filling in the cavity. Triple glazing, also called HR+++ glazing, has three panes and two cavities. That extra layer slows heat loss considerably. In a DagLichtDesign roof lantern HR++ is standard and triple is an option.

You can see the difference in the U-value: the amount of heat in watts lost per square metre of glass for each degree of temperature difference between inside and outside. The lower, the better. You can read more about this figure in the U-value of glass explained.

• HR++: U-value around 1.1 W/m²K, sound reduction about 32 dB
• Triple: U-value around 0.5 W/m²K, sound reduction about 36 dB
• Both available clear, frosted, solar control or acoustic
• Every quote shows HR++ and triple side by side

What does triple glazing save in gas and money?

On a 4 m² roof lantern, triple glazing saves 65 to 90 m³ of gas in an average winter season, according to our own sums. At a gas price of roughly €1.40 per m³ that is roughly €90 to €125 a year. Check this against your own tariff, as it differs per contract.

Set the extra cost of triple against that. It depends on size and glass type and is shown in your quote. If the extra cost is more than about ten times the annual saving, you will not earn it back on gas alone within ten years. Comfort and sound then become part of the decision. You can read more about the specification under triple glazing in a roof lantern.

A nuance that is often missing: U-values of glass are measured with the pane upright. In a sloping or horizontal roof the gas in the cavity moves more, so the real value comes out somewhat higher for both glass types. The ratio between HR++ and triple stays roughly the same.

Triple glazing or HR++: when do you choose triple?

Triple glazing is the right choice when comfort and quiet matter more than payback time. A cold glass surface above your head feels like a draught, even when nothing is draughty: your body radiates heat towards the cold surface. With triple, the inner pane stays warmer in winter, so that effect is smaller.

• Above a bedroom, for quiet and a warmer glass surface
• On a lantern facing north or west, with little solar gain
• When you are taking the house towards energy label A or better
• With a lot of traffic or aircraft noise, triple cuts about 4 dB more

When is HR++ glazing enough?

For most living rooms and open-plan kitchens HR++ glazing is more than enough, especially when the roof lantern faces south. There the sun provides free heat in spring and autumn, and HR++ lets in a little more of it than triple. That usually outweighs the slightly higher U-value. You can read more about this standard specification under HR++ double glazing in a roof lantern.

Facing south, the opposite problem is more likely: too much heat in summer. A solar control coating is then often a better use of money than a third pane. See solar control glass for a roof lantern.

Weight, light and structure

Triple glazing has an extra pane, which makes it heavier and thicker than HR++. For a roof lantern that means a heavier unit on the upstand and more work when lifting. The frame is made in our factory for the chosen glass unit, but do have your builder check whether the upstand and the roof structure can carry the extra weight.

A third pane also lets through slightly less daylight. The difference is small, but in a dark room you may just notice it. Keep an eye on the schedule too: a mono-pitch roof lantern can be delivered from 2 weeks after approval, but specific glass choices can extend the lead time to 8 weeks.

Frequently asked questions