Posted by: Sam Swan | April 13, 2011

Common Problems with Double Glazing

The most common problems within double glazing tend to be sealed units failing, and doors and windows dropping, but why is this?

Firstly, why do sealed units fail?

Sealed units have only been around since the 1970s, so no one is completely sure how long they are supposed to last. It could be ten years, could be more but, probably not forever. If you are somebody who has had glass sealed units fail and/or have condensation formed in-between the two panes of glass then you are not alone.

A unit is generally comprised of two pieces of glass, a spacer bar filled with silica (which, separates the glass and absorbs the moisture during manufacturing), and a rubber seal around the edge which is supposed to trap the air (or sometimes the gas argon is used) inside. It is often implied that the inside is a vacuum which is false because there would be no volume and the glass would implode.

Inside a frame

The main reason for units to fail is generally down to the manufacture or installation, and nothing the home owner has done. However, there are occasions where units can crack on their own accord, often because of differences in temperature between the glass inside and the glass outside the house. This is especially true in humid rooms like bathrooms and kitchens but, these cracks can be easily avoided.

How? Don’t place any large surface area items up against the glass because if heat emerges it can cause cracks.

Why? The air can expand in-between the panes of glass to an extent that combined with the force of hot glass would result in the glass giving in because it won’t be able to take the stress.


These are the five main reasons why glass sealed units tend to fail and because units are not designed to be disassembled, they therefore must be replaced. To attempt to split a unit, clean it and re-seal it again on site would be far more costly in labour terms, as well as almost certainly failing again in the short term.

1.) Incorrect rubber seal

The number one problem for fogged or misted up units is usually down to the hot black rubber seal around the edges which have not been added properly. The rubber seal is like blocks of tar and when the hot melted rubber is gunned down to the sides of the units, it will cool and solidify.

Black Rubber Seal

If any parts however, are missing, added at the wrong temperature or solidified from old stock then chances are there will be long and even, short term failure. A pin hole sized break in the seal only has to develop and it’s enough to draw in unwanted moisture, which is likely to dry out again but, as the problem now exists the moisture will return inside the unit which, over time will cause it to fail.

2.) Incorrect Packing

If glass is unsupported it can easily slip down 10-15mm in relation to the other piece of glass and if this happens it will break the seal causing the unit to mist up. Therefore, it is very important packers are used in order to support both panes of glass, (even if the window appears solid at first) and even more crucial for larger sized windows.

3.) Poor or No Drainage

Units with poor or no drainage may seem like a less obvious cause of failure and is not always a continuous problem, especially in the drier months but, believe it or not but frames are designed to leak. If the unit fails and the drainage is inadequate and the bottom of the glass is sitting in water, then it acts as a source, drawing the water into the unit.

It may seem logical that the outer rubber seal between the glass and frame is water tight but, it’s only meant to keep the glass in the frame and most of the water out which is where drains come into the equation. Drains are usually located either underneath the frames or on the front of the frames which are covered by little plastic covers. The type depends on circumstances but both can become blocked and lead to water setting in the frame.

4.) Movement of Framework:

Movement of the framework can often be a long term cause, usually due to distortion of the frame over time. Plastic in particular will expand and contract by surprising amounts depending on temperature changes.

A window will often not retract to its original position and/or size, and eventually distortion will occur. If part of the frame begins to touch and put pressure on the glass, the seals can break, or in very extreme cases, the glass can crack. If you can hear conservatory roofs creaking that is a sign of movement within the framework.

5.) Corroded Fixings:

Screws can often become rusted over time which will cause them to expand and be pushed into the side of the unit. If large screws or bolts have been used then they tend to become loose over time and be pushed into the side of the unit, which will cause them to fail.

If you have a damaged and/or stuck screw read the post below for more information.

Secondly, why do doors and windows drop?

The main problems that occur from doors and windows dropping is due to poor installation where doors or sashes skim when closed or become permanently jammed shut. Doors and window will over time settle to a degree but, if installation is carried out correctly then the amount should be minimal and not affect the overall performance.

When it comes to installing doors and sashes there are two important golden rules:


Rule 1: The frame must be square and level

Of the three materials used in window construction – plastic, metal and wood – the former is the least forgiving, therefore it is vital to keep the frames square.

If the cill is sloping from one end to the other then everything else will be out and later attempts to get sashes or doors to close properly will be a challenge in itself. One area which can have a slight tolerance of error is in the level of the frame in and out, as viewed side on.

Remember, the sides must be level in relation to the cill (when looking towards or away from the window) meaning all angles must be 90 degrees, to make a perfect rectangle. However, the top of the frame can be in or out slightly in relation to the bottom, side on; although this is on the proviso that both sides are out of square the same, so there is no twist.

Rule 2: Correct packing must be used

If the frame is fitted as described, then doors and windows can only drop due to incorrect packing, which is a relatively simple process. This issue is generally more important with plastic doors as there is nothing to keep the door square until the glass or panel have been installed.

Packing involves placing special thin plastic strips between the frame and the glass in FOUR crucial locations, on the bottom corner of the hinge side and the top corner of the handle side. The packing should keep the glass as even as possible within the opening and therefore it will follow that the door or window sash will also be square and have equal clearance all around in the frame. It’s that simple, but without it, you will experience problems sooner or later.

© Double Glazing Doctor

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