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.
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.
3 min read
Air-tightness is tested separately from thermal insulation, using standardised air permeability tests that measure how much air passes through a closed window or door under a given pressure difference. It has a direct and noticeable effect on comfort, particularly on openable casements and sliding sashes exposed to wind.
This guide explains how air permeability is classified, why it matters more in Cumbria's exposed locations than in sheltered urban settings, and what to check for beyond the headline U-value.
What air permeability testing measures
Air permeability is tested to BS EN 1026 and classified under BS EN 12207, measuring the volume of air (in cubic metres per hour, per square metre or per metre of joint) that passes through a closed window under increasing pressure. The result is expressed as a class from 1 to 4, with Class 4 representing the best resistance to air leakage.
- Class 1
- Basic resistance to air leakage
- Class 2
- Moderate resistance
- Class 3
- Good resistance, suitable for most UK exposures
- Class 4
- Best available, recommended for exposed or coastal sites
Why exposure category matters here
Wind-driven rain and pressure exposure vary significantly across Cumbria, from sheltered valley villages to open fellside and coastal positions around Morecambe Bay and the west coast. A frame and seal system rated to Class 3 may be entirely adequate in a sheltered courtyard position but under-specified on an exposed gable facing the prevailing weather. This is a genuine engineering consideration, not an upsell — an under-specified seal system on an exposed elevation will let in wind noise and draughts regardless of how good the glazing U-value is.
Sliding doors and air-tightness
Sliding doors inherently rely on a running track and compression seals rather than a fully compressed gasket on all four sides, as a hinged casement does. Quality sliding systems close this gap with multi-point locking that draws the sash tight into the frame and interlocking weather seals, but it remains worth checking test data specifically for sliding products rather than assuming casement figures apply.
- Multi-chamber gaskets compressed by multi-point locking
- Interlocking meeting stiles on sliding and bifold panels
- Brush or blade seals at sill level on sliding tracks
- Threshold detail — a low-profile weathered threshold trades a small amount of air-tightness for step-free access
Draughts are not the same as ventilation
A very air-tight home still needs controlled ventilation, provided through trickle vents or mechanical ventilation under Part F of the Building Regulations, rather than uncontrolled leakage through poor seals. Sealing a home tightly without addressing background ventilation can increase condensation risk — see our condensation guide for how the two interact.
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