Flagship guide · 7 min read
U-Values and Thermal Performance Explained
The single figure most quoted in glazing brochures, and the one most often misunderstood. Here is what a U-value actually measures, and what to do with it.
Knowledge Centre
Technical Advice
U-values, g-values, air tightness, thermal bridging, condensation and frame materials — explained so you can read a specification sheet with confidence.
Technical Advice · 3 min read
Whole-Window Thermal Performance Explained
U-value is one input. Real thermal performance in a finished home also depends on frame design, installation quality and how the window is used.
Technical Advice · 3 min read
Air-Tightness in Windows and Doors Explained
A well-insulated window with poor seals will still feel draughty. Air permeability is the figure that describes how much air leaks through in real conditions.
Technical Advice · 3 min read
Solar Gain and g-Values Explained
Large glazing brings welcome light and warmth in winter, but the same solar gain can overheat a south-facing room in summer without the right glass specification.
Technical Advice · 3 min read
Window Frame Materials Compared
Frame material shapes thermal performance, sightline width and structural strength together. Here is how the main options compare on the numbers.
Technical Advice · 3 min read
Structural Glazing and Large Spans Explained
Slim sightlines on a large sliding door or frameless corner window are a structural engineering outcome, not just an aesthetic choice.
Technical Advice · 3 min read
Glass Performance Explained: Strength, Safety and Acoustics
Beyond thermal figures, glass performance is also about strength, breakage behaviour and sound — each governed by different standards and specifications.
Technical Advice · 3 min read
Window Energy Ratings (WER) Explained
A Window Energy Rating combines several performance figures into a single familiar A to G band, similar to appliance energy labels.
Technical Advice · 3 min read
Condensation on Windows Explained
Condensation on a window is not always a fault. What matters is where it forms and what that tells you about ventilation, insulation or seal failure.
Technical Advice · 4 min read
Thermal Bridging in Windows and Reveals Explained
A thermal bridge is a localised weak point in the insulation of a building element, and window openings are one of the most common places they occur.
Technical Advice · 3 min read
FD30 vs FD60 Explained
Two numbers, one principle. Here is what each rating is tested to do, and what usually drives the choice between them.
Technical Advice · 4 min read
The Best Glass for Large Sliding Doors
The frame determines how wide a sliding door can be. The glass determines how comfortable the room feels through every season the door has to work in.
Technical Advice · 4 min read
How Wide Can Aluminium Sliding Doors Be?
There is no single maximum width for an aluminium sliding door. The system chosen and the structural opening it sits in decide the practical limit together, not either one alone.
Technical Advice · 4 min read
How Large Can Aluminium Windows Be?
Aluminium's strength-to-weight ratio makes very large window openings possible, but 'large' means something different for a fixed light than it does for an opening casement.
Technical Advice · 3 min read
Level Thresholds for Sliding Doors
A level threshold looks like a small detail on a drawing. Getting it wrong is one of the most expensive mistakes to fix once a floor slab has already been poured.
Technical Advice · 3 min read
Window Sightlines Explained
Sightline is the single spec figure that most visibly separates a good glazing installation from a mediocre one, and it is widely misunderstood.
Technical Advice · 3 min read
Dual-Colour Aluminium Windows Explained
A dual-colour aluminium window gives you a different powder-coated finish inside and out from the same frame, letting an exterior make a statement while the interior stays calm.
Technical Advice · 6 min read
Gable and Apex Glazing in Cumbria and the Lake District
A gable end apex is one of the most dramatic places to put glass in a home, and one of the least forgiving of a design that has not thought through structure and glare together.
Technical Advice · 4 min read
Corner Glazing Explained
Corner glazing looks simple from the outside — glass meeting glass with no visible support — but achieving that effect within realistic spans is a genuine structural engineering exercise.
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Other areas of the library
- Buying GuidesEverything a homeowner should know before purchasing.
- Cost & Budget GuidesRealistic investment guidance, written plainly.
- Product ComparisonsTwo good options, side by side.
- Glass AdviceSpecifying the pane, not just the frame.
- Colours & FinishesChoosing a colour you will still like in twenty years.
- Design InspirationIdeas grounded in real Cumbrian buildings.
- Planning & RegulationsPermission, conservation, listed buildings and Building Regulations.
- InstallationWhat actually happens, week by week.
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