Condensation on a roof lantern: causes and solutions

Condensation on a roof lantern forms when moist air cools against a cold surface. Where it appears tells you the most. On the inside it usually points to too much moisture in the house or too little ventilation. On the outside it is a sign of good insulation. Between the panes it means the sealed unit has failed and needs replacing.

What is condensation on a roof lantern?

Condensation is water vapour from the air that settles as droplets on a surface colder than the dew point. The dew point depends on temperature and humidity. At 20 °C and a relative humidity of 60 percent it is around 12 °C: any surface colder than that gets wet. At 50 percent it is around 9 °C.

A roof lantern is often the coldest surface in the room, and it also sits at the top, where warm, moist air rises. That is why you see condensation there sooner than on a window in the wall. How warm the inner pane stays is related to the U-value, see the U-value of glass explained.

Condensation on the inside of a roof lantern

Condensation on the inside is almost always caused by too much moisture in the air. A household produces litres of water vapour a day through cooking, showering, breathing and drying laundry. If it cannot escape, it seeks out the coldest surface.

A new extension also contains a lot of construction moisture. A cement screed and plasterwork release moisture for months. Condensation in the first winter is therefore normal and decreases once the floor and walls have dried out. Ventilate extra during that period.

Condensation only along the edge, at the upstand, often points to a cold bridge: a spot where the insulation is interrupted. The frame is thermally broken, but the upstand below it must be well insulated too, see the upstand of a roof lantern.

• Keep humidity roughly between 40 and 60 percent
• Keep trickle vents open, in winter too
• Use the cooker hood when cooking and air the room after a shower
• Do not dry laundry indoors in the room under the lantern

Condensation on the outside

Condensation on the outside of a roof lantern forms on clear, cold mornings, especially in spring and autumn. Well-insulating glass lets so little heat escape that the outer pane cools below the dew point of the outside air. It is a sign of good insulation, not a defect. With triple glazing you see it more often than with HR++. It clears by itself as soon as the sun reaches it.

Condensation between the panes

Condensation between the panes means the edge seal of the insulated glass is no longer airtight. Moist air gets into the cavity and the gas leaks out, so the glass also insulates less well. Cleaning will not fix this; the sealed unit must be replaced.

With a roof lantern this can be done per glass panel. Note the dimensions and the glass type, and check whether there is a code on the spacer between the panes. That allows a replacement unit to be made to measure.

Condensation or a leak?

Droplets under a roof lantern are often mistaken for a leak. A simple test: condensation appears spread across the glass, mainly on cold mornings and after cooking or showering, even when it is dry outside. A leak comes back after rain in the same spot, often along an edge or in a corner. If you suspect a leak, read what to do if your roof lantern leaks.

Frequently asked questions