When choosing new windows, thermal performance is often one of the first considerations. U-values and energy ratings are widely discussed — but for homes with south- or west-facing elevations, another important factor comes into play: how materials behave in prolonged heat and direct sunlight.
At KP Glass & Windows, homeowners frequently ask whether aluminium windows or uPVC windows perform better in sunny aspects. The answer lies not just in insulation values, but in material stability and long-term performance.
Understanding Thermal Ratings: Aluminium vs uPVC
Both modern aluminium and uPVC windows are designed to meet strict UK building regulations, with thermally broken frames and high-performance glazing.
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uPVC windows rely on multi-chambered plastic profiles to reduce heat transfer
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aluminium windows use a thermal break — usually a polyamide barrier — to separate internal and external metal surfaces
In terms of U-values alone, both materials can achieve excellent results when paired with modern double or triple glazing.
However, thermal ratings only tell part of the story, particularly for properties exposed to high levels of sunlight.
Heat Exposure & Material Stability
How uPVC Performs in Strong Sunlight
uPVC is a plastic-based material, and while modern formulations are significantly improved, prolonged exposure to high temperatures and intense sunlight can cause:
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Expansion and contraction during hot and cold cycles
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Increased risk of frame movement on large openings
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Potential long-term warping on darker colour finishes
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Greater stress on seals and hardware over time
In very sunny elevations, especially where windows are large or dark coloured, this movement can gradually affect alignment and operation.
Why Aluminium Is More Stable in High Temperatures
Aluminium, by contrast, is dimensionally stable. It does not soften, warp, or deform under heat in the same way plastic-based materials can.
This makes aluminium windows particularly well suited to sun-exposed elevations, where temperatures at the frame surface can be significantly higher than ambient air temperature.
Key advantages of aluminium in sunny aspects include:
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Minimal expansion and contraction, even in high heat
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Maintains alignment on large or wide window spans
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Better long-term performance of seals and hardware
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Ideal for darker colours and contemporary finishes
This stability allows aluminium windows to retain their performance and appearance year after year, even on south-facing elevations.
Thermal Comfort: It’s Not Just About Heat Loss
A common misconception is that thermal performance is only about keeping heat in. In reality, managing heat gain is just as important in sunny rooms.
Modern aluminium windows combine:
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Thermally broken frames
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High-performance glazing
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Solar control or low-E glass options
This means aluminium windows can help regulate internal temperatures, reducing overheating while still delivering excellent insulation in winter months.
Large Glazing Areas & Solar Exposure
Homes with large windows, sliding doors, or picture windows on sunny elevations benefit significantly from aluminium frames.
Because aluminium is stronger than uPVC, it allows for:
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Slimmer frames
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Larger panes of glass
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Better weight distribution without distortion
In high-sun environments, this structural strength ensures windows remain square and operational, even as temperatures fluctuate.
Aluminium or uPVC: Which Should You Choose?
Both materials have their place, but for properties with:
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South- or west-facing elevations
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Large glazing areas
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Dark-coloured frames
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Modern or minimalist designs
Aluminium windows are often the more stable, durable choice in the long term.
For shaded elevations or smaller window openings, uPVC remains a cost-effective and energy-efficient solution. The best choice often comes down to orientation, design, and long-term performance expectations.
Expert Advice from KP Glass & Windows
At KP Glass & Windows, we help homeowners choose the right window material based on how their home is positioned, not just brochure specifications.
Whether you’re considering aluminium windows for sunny elevations or comparing them against uPVC for your property, our experienced team can offer honest, practical advice tailored to your home.
📞 Call us on 01536 525140 to discuss your requirements or request a free quote.