The answer
Rooflight condensation forms when warm, damp room air touches glass or frame colder than the air can stay damp against. In older Leeds terraces common causes are a cold upstand or reveal, tired glazing and too little ventilation. Mist between the panes is another matter altogether: it points to a failed sealed unit, and then the glass needs replacing.
Why the moisture ends up on the glass
Air in a house carries water vapour from cooking, bathing, drying washing and breathing. Warm air holds more of it than cold air, so when that air meets a cold surface it cools, can no longer hold all its moisture, and leaves droplets behind. Of every surface in the room, the rooflight is frequently the coldest, because it faces the open sky and has nothing round it to hold heat in. The pane, the frame and the edge of the opening are the places where the droplets appear.
Where the droplets sit tells you something. A film across the whole of the inner pane points to damp air and a cold pane. A ring of beads or a dark line round the edge points to a cold frame or a cold edge to the opening. Water running down the reveal into the plaster points to both, and left alone it can stain the paint and encourage mould on the lining.
What older Leeds terraces add to it
A red-brick through-terrace or a gritstone cottage in the Aire valley may carry a rooflight that went in years ago. The glass in such a unit may be a single sheet or an early sealed unit with a metal spacer, and the upstand may be bare timber with little or no insulation behind it. Each of those gives cold surfaces for moisture to settle on.
The reveal is the light well running from the ceiling up to the glass. If its lining was fitted without insulation behind it, the lining takes the temperature of the roof space above, and the water collects there instead of on the pane. A flat rooflight on a kerb has the same weakness where the kerb is thin: the upstand becomes a cold ring round the frame.
Newer units are made to avoid this. Four details help the room-side surface stay warm:
- a low-emissivity coating on the inner glass
- a warm-edge spacer in place of a metal bar
- a frame with a thermal break
- insulation in the kerb or behind the reveal lining, tied into the roof insulation either side
A replacement unit installed with these details keeps the room-side surface much warmer than an old one, and the lining no longer sits cold. Our answer on whether skylights are energy efficient goes through what each of them does for heat loss.
Ventilation that keeps the moisture down
A warmer pane cuts condensation, and drier air cuts it again. Steamy rooms need a way out for the steam. An extractor fan in the bathroom and kitchen carries it outdoors, and a lid on the pan helps in the kitchen. Drying washing on a rack in a closed room with a rooflight is a common cause, and moving the rack to a room with an open window removes it.
A rooflight that opens, or a roof window with a ventilation flap, gives damp air an exit at the top of the room. Electric versions can include a rain sensor that shuts the glass when a shower starts. A standard sun tunnel over an internal bathroom delivers daylight with nothing to open, so the fan still handles the steam.
One habit works against you. A blind pulled across the glass at night stops room warmth reaching the pane, which makes the pane colder and the condensation heavier. Raising it in the morning, or leaving a gap, lets the glass warm up.
In a room that stays damp, a portable dehumidifier lowers the moisture in the air while a lasting remedy is arranged. It treats the symptom, so the cold surface behind the problem still needs attention, whether that is an old unit, a bare kerb or a reveal without insulation behind it.
When condensation points to a failed unit
The position of the moisture sorts the cases.
- On the room side of the inner pane. This is surface condensation from damp air, and it clears when the glass warms or the air dries.
- On the outside of the outer pane. Very efficient glass can leave the outer face cold enough to gather dew on a clear, cold night. It is a sign that heat is staying in the room, and it clears with the morning sun.
- Between the two panes. Moist air has found its way past the rim of the sealed unit, which has stopped doing its job. No wiping from inside will shift it, and the glass will not recover.
Mist between the panes is the case for replacement. We fit either a new sealed unit or a whole new rooflight, and the guide to why the glass mists covers the cause and the choices. If you are unsure which of the three you have, message us on WhatsApp at 07880 284133 and say where the moisture sits. A house in Leeds, or anywhere within 30 road miles of it, can have the survey.