How blown windows effect efficiency

How blown windows effect efficiency

What is a Blown Window?

A blown window is most often seen as condensation between the glazing panels.  Usually a gap allows the moisture to enter, thus leading to misty windows, which initially look like they are steamed up.  The problem is that this steaming up is between the panes rather than just on the outside, making the windows look misty.

Causes of a blown window are:

Ageing windows that have deteriorated over time.

Damaged window seals due to wear and tear or accidental damage.

Badly fitted windows that were poorly installed in the first place.

Whatever has caused the window panel to become blown and in turn causing the misty window, you need to get the window repaired as soon as possible.  Failed panels are no longer energy efficient, which is probably one the main reasons behind your double glazing purchase and steamed up windows are not very pretty to look at, especially if you are trying to sell your house.

A typical house can lose 10%, or more, of its heat through the windows so having energy efficiency in this department is quite sound advice.  A blown window pane will reduce the efficiency of the pane so the heat loss will be greater, thus costing more to heat the room.

A study conducted by the IEA (International Energy Agency) found that buildings use 28% of all global energy – and windows use half of that. Heading down to an individual scale, old leaky windows could be responsible for most of your home heating costs.  The less efficient your window, i.e. blown window units, the higher your energy costs will be.

Old father time does not help when it comes to windows.  You may be paying to heat the air which is trapped between the panes instead of your home.  The remaining heat loss being due to possible leaks in the frame and seal of the window itself.


Condensation is formed when the warm moisture in the air has nowhere to go so it looks for a cool surface where it can turn into the liquid on the surface so we can see it.  The nicest option for condensation is on the side of the glass of cool beer or wine, the opposite of this is the moisture running down the inside of the misty window looking for somewhere to go.

And the challenge is?

Building inspectors and experts alike are likely to wax lyrical about the assorted molds and mildews that can form as problems arising from condensation.  As the sun comes up it can dry out the steamed up panes so the problem goes away, temporarily.  But what is happening is that the moisture has now been evaporated by this heat and is back in the atmosphere of the room raising the humidity levels.

But then dampness is caused by condensation which can affect the wall around windows particularly if it is affected by mold and mildew presenting the appearance of little black dots around the windows and in the corners of rooms.

The Housing Health and Safety Rating System ( HHSRS ) which is something that all landlords must abide by puts emphasis on the fact that landlords must ensure mold does not affect a tenant’s physical and mental health.  Mold is a nasty fungus which is known to cause breathing difficulties.  Tenants who suffer from asthma or rhinitis conditions, or are taking any cancer treatment, may suffer serious health problems if exposed to it.

Window Technology

There are a few different factors which you should consider when choosing your replacement windows.

The U-Factor – this is the rate of heat loss for an entire window.  Lower U-factors mean more efficient windows with less heat loss. But this has proven to be something of a blunt instrument.

G-Value – this measures the degree to which glazing blocks heat from sunlight and is expressed as a number between 0 and 1. The lower the better.  In most domestic situations the specification of glazing concentrates on admitting solar energy whilst preventing energy from being re-radiated from inside.

This crudeness of measurement has led, in the last few years, to a system developed by the British Fenestration Rating Council ( that takes into account the multiplicity of factors determining a windows performance and rates on the basis of a nominal energy balance from A-G. Most importantly for the window specifier, it encourages manufacturers to publish verifiable data on Ux, g-value and air permeability.

Though u-values are still often quoted, the BFRC rating system is gaining popularity through its citing by the Building Regulations in England and Wales

There are differing options for glazing panels and changes over the years.

Secondary Glazing

This is a fairly easy type of double glazing, whereby a second pane of glass is somehow attached to the frame to cover the existing pane.  A seal is made from a plastic sleeve which goes around the edges of the new pane of glass, but this seal is likely to be poor.

Thankfully technology has moved on from this and better options are now on the market.

‘Low-E’ Glass

This is quite a significant development within the glazing market.  It is basically a coating on the face of the inner pane of glass with metal or metal oxide, short wave radiation from the sun is permitted to enter the building, whilst long wave radiation in the form of heat from the inside is reflected back.

Gas Filled Units

Filling the gap between the glass panes with low conductivity gasses such as argon or krypton improves the window efficiency by conductive and convective heat transfer.

Low Iron Glass

Removing the iron content from glass increases its light transmittance and therefore solar gain.  Low iron glass is commonly used in the outer pane of the multi-glazed unit, low-e glass being utilised for the inner pane.

In conclusion

We need to keep our glazing units free from condensation.  To admire the view from the window across the lawn as the kids are playing in the sunshine, to watching the snow fall pre Christmas.  We want to be able to keep snug, warm and healthy and blown window units, with its misty window panes, will not help with the efficiency of the central heating, because we have to run it for longer to maintain the warmth to keep out the frost or keep the condensation down.

If the condensation is not kept in check then it can lead to health issues due to the mold that is determined to grow.


International Energy Agency


British Fenestration Rating Council

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Photo by Sebastian Staines on Unsplash