Blown Sealed Units: What Failed and What Happens Next

Nothing has actually exploded

The word blown is unhelpful because it suggests a bang. What has happened is quieter and slower. A sealed unit is two or three panes held apart by a spacer bar and bonded around the whole perimeter with a sealant designed to keep the cavity airtight for many years. Inside the spacer sits a desiccant whose job is to absorb the small amount of moisture present when the unit is made.

When that perimeter seal degrades, the cavity starts exchanging air with the outside. The gas fill escapes, damp air enters, the desiccant eventually saturates and moisture starts appearing on the inner faces of the glass. That is a blown unit: an airtight assembly that has lost its airtightness.

Why seals fail in the first place

Sealed units live a hard life. They expand and contract every single day as the sun hits them and the temperature drops again at night, and the seal has to accommodate that movement for decades. Given long enough, every unit fails, so age alone is a common answer.

Beyond age, the pattern we see on site is fairly consistent. Units sitting in standing water because the frame drainage is blocked fail early, because the sealant is constantly wet. South facing elevations with heavy solar gain work the seal harder. Poorly packed units carrying the weight of the glass on the sealant rather than on packers fail early too. On exposed coastal frontages the combination of driven rain and salt does not help either.

• Age and normal daily thermal cycling
• Blocked drainage holes leaving water standing on the seal
• Heavy solar gain on south and west elevations
• Poor packing so the glass weight bears on the sealant
• Physical distortion of the frame putting the unit under strain

What it costs you beyond the appearance

The visible haze is the annoying part, but it is not the only consequence. Most modern units are filled with argon, which conducts heat less readily than air, and that fill is a real part of the insulating performance the window was rated on. Once the seal has gone the argon has gone, and the unit no longer performs as it did when it was fitted.

You may also notice the affected pane feeling colder, more surface condensation forming around it, and in bad cases water pooling along the bottom edge inside the cavity. That standing water can stain the glass permanently and sits against the glazing rebate, which is not good for the frame either.

How the replacement actually works

The good news is that the failure is confined to the glass. The usual route is to measure the existing unit carefully, including its thickness and glass build up, order a new unit made to match, then remove the beading, take out the failed unit and fit the new one with fresh packers and gaskets. The frame, hinges, handle and locks are untouched.

It is worth using the opportunity to check the specification rather than simply copying it. Depending on the window, you may be able to move to a better performing glass build up, a warm edge spacer instead of an aluminium one, toughened or laminated glass where the location calls for it, or obscure glass in a bathroom. The frame limits what will physically fit, and a surveyor should tell you what those limits are.

• Accurate sizes and unit thickness taken from the existing glazing
• Glass build up matched or upgraded within what the frame allows
• Warm edge spacer considered to reduce cold bridging at the perimeter
• Safety glass where required by the location of the pane
• Drainage in the frame checked and cleared before the new unit goes in

Several blown units at once

One failed unit is a repair. Several across the same elevation, all of similar age, is a decision. At that point it is fair to compare the cost of doing the glass now, and probably more of it in a couple of years, against replacing the windows and resetting everything including the frames, gaskets, hardware, ventilation and energy rating.

Neither answer is automatically right. It depends on the condition of the frames and how long you plan to stay. What we will not do is quote a whole house of windows when you have one cloudy pane in a sound frame.

Guarantees and paperwork

Sealed units carry a guarantee period of their own, distinct from the frame guarantee, and the term varies between manufacturers and installers. If your windows are relatively recent it is well worth digging out the original paperwork and going back to the company that fitted them before paying for anything.

Where that is not possible, it becomes an ordinary repair. We are CERTASS accredited and will provide the appropriate paperwork for work carried out under that scheme.

Having it assessed on the Wirral and in Chester

We survey free of charge across the Wirral, Chester and about thirty miles from base. Bring us the pane you are worried about and we will tell you whether it is a failed unit, surface condensation or something to do with the frame, then price the smallest sensible fix.

Questions we get asked on site

What does a blown sealed unit mean exactly?

It means the airtight seal around the edge of the double glazed unit has failed, letting the gas fill escape and damp air in, which is why moisture appears between the panes.

How long should a sealed unit last?

Every unit fails eventually, and lifespan varies considerably with exposure, drainage and how the unit was packed and fitted, so age is only part of the picture.

Does a blown unit make my heating worse?

Yes, to a degree. The argon fill is lost and the unit no longer performs as rated, so that pane feels colder and tends to attract more surface condensation.

Can the new unit have better glass than the old one?

Often it can, within the thickness the existing frame will accept, so it is worth asking about warm edge spacers and improved glass build ups rather than simply matching the original.

Do I need to replace matching windows at the same time for appearance?

Not usually. A correctly specified replacement unit in the same frame should be visually consistent with the panes around it.