Why rooflights mist up between the panes

Look along the edge of a clear unit in strong light and you can usually see the spacer, a thin bar running round the rim between the two panes. It holds them a fixed distance apart. Sealant bonds the glass to the bar on both faces, and a second layer often sits outside it, around the very edge. The gap holds dry air or a gas such as argon, which slows the heat passing through.

Spacer bars are often hollow and carry a drying agent, the desiccant, that soaks up the small amount of moisture sealed in at manufacture. The desiccant is a safety margin with a limited capacity. While the seal holds, that capacity is never tested. Once vapour begins to creep past, the desiccant takes it up for a time, which is why a failing unit can look clear for months before the first haze shows.

A roof window in a south slope can be too hot to touch by midday and frosted over a few hours after dark. Each swing makes the glass, the spacer and the sealant expand and contract at different rates, and the bond at the edge has to absorb the difference. Ultraviolet light adds to the strain, because the outer edge of the sealant is the part nearest the sky.

Pressure plays a part as well. The gas in the cavity warms and cools with the weather and the sealed pane flexes a little in response, a movement sometimes called breathing. Over the life of a unit that adds up to a great many small cycles, and a seal that is almost sound will be the first to give.

Water that stays put

A tilted pane sheds rain towards its lowest edge, and what happens next depends on the frame. Where the rebate drains freely, the water leaves. Where the drain paths are blocked by dirt, paint or a badly seated bead, water sits against the seal after every shower and works into the sealant over the years. Clearing a blocked drain path on the old frame does not reseal a cavity that has already let moisture in. A flat rooflight on a low kerb collects leaf litter and moss along its lower edge, and a damp mat of it can hold water on the seal for days. That edge is one of the first places we look from the roof.

Knocks and movement

A ladder leant on the frame, a branch in a gale, hail, or a frame that has twisted as the roof timbers moved can crack the bond. The mist then starts at one corner and spreads from there across the pane.

The first haze rarely arrives for good. On a cold morning the glass nearest the sky is the coldest surface in the cavity, so any vapour that has got in condenses there as a thin film. By lunchtime the pane has warmed, the film evaporates back into the gap, and the unit looks clear again. The pattern can repeat for weeks, or for a winter or two.

Every cycle draws in a little more moist air through the damaged seal. The desiccant fills, the dew point inside the cavity rises, and the clear spells shorten until the glass stays cloudy all day. Droplets then run to the bottom of the pane, and when they dry in summer they leave a mineral stain, the tide-line, where the dissolved dirt in the water has been left behind.

This slow build is why the problem is noticed late. A landing or an en suite that depends on one rooflight for its daylight shows it sooner, since the room is looked at in every weather. A spare bedroom with a second window may go months before anyone glances up.

Seals wear out, and a unit of any age can reach that point. Still, the pattern on a roof gives clues to the cause, and the cause shapes the repair. When two matching windows sit side by side in the same slope and only one has misted, the difference is usually in what that one has to put up with: a blocked drain path, a patch of moss, a slightly different angle to the sun, or a knock from a ladder.

What we look at on the roof

From above we see where on the pane the mist begins, whether the lower rebate drains, whether the covering has built up against the frame, and whether the frame has moved. On a flat roof we check that the kerb is firm, that the covering is bonded to it and that nothing holds water against the glass. That tells us whether a new sealed unit will be safe in the old frame or whether the frame is part of the cause.

If the unit is fairly new

Look out the invoice and any paperwork from the original supplier. A failure soon after fitting can point to how the unit was made, stored or seated, and the original supplier's terms are a separate matter from ours. Send us photographs as well, since they help us judge what sort of failure it is.

A garage conversion that borrows all its light from one flat rooflight shows the loss at once. The haze scatters daylight, the sky turns to a milky patch, and the lamps go on in the middle of the day. In a room with a wall window as well, the effect is gentler, though the part of the floor under the glass is dimmer than it should be.

Heat goes the same way. A dry gas fill and a low-emissivity coating are doing work in a healthy unit, and once the cavity is wet, the pane loses more heat on a winter night and the glass feels colder to the touch. Our page on energy-efficient rooflights covers what the glass is meant to achieve.

Condensation on the room side is a separate matter

A film of water on the inside face that wipes away points to humidity and ventilation. It forms in bathrooms, utilities and kitchens. A new sealed unit will not cure it, though a warmer pane often forms less of it. Stains on the lining or timber point to water entering from outside, which is the subject of our answer on whether a misted skylight is leaking.

Leave the edges of the glass alone. Sealant brushed on from outside cannot reach moisture that is already inside the cavity, and it can hide the very marks we use to read the fault. Misted glass cannot be cleared for good, and our answer on whether misted rooflight glass can be cleared gives the reasons.

The cure is a new sealed unit in a sound frame, or a whole new rooflight where the frame, flashing or kerb has aged along with the glass. Our guide to glass only or whole unit sets out how that choice is made, and how long rooflights last explains which parts wear out first. The replacement meets the current energy ceiling for its type, which the guide to Part L and rooflights explains.

We replace misted units across Maidstone and mid-Kent. A survey is free and the quote you receive is fixed and written down, with a 10-year workmanship guarantee on what we fit. When you are ready, use the quote form, or send photographs by WhatsApp on 07880 284133 or to info@orchardskylightsmaidstone.co.uk.