What Are Energy Efficient Windows?

 

Energy efficient windows are designed to reduce the transfer of heat between the inside and outside of the property.

​

Their performance depends on more than the number of panes of glass.

​

A modern window can include:

​

• double or triple glazing;

• low-emissivity glass;

• insulated glazing cavities;

• warm-edge spacer bars;

• thermally efficient frames;

• improved seals;

• suitable gas fills between panes; and

• carefully designed installation details.

​

The complete window, including the frame and the way it is fitted into the wall, determines how well it performs.

​

​

How Do Energy Saving Windows Work?

 

Heat can be lost through windows by conduction, air leakage and radiation through the glazing.

​

Modern energy saving windows are designed to reduce these losses.

​

Multiple panes of glass create insulating spaces between the inside and outside surfaces. Low-emissivity coatings can reduce radiant heat loss, while improved frames and seals help reduce draughts and unwanted air leakage.

​

The result can be a warmer internal surface and a room that feels more comfortable even when the thermostat temperature remains unchanged.

​

​

Double Glazing

 

Double glazing consists of two panes of glass separated by a sealed cavity.

​

It has become the normal standard for replacement windows in many UK homes and can provide a significant improvement over traditional single glazing.

​

Modern double glazing can incorporate:

​

• low-emissivity glass;

• improved spacer bars;

• insulated frames;

• gas-filled cavities; and

• high-performance seals.

​

The quality of one double-glazed window can differ considerably from another, so homeowners should compare the specification of the complete product rather than simply asking whether it is double glazed.

​

​

Triple Glazing

 

Triple glazing adds a third pane of glass and another insulating cavity.

​

This can provide improved thermal performance and may be particularly attractive for highly insulated homes or properties undergoing comprehensive energy upgrades.

​

However, triple glazing is not automatically the best choice for every house.

​

It can be:

​

• heavier;

• more expensive;

• more demanding on frames and hinges; and

• unnecessary where other parts of the building remain poorly insulated.

​

At APN Green Homes, our whole-home approach considers whether the additional investment in triple glazing is justified relative to other improvements that could be made to the property.

​

​

What Are the Best Energy Efficient Windows?

 

The best energy efficient windows are not necessarily those with the most glass or the highest price.

​

The right choice depends on the property, including:

​

• existing wall construction;

• insulation levels;

• orientation;

• size of window openings;

• architectural style;

• ventilation requirements;

• exposure;

• planning restrictions; and

• available budget.

​

A very high-performance window may provide limited overall benefit if the surrounding walls, roof and floor continue to lose large amounts of heat.

​

The best solution should therefore be considered as part of the building as a whole.

​

​

Window Frames and Energy Efficiency

 

The frame is an important part of the thermal performance of a window.

​

Common frame materials include:

​

• uPVC;

• timber;

• aluminium;

• composite systems; and

• specialist thermally broken frames.

​

Each has different advantages in terms of appearance, durability, maintenance and thermal performance.

​

Modern aluminium windows, for example, commonly incorporate thermal breaks to reduce heat transfer through the metal.

​

Timber can offer good thermal performance and may be particularly appropriate for period or traditional properties.

​

​

Low-E Glass

 

Low-emissivity, or Low-E, glass incorporates a very thin coating designed to reduce heat transfer through the glazing.

​

The coating allows daylight to enter while helping retain more heat inside the property.

​

Low-E glass is commonly incorporated into modern energy efficient glazing systems and can improve performance without substantially changing the appearance of the window.

​

​

Most Energy Efficient Windows

 

Homeowners searching for the most energy efficient windows should look beyond marketing claims and consider the performance of the complete window.

​

Important factors can include:

​

• glazing specification;

• frame performance;

• air leakage;

• installation quality;

• seals;

• spacer bars;

• thermal bridging; and

• the relationship between the window and surrounding wall.

​

A high-performance window that is badly installed can lose much of the benefit expected from the product.

​

Installation quality therefore matters just as much as the specification on the brochure.

​

​

Window Energy Ratings

 

Replacement windows may be supplied with an energy rating designed to help homeowners compare products.

​

These ratings can provide a useful indication of overall performance, but they should not be considered in isolation.

​

The appropriate window still needs to suit the architecture, orientation and energy strategy of the property.

​

For a whole-house refurbishment, the relationship between glazing, insulation, ventilation and heating can be more important than simply choosing the highest-rated product available.

​

​

Heat Loss Through Windows

 

Windows traditionally perform less efficiently than well-insulated walls and roofs.

​

This means large glazed areas can have a significant influence on heat loss.

​

The effect can be particularly noticeable in:

​

• older houses;

• properties with single glazing;

• large bay windows;

• conservatories;

• glazed extensions;

• floor-to-ceiling windows; and

• rooms with substantial areas of external glazing.

​

Improving these windows can increase comfort as well as reduce energy demand.

​

​

Draughts Around Windows

 

Some of the discomfort associated with older windows comes from draughts rather than heat passing directly through the glass.

​

Air leakage may occur around:

​

• opening casements;

• damaged seals;

• poorly fitted frames;

• gaps between the frame and wall;

• sash windows;

• trickle vents; and

• deteriorated surrounding finishes.

​

In some cases, repairing and draught-proofing an existing window may provide a worthwhile improvement without complete replacement.

​

The condition of the existing windows should therefore be assessed before deciding what work is necessary.

​

​

Energy Efficient Windows and Insulation

 

Window replacement works best when considered alongside the insulation of the surrounding building.

​

If the walls, roof or floor remain poorly insulated, replacing windows alone may not provide the transformation a homeowner expects.

​

Similarly, insulation work around window openings needs to consider thermal bridging at:

​

• reveals;

• lintels;

• sills; and

• junctions between frames and walls.

​

A coordinated design can help reduce cold spots around the window opening.

​

​

Energy Efficient Windows and Heat Pumps

 

Improved glazing can be particularly relevant where an air source heat pump is being considered.

​

Heat pumps generally perform best when the heating demand of the property is controlled and unnecessary heat loss is reduced.

​

Replacing inefficient windows can contribute towards a lower overall heat requirement, particularly where the existing glazing is old or in poor condition.

​

However, window replacement should be considered alongside roof, wall and floor insulation rather than automatically assuming it should be the first improvement undertaken.

​

​

Windows and Solar Gain

 

Windows do not only lose heat.

​

They can also allow solar energy into the property.

​

During winter, sunlight through suitable glazing can provide useful passive heating.

​

During summer, excessive solar gain can contribute to overheating.

​

This is particularly relevant for:

​

• south-facing rooms;

• west-facing glazing;

• large sliding doors;

• glazed extensions;

• rooflights; and

• loft conversions.

​

At APN Green Homes, we consider both winter heat loss and summer comfort when reviewing glazing.

​

​

Energy Efficient Windows and Summer Overheating

 

Increasing the thermal performance of a home should not mean ignoring the risk of overheating.

​

Large modern windows can create significant solar gain during warm weather.

​

Possible solutions may include:

​

• solar-control glazing;

• external shading;

• internal blinds;

• shutters;

• appropriate orientation;

• ventilation; and

• air conditioning where necessary.

​

A greener home should remain comfortable throughout the year, not simply retain more heat during winter.

​

​

Windows and Ventilation

 

Replacing old draughty windows can make a property considerably more airtight.

​

This is generally beneficial for energy efficiency, but suitable ventilation still needs to be maintained.

​

A home may require:

​

• trickle ventilation;

• extract fans;

• continuous mechanical ventilation; or

• MVHR.

​

Simply sealing every source of air movement without considering ventilation can contribute to condensation, humidity and poor indoor air quality.

​

Window replacement should therefore form part of a wider ventilation strategy.

​

​

Energy Efficient Windows for Period Homes

 

Older and period homes require particular care.

​

Victorian, Edwardian and Georgian properties often have windows that contribute significantly to the architectural character of the building.

​

Complete replacement may not always be the most appropriate solution.

​

Options may include:

​

• repair of existing timber windows;

• draught proofing;

• secondary glazing;

• slim-profile double glazing;

• sympathetic replacement windows; and

• improvements to shutters or surrounding fabric.

​

The right approach needs to balance energy efficiency with the character and construction of the property.

​

​

Energy Efficient Windows in Conservation Areas

 

Properties in conservation areas may be subject to additional controls over window replacement.

​

The local authority may be particularly concerned with:

​

• frame material;

• glazing-bar proportions;

• opening style;

• colour;

• window depth;

• historic detailing; and

• appearance from the street.

​

Modern replacement windows that differ substantially from the originals may require planning consideration.

​

It is therefore sensible to establish the position before ordering expensive windows.

​

​

Energy Efficient Windows for Listed Buildings

 

Window replacement on a listed building requires even greater care.

​

Original windows may form an important part of the historic significance of the property and removing them may not be acceptable.

​

Energy-efficiency improvements might instead involve:

​

• repair;

• draught proofing;

• secondary glazing;

• shutters; or

• carefully designed specialist glazing.

​

Where alterations are proposed, listed-building consent may be required.

​

APN Green Homes can identify where planning and design issues should be investigated before the project proceeds.

​

​

Replacement Windows and Planning Permission

 

Many ordinary replacement-window projects can be relatively straightforward, particularly where the appearance remains substantially unchanged.

​

However, planning issues can arise for:

​

• listed buildings;

• conservation areas;

• flats;

• properties affected by Article 4 Directions;

• significant changes in material or appearance; and

• alterations to window openings.

​

Where there is uncertainty, the planning requirements should be checked before manufacture and installation.

​

​

Building Regulations for Replacement Windows

 

Replacement windows can also be subject to Building Regulations requirements.

​

These can relate to matters such as:

​

• thermal performance;

• ventilation;

• safety glazing;

• means of escape;

• structural support; and

• installation.

​

Where window openings are being enlarged, reduced or altered, additional design and structural considerations may also arise.

​

​

Energy Efficient Windows During an Extension

 

An extension or refurbishment can provide an ideal opportunity to consider glazing across the whole property.

​

Rather than selecting windows independently from the rest of the project, the design can consider:

​

• insulation;

• heating;

• ventilation;

• solar gain;

• summer overheating;

• appearance;

• daylight; and

• future energy use.

​

This is particularly important where extensions include large sliding or bifold doors and roof glazing.

​

The glazing design can have a major influence on both the appearance and energy performance of the finished space.

​

​

How Much Do Energy Efficient Windows Cost?

 

The cost of energy efficient windows varies considerably.

​

Factors affecting price may include:

​

• number of windows;

• window sizes;

• frame material;

• double or triple glazing;

• opening arrangements;

• glazing specification;

• access;

• installation;

• scaffolding;

• decorative detailing; and

• whether existing openings are being altered.

​

Specialist timber windows or replacement glazing for period properties may cost considerably more than standard modern units.

​

Homeowners should compare quality, performance, warranty and installation standards rather than selecting replacement windows purely on price.

​

​

Are Energy Efficient Windows Worth It?

 

That depends on the existing windows and the wider condition of the property.

​

Replacing poor single glazing, badly deteriorated frames or extremely draughty windows may provide a noticeable improvement in comfort.

​

Replacing relatively modern functioning double glazing purely to achieve a small increase in thermal performance may be much harder to justify financially.

​

This is why APN Green Homes considers where your money is likely to make the greatest difference rather than automatically recommending replacement.

​

​

Repair or Replace?

 

Not every older window needs to be removed.

​

In some properties, repair may provide a better solution.

​

Possible improvements can include:

​

• new seals;

• draught proofing;

• frame repairs;

• replacement glazing units;

• secondary glazing;

• refurbishment of sash windows; and

• improved ironmongery.

​

The decision should consider the condition, character and performance of the existing window as well as the cost of complete replacement.

​

​

Energy Efficient Windows as Part of a Green Home Plan

 

Windows are only one part of the building envelope.

​

Insulation reduces heat loss through roofs, walls and floors.

​

Energy efficient windows reduce glazing losses and draughts.

​

Ventilation maintains healthy indoor air quality.

​

A heat pump can provide efficient heating.

​

Solar panels can generate renewable electricity.

​

Battery storage can help manage electricity use.

​

Considering these improvements together can create a more effective overall energy strategy.

​

​

Independent Window and Glazing Advice

 

Window companies naturally focus on selling replacement windows.

​

APN Green Homes takes a wider approach.

​

Through a Green Home Survey, we can consider whether window replacement should be a priority, whether repairs or secondary glazing deserve consideration and how glazing improvements fit alongside insulation, heating, ventilation and other planned upgrades.

​

For some homes, replacement windows may make an important difference.

​

For others, the available budget may produce greater benefit elsewhere.

​

The objective is to help you understand that difference before committing to major expenditure.

​

​

Start With the Whole Property

 

The best energy efficient windows are those that work with the rest of the home.

​

Before replacing glazing, it makes sense to understand the existing windows, surrounding walls, insulation, ventilation, heating requirements and the other improvements you may be considering.

​

​

NEXT STEPS

Every home is different, so the right next step depends on your property, your budget and what you want to achieve. APN Green Homes can help you turn the information you have just read into a practical plan for your own home. Whether you are considering one improvement or looking at a complete whole-home energy upgrade, we can help you understand what is suitable, what should be prioritised and where your money is likely to make the greatest difference.

​

Choose one of the options below to get the ball rolling:

​

BOOK A GREEN HOME SURVEY
Get a detailed assessment of your home with practical, property-specific recommendations and a clear plan for the improvements worth considering.

​

BOOK A PHONE CONSULTATION
Talk through your property, ideas and priorities with us and get some initial direction on the best way forward.

​

EMAIL US
Send us details of your property, photographs or any questions and we can advise you on the most appropriate next step.

​

GIVE US A CALL
Prefer to speak to someone? Give us a call and tell us what you are hoping to achieve.

​

Your greener, more efficient home starts with understanding the right improvements for your property — and putting them in the right order.