If you’re researching new windows for your home, you’ve probably come across terms like U-factor, Low E coating, and gas fill, and wondered what they actually mean in practical terms. As installers working across Portsmouth and the surrounding Hampshire area, we help homeowners cut through this technical energy efficiency jargon every day. Here’s what genuinely matters when choosing energy efficient uPVC windows, and why these factors make such a difference to your home’s comfort and your energy bills.
What to Look For in Energy Efficient uPVC Windows
- U-factor (or U-value) measures heat loss, lower numbers mean better insulation
- UK Building Regulations set a maximum whole window U-value of 1.4 W/m²K for replacement windows in existing homes
- For new build properties, Part L requirements are stricter and typically call for a U-value of around 1.2 W/m²K or better
- Low E (low emissivity) glass coatings reflect heat back into the room
- Gas filled units use argon or krypton between the panes to reduce heat transfer
- Vacuum glazing removes the gas entirely, offering exceptional performance in a slim profile
- Check the Window Energy Rating (WER), aim for A rated or higher
- Multi chamber uPVC frames and warm edge spacer bars improve insulation beyond the glass itself
Understanding U-Factor and UK Regulations
The U-factor, sometimes called the U-value, is one of the most important figures to understand when comparing windows. It measures how much heat passes through the window, covering the frame, glass, and spacer bar together as a complete unit. The lower the U-value, the better the window is at keeping heat inside your home during winter and keeping unwanted heat out during summer.
Under current UK Building Regulations Approved Document L, replacement windows fitted into existing dwellings must achieve a whole window U-value of 1.4 W/m²K or better. This is a backstop figure, meaning it’s the absolute minimum standard that any installer must meet by law, and it applies whether you’re replacing a single window or undertaking a full house refit.
For new build properties, the standards are tighter still. Part L requirements for new dwellings typically expect a U value around 1.2 W/m²K or lower as part of the wider whole house energy performance calculation, often referred to as the SAP assessment. Many manufacturers now produce uPVC windows that comfortably exceed both of these figures, with quality double glazed units regularly achieving U-values between 1.0 and 1.2 W/m²K, and high performance options pushing below 1.0 W/m²K.
When comparing quotes, always ask for the whole window U-value rather than just the glass specification. A FENSA or competent person scheme registered installer, which is a requirement for most window replacement work, will need to provide certification confirming the installed windows meet the relevant U-value standard, so this paperwork should form part of any legitimate installation. At BJH, we are fully registered FENSA double glazing installers, and would be more than happy to help you find windows or doors that suite your energy efficiency requirements.
Low E Glass Explained
Low E, short for low emissivity, refers to a microscopically thin metallic coating applied to one of the internal glass surfaces within a sealed unit. This coating is virtually invisible but works by reflecting long wave infrared heat back into the room, while still allowing short wave solar energy and visible light to pass through.
In practical terms, this means the heat generated by your central heating, your appliances, and even your own body warmth is reflected back into the living space rather than escaping through the glass. During colder months, this can make a noticeable difference to how quickly a room loses heat once your heating switches off.
Most reputable double glazing units now come with Low E coatings as standard, but it’s worth confirming this when getting quotes, as it’s not something you can identify just by looking at the glass. The coating is applied during manufacturing and forms part of the sealed unit’s specification, and it’s a key contributor to achieving the U-values required under current regulations.
Our Residence 9 windows for example can come with solar control low E coatings as well as double or triple glazing, making it an ideal choice for optimal energy efficiency requirements.
Gas Fill: Argon, Krypton, and Vacuum Sealing
Between the panes of glass in a sealed unit, there’s a cavity that’s traditionally filled with air. However, modern energy efficient units replace this air with inert gases, or in some cases remove it entirely, to significantly reduce heat transfer through the cavity.
Argon Gas
Argon is the most widely used gas fill and remains the standard choice for the vast majority of double glazed uPVC windows. It’s denser than air, which reduces convective heat transfer within the cavity, and it’s completely safe, colourless, and odourless. Argon offers a good balance of performance and cost, and units filled with argon comfortably help achieve the U-values required for compliance with current building regulations.
Krypton Gas
Krypton performs even better than argon, particularly in narrower cavity widths where it becomes more efficient at reducing heat transfer. This makes krypton a useful option for slimline double glazed units or for triple glazed units where multiple narrow cavities are used. Krypton is more costly to produce than argon, so it tends to appear in premium specifications or where a slimmer overall unit thickness is needed without compromising on thermal performance, such as in heritage style properties with shallower frame rebates.
Vacuum Glazing
Vacuum glazing is one of the more recent developments to reach the UK market and works on a different principle entirely. Rather than filling the cavity with gas, the space between two panes is evacuated to create a vacuum, with tiny support pillars keeping the panes apart. Because there’s no gas or air to conduct or convect heat, vacuum glazed units can achieve U-values comparable to triple glazing while remaining as slim as a standard double glazed unit, often around 6 to 8mm thick overall.
This makes vacuum glazing particularly useful for period properties or conservation areas around Portsmouth where slim sightlines need to be maintained but improved thermal performance is still required. It’s a newer technology and tends to come at a higher price point, but for the right application it offers a genuinely impressive specification.
Across all of these options, the spacer bar and edge seal quality matters just as much as the gas fill or vacuum itself, since this is what maintains the cavity and prevents gradual gas loss or vacuum degradation over the lifespan of the unit.
The Role of Warm Edge Spacer Bars
The spacer bar sits around the perimeter of the glass unit, separating the panes and maintaining the gap that holds the gas fill or vacuum. Traditional spacer bars were made from aluminium, which conducts heat readily and creates a cold bridge around the edge of the glass, often leading to condensation forming on the inside edges during cold weather.
Warm edge spacer bars, typically made from reinforced plastic or stainless steel composites, reduce this heat transfer significantly. The result is a more consistent surface temperature across the entire glass area, less risk of edge condensation, and a meaningful contribution to the overall whole window U-value, helping ensure compliance with the 1.4 W/m²K backstop and getting closer to the lower figures seen in higher specification units.
Window Energy Ratings (WER)
To make comparing windows easier, the British Fenestration Rating Council operates a Window Energy Rating scheme that grades windows from A++ down to E, similar to the energy labels you’d see on appliances. This rating takes into account the U-value, solar heat gain, and air leakage all together, giving you a single figure to compare different products against.
For most homes, we’d recommend looking for windows rated A or above, with A++ representing the most efficient options currently available. A higher rated window will typically combine a low U-value, an effective Low E coating, and good gas fill or vacuum retention, so this rating is a useful shortcut if you don’t want to compare every individual specification separately. Windows achieving the current regulatory minimum of 1.4 W/m²K will generally sit around a B rating, while units pushing toward 1.0 W/m²K or below, particularly those using krypton or vacuum glazing, can achieve A or A++ ratings.
Frame Design and Multi Chamber Construction
While glass specification gets a lot of attention, the uPVC frame itself plays a significant role in overall window performance, particularly when it comes to meeting the whole window U-value figures required by current regulations. Modern uPVC profiles use multi chamber construction, meaning the frame is divided internally into several separate sections or chambers.
These chambers trap air and create additional barriers to heat transfer through the frame material itself. Profiles with five, six, or even seven chambers are now common among quality manufacturers, and generally speaking, more chambers correlate with better thermal performance from the frame, helping the overall unit reach lower U-values even when paired with standard argon filled double glazing.
Reinforcement within the frame, usually steel or aluminium inserts in larger sections, also affects how well the frame holds its shape over time, which matters for maintaining a tight seal and preventing drafts developing as the window ages.
Choosing the Right Specification for Your Home
For most properties in the Portsmouth area, well specified double glazing with Low E coatings, argon fill, and warm edge spacers already exceeds the regulatory minimum and delivers excellent performance. However, for period properties, north facing rooms, or homes where slim sightlines are important, krypton filled units or vacuum glazing may offer worthwhile improvements that are better suited to the property’s specific needs.
It’s also worth remembering that installation quality affects energy efficiency just as much as the products themselves. Even the best specified window won’t perform as expected if it’s not fitted correctly, with proper sealing around the frame and attention to detail during fitting. This is where choosing an experienced, registered local installer like BJH makes a genuine difference, both to performance and to ensuring your installation meets current building regulations with the correct certification.
Speak to BJH Windows & Conservatories About Energy Efficient uPVC Windows
At BJH Windows & Conservatories, we supply and install uPVC windows across Portsmouth and the surrounding areas of south England, specifying high performance glass with Low E coatings as standard, alongside argon, krypton, or vacuum glazing options depending on your property’s needs. All our installations meet or exceed current UK U-value requirements, and we provide the relevant certification confirming compliance. We’re happy to talk through the specifications in plain terms, help you understand what’s right for your home, and ensure your new windows are fitted to the highest standard for long lasting comfort and efficiency. Contact us today to arrange a no obligation quote for your home.
About the Author
Oscar Hamilton
Trainee ManagerI am the third-generation member of the BJH Windows family! I have a deep-rooted passion for the business. I achieved a First-Class degree in Business Management and Leadership from the University of the West of England. Having worked summers at BJH since my childhood, I have hands-on experience in the industry, even starting as a fitter’s mate. I am a fully qualified FENSA surveyor, combining my academic knowledge and practical expertise to deliver exceptional service, ensuring the family legacy of quality and customer care continues.
