Secondary Glazing – The Basics

Variously called Secondary Glazing, Secondary Double Glazing, Secondary Windows, Storm Windows and Internal Double Glazing – these are all just different descriptions for one and the same thing!

Secondary glazing is a slim, completely separate internal window or door, fitted to the inside of the existing window or door.

Our Secondary Glazing system is very discreet – made and fitted to blend in to the existing windows and doors, it is:

  • Manufactured from slim extruded aluminium frames
  • As standard has a durable white gloss finish & other colours are available
  • Glazed with various types of 4, 6 or 6.4mm glass
  • Robust & functional, as well as complementary & unobtrusive once installed
  • Available in different unit types to match different types of existing windows and doors
  • Designed to allow easy access to the existing primary windows for ventilation & cleaning
  • Suitable for installation to almost all types of existing windows & doors
  • Mainly specified for Heat Insulation, Soundproofing – or a combination of the two

Once fitted, Internal Double Glazing has been created – it is a misconception that Secondary Glazing is itself Double Glazed.

For those of a more technical nature, or who would like more in depth details, information and specifications, please visit our Product Range and Technical Section.

Secondary Windows – Applications & Benefits

Balanced Vertically Sliding Secondary Glazing for Sash Windows, London Secondary Glazing

Secondary glazing is a versatile product that offers a host of benefits – but that does not make it the best product for every situation or for every customer.

The most common question that people ask is:

  • “why secondary glazing, rather than replacement double glazing?”

We reply that:

“if your existing windows are essentially in a good, sound condition – then installing secondary windows will provide you with – excellent energy insulation – and maximum soundproofing – at a greatly reduced cost – when compared to replacement windows.”

Conversely, if your existing windows are so rotten or rusted or broken as to be beyond economical repair, then clearly you are going to have to replace them.

Each situation is different – so each customer must weigh the pros and cons to make their decision.

To aid you in that process, the following are all of the benefits provided by secondary glazing and all of the applications to which it is best suited:

  • Thermal Window Insulation – especially of existing windows that do not need replacing
  • EPC Compliance – improves a property’s energy efficiency rating
  • Noise Insulation – secondary glazing is always more effective than Replacement Double Glazing
  • Above First Floor – no need for scaffolding for external access
  • Conservation Areas, Listed Properties & Historic or Character Properties – maintains the external appearance
  • Flats & Apartments in Blocks – usually have restrictions on changing the windows
  • Cost Considerations – much cheaper than replacing windows
  • Environmental Impact – produces significantly less carbon dioxide emissions during the manufacturing process than replacement double glazing – extends the life & reduces the carbon footprint of the existing windows – keeps them out of land fill
  • Security – is improved by adding a second layer of glass & it can only be opened from inside
  • Condensation – is usually eliminated by installing secondary glazing
  • Draughts & Dust Ingress – are stopped

Secondary Double Glazing – Product Range

Slim Matching Frame Sections of Lift Outs and Sliders are Ideal for Bays, London Secondary Glazing

We have a comprehensive range of styles to suit just about any application:

Standard Secondary Glazing Types Application
  • 2, 3, 4 & 5 part Horizontally Sliding Units
  • Balanced Vertically Sliding Units
  • Easy Clean Tilt-in Balanced Vertically Sliding Units
  • Hinged Units
  • Lift-Out Units where regular access is not required
  • Fixed Units
  • Hinged & some Sash Windows & Doors
  • Sash Windows
  • Sash Windows
  • Casement Windows & Doors
  • Small Casement & Sash Windows
  • Glazed Doors & Non-Opening Windows

Special & Bespoke Secondary Glazing
  • Curved
  • Semi-Circular
  • Circular
  • Arched
  • Shaped
  • Incorporating Fly Screens
  • Incorporating Trickle Vents
  • Any Colour Finish
  • Specialist Glasses
  • Specialist Film Applications

Our standard frame finish is White, we also stock Brown and we can paint finish to any RAL colour to match or complement your internal colour scheme and decoration.

Special Applications:

For Secondary Glazing Sash Windows

Spiral Balanced Vertically Sliding Secondary Windows, London Secondary Glazing

If you have ever had a price for replacing sash windows, then you know just how horribly expensive they are.

Additionally, many are in Listed Buildings, Conservation Areas and Apartment Blocks, where double glazing is prohibited.

We have the solution:

Spiral Balanced Vertically Sliding Secondary Windows

Due to the characteristics of the existing primary sash window, the Balanced Vertical Slider is the most complex type of secondary glazing system.

The Spiral Balance System

The spiral balance system allows the secondary glazing to mirror the normal up and down movements of a traditional sash window – including leaving it in the open position for ventilation.

Additionally, if the gap between the secondary and the existing sash window is too small for easy cleaning access, then the secondary sashes can be fitted with our Easy-Clean ® Mechanism so that you can safely release them and tilt them through 180 degrees into the room for cleaning the other side of the glass.

This can be particularly useful for windows above the first floor, above conservatories and above single-story extensions and any other areas where external access is difficult.

How it works

The individually pre-tensioned spiral spring balances control the vertical motion of the sashes, enabling easy fingertip positioning and repositioning.

The design provides ease of movement and has been specifically developed to withstand decades of wear and tear.

For Heritage Windows & Listed Building Double Glazing

Recommended for Double Glazing Listed Buildings and Frames Can Be Colour Matched, London Secondary Glazing

If you have a heritage property and/or do not have permission to change the existing windows, then secondary glazing is the best way to improve the property’s energy efficiency and soundproofing whilst still retaining your home’s character and authenticity.

We are fully experienced in installing secondary glazing to Heritage, Listed, Conservation and Character properties.

The Benefits of Secondary Glazing for Historic & Listed Buildings

  • Secondary Glazing does not alter the external façade
  • Accepted by & specified by Planning & Conservation Officers
  • An economic solution
  • Protects the value of the property
  • Colour matching to any RAL Colour
  • In most instances does not require planning permission

For Condensation

Secondary Double Glazing Will Normally Result in the Total Elimination of Condensation, London Secondary Glazing

In the majority of cases the installation of secondary double glazing will result in the total elimination of condensation.

In most instances, condensation is caused by how we live and how we ventilate our living space

A tightly sealed house or flat, where a lot of water vapour is produced that cannot escape via good ventilation, will inevitably result in the inside air becoming saturated to the point that the excess water vapour starts to condense onto colder non-porous surfaces – glass, mirrors, window frames, gloss paintwork, etc.

This is known as the “Dew Point” – the point at which the air cannot hold any more water – naturally the air circulates in a room and as it passes cooler surfaces such as windows, it cools and releases moisture in the form of condensation onto those cooler surfaces.

The Primary Causes of Condensation

The figures quoted are the daily averages for a family of four:

  • Breathing – adds nearly 10 pints of moisture to the air every day!
  • Cooking – adds over 5 pints per day
  • Baths & Showers – add nearly 2 pints each day
  • Washing & Drying Clothes – adds more than 9 pints per day
  • Heating – especially flue-less gas heaters – in fact every carbon fuel produces some amount of water when used

Aids to Eliminating Condensation

  • Install secondary glazing
  • Ensure sufficient ventilation – without it, the moisture can’t get out!
    • open chimney flue dampers or vents
    • vent all gas burners
    • fit extractor fans in kitchens & bathrooms
    • air the house daily for just a few minutes by opening windows & doors
    • shut kitchen, bathroom & laundry doors during use – then open the window for a few minutes afterwards
  • Consider using a dehumidifier

Our Services

Residential Secondary Glazing Installations throughout London and the Home Counties, London Secondary Glazing

Secondary Glazing for Window Insulation, London Secondary Glazing

Acoustic Soundproofing Window Treatments, London Secondary Glazing

Residential Installations

We install to all types of properties throughout London including the City and in the Home Counties

Commercial Installations

We install for:

  • Landlords
  • Letting Agents
  • Property Maintenance & Management Companies
  • Offices
  • Architects
  • Specifiers
  • Builders
  • Churches, Village Halls, Mosques & Temples
  • Hotels & Motels
  • Schools & Universities
  • Dance & Recording Studios
  • Historic, Listed & Conservation Buildings
  • Care & Rest Homes & Hospices
  • Pubs & Clubs
  • Charities
  • Museums & Galleries

Supply Only

We supply only to:

  • Professional Installers, Builders Carpenters

Our products are not “DIY Kit” type products and require a professional installation.


Aluminium Frames and Sashes are extruded from 6063T6 alloy in accordance with BS1474, London Secondary Glazing


Aluminium alloys are extruded from 6063T6 alloy in accordance with BS1474

Assembly & Site Fixings

Dependent upon the type of secondary glazing unit, a combination of concealed aluminium cleats, BZP & stainless-steel screws


Sub-Frames are manufactured from sustainable, seasoned timber

Finishing Trims

Finishing Trims are manufactured from timber or uPVC, according to the application & internal décor


All aluminium sections are polyester powder coated in a white high gloss finish, produced in accordance with BS6496.

Stock finishes are held in White & Brown

Any Other RAL colour is available upon request

Weather Piles

Weather piles are Polypropylene Draught Strips to BS7386.


All gaskets are black rubber extruded to BS2782


A combination of decorator’s caulk permanently flexible bedding compound & colour matched silicone sealant

Coupling & Stacking of Secondary Glazed Units

All products can be coupled or stacked with other products in our range using matching interlocks & couplers


All secondary glazing is glazed in accordance with Building & Safety Regulations

Opening & Sliding Units

Use rollers, glides, hinges or spiral balances


Every product and installation is Fully Guaranteed for 10 Years GUARANTEE pdf icon

Our Customers are the Guardians of Our Reputation, please take a moment to see our Customer Reviews

Thank you for visiting London Secondary Glazing! – please Contact Us if we can be of further assistance.