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Secondary Windows, Single Glazing Heat Insulation for Energy Efficient Windows



Secondary Windows, are they the Right Choice for Insulating Your Single Glazed Windows?

Single glazing insulation, to create energy efficient windows, is a “must do” these days for both Residential and Commercial properties - rising energy costs plus the comfort factor make it essential.

At around 30% of a property’s total poor insulation heat loss - insulating single glazed windows offers probably the biggest single saving and the fastest capital cost payback.

And in the summer, secondary windows help to keep the heat out and further reduce energy costs where air conditioning is in use.

Replacement Double Glazing or Secondary Windows?

Bay Window Good Condition Cold Draughty Noisy Issues Resolved 30 of Replacement Costs Secondary Glazing Lo

As well as supplying and installing Secondary Energy Windows, we also replace and re-glaze all types of wooden, metal and uPVC windows and doors - that allows us to offer you Best Advice - without being limited by our product range.

Over recent years, the main approach to heat insulation for windows has been to replace them with double glazing - but that can be unnecessarily expensive and there are many situations where modern double glazing is not allowed or would be inappropriate.

In the following section, we present the information and considerations for you to be able to make an informed choice.

If you have already decided that Secondary Windows are the Right Choice for you, then please go straight to Insulation Values to see what we can achieve!

Secondary Glazing Windows? How to Decide!

Secondary Glazed Windows eliminate Draughts and Provide Heat Insulation comparable to Replacement Double G

With over 30 years of dealing with these issues, we have found that these are the most important considerations when deciding if Secondary Windows are the best option:

  • our standard range of Secondary Glazed Windows provide excellent Window Insulation, completely eliminate Draughts and will provide you with a quick, clean, low cost (compared to replacement double glazing) upgrade to Energy Efficient Windows. Adding Low E Glass will achieve Heat Insulation for Windows that is comparable to modern Replacement Double Glazing!
  • because of the lower installation cost, the payback of Capital Investment via Reduced Energy Bills will always be much quicker with Secondary Glazing Windows
  • when the Original Windows are still Basically Sound but in need of upgrading to provide a modern standard of Insulating Windows, then Secondary Windows will provide an effective and very economical solution
  • for Leasehold and Joint Freehold properties and those subject to Conservation Area or Listed Building status, all very common in and around London, Secondary Window Glazing will often be the only allowable upgrade and certainly the quickest, least stressful and cheapest option

Secondary Glazed Energy Windows - Heat & Energy Insulation Values

Our Secondary Glazing with 6mm Low E Glass provides Better Insulation than Replacement Double Glazing Sec

To put this into perspective, heat insulation for windows is expressed as the U Value - the lower the U Value the better the window insulation.

For those of a technical nature, the U Value is the measure used for Thermal Transmittance, in this case Heat Loss, of the rate of transfer of heat (in watts) through one square metre of a window, divided by the difference in temperature across the whole window. It is expressed in watts per square metre kelvin, abbreviated to W/m²K.

A window with a U value of 4.0 (W/m²K) will lose twice as much heat as one with a U value of 2.0 (W/m²K)

Or, conversely, a window with a U value of 1.5 (W/m²K) will conserve twice as much heat as one with a U value of 3.0 (W/m²K)

Single, Double or Secondary Glazed       W/m²K
Metal 4mm Float - Single Glazed       5.7
Wood 4mm Float - Single Glazed       4.8
Metal 4mm Float - Single Glazed 25 up to 50mm 4mm Float Secondary Glazing   3.2
Wood 4mm Float - Single Glazed 25 up to 50mm 4mm Float Secondary Glazing   2.5
London Secondary Glazing - Tested:
Wood 4mm Float - Single Glazed 75 up to 100mm 4mm Low E Secondary Glazing   1.9 ***
Wood 4mm Float - Single Glazed 25 up to 50mm 4mm Low E Secondary Glazing   1.8 ***
Building regulations Doc L1b 2010 Amended 2016:
The Requirement for Commercial - Double Glazed Replacement Windows is     1.8 #
# If the building is essentially residential in character, then apply:-     1.6
The Requirement for Residential - Double Glazed Replacement Windows is     1.6
London Secondary Glazing - Calculated:
Wood 4mm Float - Single Glazed 25 up to 50mm 6mm Low E Secondary Glazing   1.5

*** See the London Secondary Glazing test report pdf

Secondary Glazing Windows - How Much Money Will I Save?

When its Cold Outside Secondary Glazing Keeps You Warm and Saves You Money Secondary Glazing London

There are so many variables - area and the quality of sealing of your windows, floor area, type of heating, type of property - that it is difficult to be exact, but based upon our many years of experience and taking average calculations, we can give you a pretty good idea.

The Government’s annual English Housing Survey gives us average sizes for different types of properties and the Statistical Office publishes Annual Domestic Energy Bills for both electricity and gas, used as fuel.

With the above information and using both tested and calculated reduction in heat loss for our secondary insulating windows, the following chart provides you with a very good guide:

Just look for your property type - or, if you want to be more accurate - use the floor area of your property and multiply by the m² or ft² factors at the end of the chart.

    Secondary Glazing With 25/50mm Cavity and:
    4mm Float Glass
4mm Low E Glass
6mm Low E Glass
Average of All Properties   92.3m²   993.51ft² £481.90 £ 628.93 £ 691.51
Flat   56.9m²   612.47ft² £297.07 £ 387.72 £ 426.30
Terraced House   82.6m²   889.10ft² £431.25 £ 562.84 £ 618.84
Semi Detached House   93.2m² 1003.20ft² £486.60 £ 635.06 £ 698.25
Detached House 151.7m² 1632.88ft² £792.03 £1033.68 £1136.54
To Calculate Multiply PFA m²
multiply by: £5.221/m² £6.814/m² £7.492/m²
Property Floor Area PFA ft² multiply by: £0.485/ft² £0.633/ft² £0.696/ft²
* All figures based upon latest available data as at 7th April 2017 (updated annually)
** Above calculations do not take into account any Summer Savings where air conditioning is installed

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