Water on the inside of an atrium roof gets reported as a leak more often than anything else, and about half the time it isn’t one. It’s condensation, which is a different fault with a different fix, and telling them apart early saves a pointless search for a hole that was never there.
The useful part is that condensation is diagnostic. Where it forms tells you something specific about the glass, the building or the way the space is being used.
The physics, briefly
Warm air holds more moisture than cold air. Push warm moist air against a cold surface and the air in contact with it cools, hits its dew point, and the moisture comes out as liquid.
In a building, the coldest surface at high level on a winter morning is almost always the glass. So that’s where it lands.
Which means the question is never “why is there condensation” in the abstract. It’s why that surface is cold enough, or that air wet enough, for it to happen there.
Three places it forms, and three different stories
On the room-side face of the inner pane, it’s a building issue rather than a glazing failure. The glass is doing what glass does; the air below it is carrying more moisture than the surface temperature can tolerate. Swimming pools, kitchens, planted atria, anywhere with a lot of people and not much ventilation. Early morning in winter is the classic time, and it usually clears as the space warms.
Persistent condensation in this position still matters, because water running off glass and down framework finds its way into junctions and sits there. Over years that’s corrosion, mould at the perimeter and staining on the ceiling below. The fix is ventilation, heating strategy or thermal performance, not sealant.
Between the panes of a sealed unit, it’s a failed unit. Full stop. A double-glazed unit has a sealed cavity with a desiccant in the spacer bar, and when the edge seal goes, moist air gets in, the desiccant saturates, and you get misting inside the glass that you can’t wipe off. It typically appears first at the bottom edge and spreads. There’s no repair for it. The unit gets replaced.
On the outside face, it’s harmless and often a good sign. Exterior condensation on a cold clear night usually means the low-emissivity coating is doing its job, so heat stays inside instead of warming the outer pane. It clears with the morning sun and tells you nothing bad about the roof.
How to tell condensation from a leak
Four tests, and between them they settle most cases.
Timing is the first. Condensation appears on cold mornings, often without rain, and clears as the day warms. A leak appears during or shortly after rain and doesn’t care about temperature.
Distribution is the second. Condensation forms broadly across cold surfaces and concentrates at the coldest points, typically along glazing bars and at the perimeter where there’s a thermal bridge. A leak runs from a point.
Cleanliness is the third. Condensate is clean water. Water that has come in from outside has usually travelled over dirty surfaces and leaves a mark.
Volume is the fourth. Condensation is a film and drips. A leak in heavy rain is a flow.
Where the two overlap, and they do, the sensible move is a close inspection rather than a guess. Water that has come in from outside tracks along steelwork before it appears, so the position it shows up in is evidence rather than a location.
What the pattern tells you about the glass
Condensation that forms in lines along the glazing bars but not across the middle of the panes points at the framework, not the glass. Thermally broken aluminium systems keep the inner face warm enough to stay dry; older unbroken aluminium is a straight cold bridge from outside to in, and it’ll be the first thing to wet.
Condensation at the corners of panes, concentrated in a few units while the rest stay clear, is worth looking at closely. Spacer bars conduct, so corners run cooler, but a few units behaving differently from their neighbours can mean those units are on their way out.
Condensation on only part of an atrium is often about airflow rather than glass. Dead zones with no circulation hold moist air against the surface. Planting and water features in one area of an interior space raise local humidity more than people expect, and glazed internal screens around them can either help or trap it depending on how they’re arranged, which is where the way glass partition systems are specified and installed starts to matter.
What to do about it
If it’s interstitial misting, the units are failed and need replacing. Nothing else works.
If it’s surface condensation, look at ventilation and at the thermal performance of the glass and frames before anything else. Upgrading to thermally broken framework and better units is a capital job, and in a glazed atrium that’s running warm and wet it tends to pay for itself in heating as well as fixing the symptom. Guidance from the Centre for Window and Cladding Technology covers how glazed envelopes are assessed for this.
Either way, get someone with close access to look before you spend. Misting and surface condensation look identical from a lobby floor forty feet below, and they have nothing in common except the water.