Insulated Glass Units are sealed at low altitude — but installed at height. The physics of that journey creates pressure differentials that warp, stress, and define how your glass performs for decades.
Every IGU (Insulated Glass Unit / Double Glazed Unit) is manufactured and sealed at factory level — typically near sea level or at a controlled atmospheric pressure. When installed in high-rise buildings or elevated sites, the ambient pressure outside is higher than the pressure trapped inside the sealed cavity.
This imbalance doesn't resolve itself — it permanently loads the glass panes for the lifetime of the unit, causing deflection, optical distortion, and in worst cases, seal failure or glass breakage.
Cross-section: DGU cavity at lower sealed pressure vs higher ambient pressure at installation
The direction and magnitude of glass deflection depends on unit size, pressure differential, and glass thickness. Larger panes flex more — smaller, thicker units are more rigid relative to the applied force.
Higher external pressure pushes both panes inward, creating a dish-like concave appearance. Most visible on large units (>2m²).
Stiffer smaller or thicker units resist inward flex — they may appear convex or barrel-shaped when pressure and aspect ratio interact.
The glass process determines allowable stress, post-breakage behaviour, and how well the unit survives pressure and thermal loads over its service life.
Standard float glass. Lowest strength (~28 N/mm²). Breaks into large sharp shards. Use only in small, low-risk, low-altitude applications. Cheapest — but most dangerous on failure.
2× stronger than annealed (~42 N/mm²). Breaks into large fragments, not cubes — not a safety glass but resists thermal stress and pressure loads well. Ideal inner pane for medium-large DGUs.
4–5× stronger (~55 N/mm²). Breaks into small harmless cubes. Mandatory for safety glazing, overhead, and facades. Cannot be cut after toughening — all processing before furnace.
Two+ panes bonded with PVB or SGP interlayer. On breakage, shards stay bonded — true post-breakage integrity. Essential for overhead, roof lights, and fall-arrest glazing.
Outer lite = toughened or HS. Inner lite = laminated. Best-in-class for high-rise façades and structural glazing. Combines thermal performance with post-breakage safety. Premium — worth it.
Enter your unit dimensions, installation height, and glass specification. The calculator estimates pressure load, deflection, and gives a safe-to-use verdict based on simplified plate theory.
For every 300m of altitude difference between factory and site, the sealed cavity experiences approximately 35–40 Pa of additional pressure load. At 1000m difference, that is over 120 Pa — equivalent to a moderate wind load permanently acting on both glass panes.
Rule: If the altitude difference between factory seal point and installation site exceeds 500m, always specify capillary tube pressure equalisation or request altitude-compensated sealing from your IGU fabricator. Large-format annealed glass simply cannot survive this additional load long-term.
Important disclaimer: This calculator provides indicative guidance only, based on simplified Kirchhoff plate theory with conservative load assumptions. For structural façades, overhead glazing, safety-critical applications, or any project requiring compliance documentation, always commission a full structural glass engineering calculation to BS EN 16612, ASTM E1300, or equivalent national standard. Results here do not constitute a structural engineering certificate.