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: Thermal and Structural Performance
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.
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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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