Dec 2023

Choosing double-glazed windows: UPVC, Aluminium or Timber?

Compare the critical factors in this comprehensive guide to double-glazed windows.

Image of a modern house exterior with large double glazed timber windows allowing sun through.

When it comes to designing or renovating a building, the choice of double glazed window frames plays a pivotal role not just in aesthetic appeal but also overall energy efficiency, durability and comfort of the structure. The debate often boils down to three popular materials: uPVC, aluminium, and timber. Each of these materials brings its unique set of properties, benefits, and aesthetic values to the table, making the decision more than just a matter of personal taste.

Whether you are an architect, builder, or homeowner, understanding the nuances of these materials is the key to choosing window frames that align with your design objectives, budget constraints, and sustainability goals.

In this comprehensive guide, we delve into the world of uPVC, aluminium, and timber framed windows, comparing them across various critical factors. From thermal performance and maintenance requirements to cost implications and environmental impact, this detailed overview will help you make an informed decision for your next project.

A quick look:

Best for affordable efficiency

uPVC double-glazed windows

  • There is a reason uPVC double-glazed windows are used everywhere in Europe. They’re cost effective and very high performance. We recommend them to everyone.

Best for modern design aesthetics

Aluminium thermally-broken double-glazed windows

  • If you are going to choose aluminium windows, choose thermally-broken frames. Metal is a conductor of heat – exactly what you don’t want in your window. If you can’t afford thermally broken or composite, choose uPVC.

Best for total sustainability

Timber double-glazed windows

  • Everyone loves timber, and they do a great job all-round on energy and carbon – the downsides are cost, and maintenance.

Powerhaus Insight: When building a new home, we believe investing in the highest-performing windows for your budget is important. Regret about skimping on window choice is one of the most common comments we hear from clients in colder climates (along with missing air-tightness)

A few key window metrics we’ll refer to:

  1. U value. The most important, U value measures how insulative a window is. It is the inverse of R values used in walls – so lower values are better. The ranges for double glazed windows are roughly between 1–5, with values under 3 being better.
  2. Solar Heat Gain Co-Efficient (SHGC). This measures how much solar heat is transmitted through the window when receiving direct sunlight. It is measured between 0–1, with higher values letting in more heat.
  3. Air infiltration rate. This measures how much air your windows let in when they are closed. That’s right, windows are responsible for a lot of air-leakage that make your house less comfortable and cost more to run. The lower the air-infiltration rate, the better.

You can look up a database of all windows and their data at the Australian Window Energy Rating Scheme (WERS) database – or just ask your window supplier.

UPVC Double Glazed Windows: Affordable Efficiency

Unplasticized Polyvinyl Chloride, commonly known as uPVC, is a type of plastic that is widely used in the construction of window frames. It’s known for its strength, durability and resistance to weathering, making it an increasingly popular choice in the window industry. It’s the most popular window in Europe, and for good reason – it is the most energy efficient double-glazed window available in Australia.

Thermal Performance: One of the standout features of uPVC windows is their excellent thermal insulation. The material itself has low thermal conductivity, which means it does not allow much heat to pass through. The strong frames of uPVC windows also allow for thicker glass units, part of the high performance values that they achieve. These properties makes uPVC windows highly efficient in reducing heat loss from homes, contributing to better energy efficiency and lower heating costs.

Bar chart comparing the U values of different double glazed windows. Aluminium windows are the highest, followed by aluminium thermally-broken and timber. uPVC windows have the lowest U values.

Double glazed windows vary significantly in thermal performance, and uPVC are the best. Data source: WERS (data represents 75% most common window values for each frame type)

Maintenance and Durability: uPVC windows are renowned for their low maintenance requirements. They do not need painting or sealing and are resistant to rotting, warping, or corroding. This durability ensures that uPVC windows remain functional and aesthetically pleasing for many years, often outlasting their wooden counterparts. uPVC windows are also as strong as aluminium, allowing for wide window spans. When recycled, uPVC actually becomes stronger.

Airtightness: uPVC windows generally seal the best of all windows, with the best air-tightness. When we talk about airtightness, we’re referring to a closed window’s ability to stop unwanted air moving through the window components. Opening type does play a part in this – compression-fit windows like tilt & turn and awning will always do better than sliders and casement windows. uPVC windows also have the widest range of compression-fit opening styles available.

Cost Effectiveness: When it comes to cost, uPVC windows are generally more affordable than thermally-broken aluminum or timber options. This cost effectiveness, combined with their long lifespan and minimal maintenance needs, makes them a budget-friendly choice for both residential and commercial properties.

Aesthetic Flexibility: While traditionally available in white, advancements in technology have expanded the range of colours and finishes available in uPVC windows. This includes wood grain effects that mimic the appearance of natural timber (as in the highlight image above), offering more aesthetic flexibility to match various architectural styles.

The frame thickness of uPVC is important to note, however can easily be accommodated by designing slightly larger window openings and attention to opening types.

Environmental Impact: uPVC is a plastic, and its production and disposal raise environmental concerns. However, in the UK where these windows have been around longer, most uPVC is now recycled, and we expect that industry will follow in Australia. The first uPVC window recycling workshop has opened in Australia!

In terms of embodied carbon, uPVC has around half the embodied energy of aluminum windows. When you include the energy saved over the life of the house due to uPVC windows high insulative qualities and the likelihood of increasing recycling options – they are a sound environmental choice.

Table showing the embodied carbon of different window frame types: aluminium, upvc and wood.

A recent study of the embodied carbon of different window frames found that Aluminium were by far the highest.

Sound and Acoustics: uPVC windows provide good sound insulation, thanks to their tight seals and the inherent sound-dampening properties of the material. This makes them a suitable choice for homes in noisy environments.

Conclusion: uPVC windows are Powerhaus favourites. They stand out as the most affordable, low-maintenance, and energy-efficient option. Their growing range of styles and finishes, combined with their practical benefits, makes them a compelling choice for those looking to balance cost with performance and aesthetics.

Aluminium Double Glazed Windows: Durable and Modern

Aluminium, a lightweight yet strong metal, has become a popular choice for window frames in contemporary architecture. Known for its sleek, modern appearance, aluminium offers a distinct aesthetic often sought after in modern design projects, however it offers very poor thermal performance and energy efficiency outcomes both in heat transfer through frames and air movement.

Thermal Performance: Aluminium has high thermal conductivity, offering much less efficient insulation compared to other frame materials. As a result, advancements in manufacturing have lead to thermally-broken aluminium frames. These frames contain a barrier that significantly reduces heat transfer, enhancing the thermal efficiency of the windows.

However it remainds more challenging to fit thicker high-performance glass units, and Aluminium frames remain the worst thermal performers (on average) of all windows.

Maintenance and Durability: Aluminium windows are low maintenance and withstand harsh weather conditions, requiring only occasional cleaning to keep them looking new. Unlike timber, they do not need regular painting or staining, and are not prone to rot or pest infestation.

Aluminium is renowned for its exceptional strength, which allows for slimmer frames and larger panes of glass, maximising natural light and views. This strength also contributes to the durability of aluminium windows, making them resistant to warping, cracking, or twisting over time.

Airtightness: Aluminium windows are not always known for their high-quality seals and limiting unwanted airflow. Airtightness has a huge impact on the comfort of a building. If you do choose aluminium – ensure that the windows you are choosing have high-quality seals by checking they have low air-infiltration rates in testing. You can see tested data for all Australian windows on the the WERS site, and find air infiltration values for your specific windows.

Cost Effectiveness: While aluminium windows are generally less expensive than uPVC, thermally-broken Aluminium are typically more expensive. If you are set on aluminium, we strongly recommend thermally-broken frames, as they significantly increase energy-efficiency and comfort, as well as preventing condensation and mositure on frames.

Aesthetic Flexibility: Aluminium frames can be powder-coated in a wide range of colours and finishes, allowing for customisation to suit different architectural styles. Their sleek and minimalistic design is particularly appealing in modern and industrial-style buildings.

Environmental Impact: Unfortunately, aluminium manufacture is very energy intensive and aluminium frames have the highest embodied energy of any frame type. When considering a full life cycle assessment,

“Aluminium frames cause the highest burden to the environment because of the dangerous pollutants released and high energy consumption during aluminium production.” (Asif, Davidson & Muneer, 2002)

However aluminium is a highly recyclable material – which can reduce its overall life cycle impact if undertaken.

Sound and Acoustics: While not as inherently soundproof as uPVC, as metal reflects noise, the double-glazing itself will still provide noise reduction.

Conclusion: Aluminium windows offer a blend of strength, durability, and modern aesthetics.However the low thermal performance, high U value and higher average air loss, of standard frames is a deal breaker for us – we don’t recommend them.  If you must have aluminium windows, we only recommend choosing thermally-broken windows.

Timber Framed Double Glazed Windows: Classic Elegance

Timber has been a traditional choice for window frames for centuries, offering a timeless appeal that is hard to replicate. Known for their natural beauty, timber windows bring warmth and character to any building.

Thermal Performance: Wood is a natural insulator, making timber windows excellent at reducing heat transfer. This inherent property helps in maintaining a comfortable indoor temperature, contributing to the overall energy efficiency of a home. When combined with modern glazing technologies, timber windows can rival the thermal performance of more modern materials.

Maintenance and Durability: Timber windows require more maintenance than uPVC or aluminium frames. They need to be regularly treated and protected against moisture, rot and pests to maintain their appearance and functionality. However, with proper care, timber windows can last for decades, and their ability to be repaired and repainted extends their lifespan further.

Image of a kitchen with double glazed composite windows showing the timber inside and alumnium outside

Composite windows are an excellent compromise. The timber frame is sustainable and warm, while aluminium clad exterior ensures durability and reduces maintenance.  📸 Image source: Miglass high performance windows

Airtightness: Timber windows, like aluminum, are more prone to air-leakage, as timber can wear and warp slightly over time. In this case, higher-quality windows with quality seals will play a more important role in ensuring your windows do the job of keeping the house comfortable.

Cost Effectiveness: Generally, timber windows are more expensive than uPVC and can be on par with or more costly than aluminium, depending on the quality and type of wood.

Aesthetic Appeal: The natural look of wood is one of the main reasons for its enduring popularity. Timber frames can be painted or stained in a variety of colours, allowing for a high degree of customisation to match different architectural styles. Whether it’s a rustic country home or an elegant Victorian-style residence, timber windows add a touch of class and authenticity. We’ll be honest – this is why we love them.

Environmental Impact: Timber is a renewable resource and when sourced from sustainably managed forests can be an environmentally friendly choice. Additionally, timber frames are biodegradable at the end of their life, reducing environmental impact.

Timber frames are also quite insulative, although not quite as high as uPVC – ensuring energy saving over the life the home.

Sound and acoustics: Timber frames naturally provide good sound insulation due to the density of wood. This makes them an excellent choice for homes in noisy areas, where reducing external sound intrusion is a priority.

Conclusion: Timber framed windows offer unmatched aesthetic appeal and natural insulation properties, making them a desirable choice for those looking to add elegance and character to their homes. While they require more maintenance than other materials, their beauty and the warmth they bring to a property can make the extra care worthwhile. For those valuing classic aesthetics and environmental sustainability, timber windows are an excellent choice.

Aluminium-clad composite windows offer reduced maintenance, with the solid thermal performance, aesthetics and whole-of-life sustainability of timber.

Weighing the Options

So what window type should you choose? That depends on your budget, as well as your comfort and sustainability goals.

Budget-friendly and maximum efficiency choice: uPVC Windows

When it comes to balancing performance and cost, uPVC windows emerge as the clear winner. Their superior insulation properties, coupled with affordability, make them an ideal choice for projects where energy efficiency and budget are top priorities. uPVC’s durability and low maintenance requirements further solidify its position as a practical and cost-effective solution for modern building designs.

Total environmental sustainability choice: Timber Windows

If the primary focus is on environmental sustainability, timber windows are the standout option. Timber is a renewable resource and when sourced from sustainably managed forests, it represents an environmentally friendly choice. Timber frames are biodegradable at the end of their lifespan, reducing long-term environmental impact. They also offer excellent thermal insulation, contributing to the energy efficiency of the building. However it’s important to consider that timber frames require more maintenance than uPVC or aluminium and this maintenance itself can have environmental implications depending on the products used.

Timber composite windows (which have an outer lining of aluminium) reduce the maintenance cost while retaining all of timber’s other benefits.

When the aesthetic cannot be compromised choice: Thermally-broken Aluminium Windows

We understand that sometimes, the design vision prioritises aesthetics. In such cases, we encourage considering the upgrade to thermally broken and well-sealed aluminium window units. While these options may result in a higher cost, they offer the best of both worlds – the style that aligns with your aesthetic goals and the high-performance features essential for energy efficiency. These windows not only enhance the visual appeal of your projects but also ensure energy efficiency is not compromised.

Decision tree for choosing double-glazed window frames, starting with the question "I just want the best thermal performance".

Stuck on choosing window frames? Here’s a helpful decision tree – or, try the Powerhaus app to see specifics for your project.

Summary

The choice between uPVC for budget-friendly thermal performance and timber for total environmental sustainability depends on the specific priorities and constraints of each project. uPVC offers an excellent balance of cost and energy efficiency, making it ideal for budget-sensitive projects. Timber, on the other hand, stands out for its environmental credentials and aesthetic appeal, making it suitable for projects where sustainability and natural beauty are the driving factors. By carefully considering the project’s goals and context, architects and builders can make informed decisions that align with their client’s needs and values.

Discover the best window frames for your project with Powerhaus

Ready to dive deeper into the world of window frames and find the perfect match for your next project? Look no further than the Powerhaus app . Our innovative tool allows you to compare the performance of different window frames, including uPVC, aluminium and timber, in real-time scenarios. See how each option impacts the energy efficiency, sustainability and overall aesthetics of a building. With Powerhaus, making an informed decision has never been easier.

But why stop there? Join our community of forward-thinking architects and builders by signing up for our Energy Efficiency Newsletter. Stay ahead of the curve with the latest insights, trends and tips in sustainable building design. Our newsletter is your go-to resource for all things related to energy efficiency, offering valuable content that can transform the way you approach your projects.

📸 Top image: Easy windows