Misted sealed units and why they go

How it fails

The unit breathes. Air inside expands when the sun heats it and contracts when it cools, and that pressure cycle works the seal twice a day for twenty five years.

Eventually a path opens. Moist air enters, the desiccant absorbs it until it saturates, and after that any cold morning condenses moisture on the inside faces of the glass where nobody can reach it.

Why overhead fails first

More thermal cycling. A rooflight goes from cold at dawn to hot by midday, every day. The temperature swing overhead is larger than on a wall.

Standing water. Rain sits on a rooflight rather than running off. The perimeter seal is wet far more of the time than a vertical unit’s, and the edge seal is what is being asked to keep water out.

UV. Overhead glazing takes more direct ultraviolet, which degrades the sealant.

Twenty to twenty five years is a fair life. In wall glazing you might get thirty five.

Why drying it does not work

Some companies drill the unit, dry the cavity and fit small vents. The visible misting clears.

What has not happened: the seal is not restored, the argon has gone and is not coming back, and the unit now deliberately breathes outside air, so it will fog again in the right conditions and the desiccant is already spent.

On an overhead unit at height, the labour to do that is most of the labour to replace it properly. We do not offer it.

What shortens the life

Standing water against the frame from poor drainage, coastal exposure, and any installation that leaves the unit sitting in water rather than shedding it.

What a sealed unit is trying to do

Two panes of glass, held apart by a spacer bar around the perimeter, with the cavity between them filled with argon and one of the internal glass surfaces carrying a low emissivity coating. Around the outside, a perimeter seal keeps the gas in and the weather out. Inside the hollow spacer bar sits a desiccant, which absorbs the small amount of moisture inevitably sealed in at manufacture.

That assembly has to stay airtight for decades, through every temperature the British climate offers, while the gas inside it expands and contracts every day.

Why a rooflight seal works harder than a window seal

Three reasons, and together they explain why rooflight units do not last as long as the same glazing in a wall.

It lies flat or nearly flat. A vertical window sheds water. A rooflight, particularly a flat roof unit, holds it. A perimeter seal designed to shed water is not designed to sit under standing water after every shower, and on a roof where the deck does not fall away from the kerb it does exactly that.

It faces the sky. On a clear night a rooflight radiates toward the coldest thing available and its surface temperature drops well below the air temperature. On a sunny afternoon it is the hottest surface on the building. The daily swing is greater than anything a wall window experiences.

The gas pumps. Every one of those temperature swings expands and contracts the argon in the cavity, and the seal has to accommodate that movement. Thousands of cycles a decade, working the seal a fraction each time.

That daily pumping is what eventually works a seal loose. It is fatigue rather than failure.

Why misting appears suddenly, years after the seal went

This is the part that puzzles people, and the desiccant explains it.

Once the seal begins to admit air, moisture enters the cavity. The desiccant absorbs it, and it goes on absorbing it, silently, for as long as it has capacity. During that period nothing is visible: the unit looks exactly as it always did.

When the desiccant saturates, there is nowhere for further moisture to go. The next cold morning, water condenses inside the cavity and the unit mists.

So a window can go from apparently fine to visibly misted in a single winter, and the seal will have failed some years earlier. It is not sudden; it is the visible end of a slow process.

What has already been lost by then

The argon. Gone, and it goes long before the misting appears. That means the unit has been performing like plain double glazing rather than to its specification for some time, quietly, without anybody noticing the room getting colder.

The coating. Once the cavity is wet, the low emissivity coating starts to degrade. That is what produces the permanent bloom or haze that does not clear even on a warm dry day, as distinct from the condensation that comes and goes with the weather.

A unit that mists on cold mornings and clears by lunchtime has a failed seal and an intact coating. One that is permanently cloudy has both.

How long is normal

Fifteen to twenty five years for a rooflight sealed unit.

Toward the shorter end on a coastal or exposed roof, where the seal takes more wind driven water, more thermal cycling and salt.

Failure earlier than about twelve years usually points at something specific rather than at ordinary ageing.

What causes early failure

Standing water. The commonest cause on a flat roof. A kerb that is too low, or a deck that does not fall away from it, or a blocked outlet, leaves the unit sitting in a puddle. That is a design or installation fault rather than a product one.

A rigid installation. A unit clamped hard into an opening with no allowance for thermal or structural movement is under stress it was not designed for. On a building that moves seasonally, which includes any timber framed structure, that is a real consideration.

Forcing a unit into an opening it does not fit. Packing the difference where an obsolete size has been replaced with a modern one. The packing settles, the frame goes out of square, and the sealed unit ends up carrying loads it should not.

A manufacturing batch problem. It happens, it is uncommon, and it is what a manufacturer’s warranty is for.

Is it urgent

No. A misted unit is not a safety issue. Nothing is going to fall out of the roof, no water is coming into the house, and there is no structural implication.

It is a performance and appearance problem, it does not improve on its own, and you can live with it indefinitely if you choose to.

What is urgent is soft timber. If you can press the frame at the sill or the lower corners and it gives, that is decay, and decay is caused by water getting in and does not stop until the cause is corrected.

Glass or whole window

On some pitched roof windows the sealed unit is held by a removable glazing bead and can be changed with the frame in place. On others it is bonded into the sash and the sash is replaced. On most flat roof rooflights the glass and frame are a single bonded unit and the answer is a whole unit.

Whether it is worth doing is the separate question, and the arithmetic is simple: most of the cost of the job is getting to the window rather than the component.

Frame under fifteen years and sound: replace the glass. Over twenty: replace the window, because new glazing in a worn frame with hardened seals and thermal performance two generations behind buys you the shorter of the two lifespans. Any timber decay: replace the window, always.

What not to do

Drilling and drying. Some companies drill a misted unit, dry the cavity and fit small vents. The visible misting clears.

The seal is not restored. The argon is gone and is not coming back. The unit now deliberately breathes outside air, with a desiccant that is already spent, and the coating continues to degrade.

On an overhead unit at height, the labour to do that is most of the labour to replace it properly. We do not offer it.

The upgrade that comes with replacement

A unit fitted twenty five years ago has no low emissivity coating, no argon, an aluminium spacer and no solar control. The replacement has all four as standard.

The room will be warmer in winter, cooler in summer and quieter in rain, and none of that is an optional extra. The band of edge condensation an aluminium spacer produces on cold mornings disappears entirely, because a warm edge spacer runs several degrees warmer at the glass edge.

And where the window faces south or west, adding solar control glass at this point costs a modest premium because the labour and access are already being paid for.

How a sealed unit is meant to work

Two or three panes held apart by a spacer bar, with desiccant inside the spacer to absorb residual moisture, and a two stage perimeter seal: a primary butyl seal that resists vapour, and a secondary structural sealant that holds the assembly together.

The cavity is filled with dry air or an inert gas, and the whole thing is supposed to stay closed for its service life.

The mechanism of failure

The unit breathes. Every day it warms in the sun and cools at night, and the gas inside expands and contracts. The seals flex with it.

Over years that flexing works the seal, and eventually the primary seal loses its integrity somewhere along the perimeter.

From then on, air moves in and out with the daily cycle, carrying moisture with it. The desiccant absorbs that moisture until it saturates. Once it does, the water condenses inside the cavity where nothing can reach it, and that is the misting you see.

Why rooflights fail sooner than windows

Three reasons, and they compound.

Greater temperature swings. A rooflight faces the sky. It gets more direct sun in the day and radiates to a cold clear sky at night, so the daily cycle it experiences is more severe than a vertical window.

Water sitting at the bottom edge. Where the sill drainage channel blocks, water stands against the lower edge of the unit permanently, attacking the seal from outside. This is the largest avoidable factor.

Ultraviolet on the edge seal. Overhead glazing takes more of it.

What it costs you to leave it

Thermal performance, immediately, because the gas fill has gone and the cavity now contains humid air.

And light, permanently. As moisture repeatedly condenses and evaporates it leaves mineral deposits that etch and stain the inner faces. That staining does not come off, so the longer it is left the worse the glass looks in the meantime.

It is not urgent and it is not a leak. The unit still keeps rain out.

The fix that is not a fix

Drilling and venting the unit. It appears online periodically as a fix. It converts a sealed unit into an unsealed one, ends the thermal performance permanently, and does not stop the staining.

Extending the life of the next one

Clear the sill drainage channel once a year. It is five minutes and it addresses the single biggest avoidable cause.