Insulated Glass Unit Calculator — Panes, Gaps & Performance

The rigorous method follows EN 673 / ISO 10292 and accounts for surface resistances, glass conduction, and gas-gap conduction/convection. For planning purposes, use this rule of thumb: clear double 4/16/4 ≈ 2.8 W/m²K; add a soft low-E coating ≈ 1.4; add argon fill ≈ 1.1; triple low-E argon ≈ 0.7. Certified values come from NFRC or EN lab testing, not calculation alone.

12 mm and 16 mm are the standard high-performing gaps. Below 10 mm, gas conduction is too high. Above 18 mm, convection currents set in and thermal performance actually drops — the U-value gets worse, not better. Krypton can use narrower gaps (6–9 mm) because its lower thermal conductivity compensates.

Argon gives roughly 0.2–0.3 W/m²K better U-value than air fill and costs about 30% more than air. Krypton doubles that improvement again but costs 2× argon. Krypton makes sense in very narrow gaps (<9 mm) where argon convection is worse, or in passive solar designs chasing ultra-low U-values.

For most climates: surface 2 (the inner side of the outer pane) for winter heat retention, or surface 5 (the outer side of the inner pane) for summer solar rejection. Coating on surface 3 or 4 faces the gas gap and can degrade. Coating both surface 2 and 5 gives the lowest SHGC but adds cost.

Hard coat (pyrolytic) Low-E is applied during float glass manufacturing — it is durable but has higher SHGC (~0.63), making it suitable for cold climates wanting passive solar gain. Soft coat (sputtered) Low-E is applied after manufacturing inside a vacuum chamber — it has lower SHGC (~0.41) and better U-value but is more delicate (needs a cover pane).

The gas gap contributes essentially zero weight — only the glass panes and aluminum spacer bar matter. A 4/16/4 IGU (two 4 mm panes, 1200 × 900 mm) weighs about 21.6 kg from glass plus ~0.5 kg from the spacer bar. Triple glazing triples the glass weight.

Triple glazing makes sense below U ≈ 0.9 W/m²K, where the center-of-glass improvement of a better double unit starts hitting diminishing returns — frame and edge-of-glass losses dominate the overall window U-value. Passivhaus targets require triple glazing (U ≤ 0.85). For most homes in the US, UK, and central Europe, a good double low-E argon unit (U ≈ 1.1–1.4) is the cost-optimal choice.

They are independent metrics telling you different things about the same window. U-value measures heat flow through the glass (lower = better insulation). SHGC measures how much of the sun's energy gets through (lower = less solar heat gain). In hot climates you want low SHGC; in cold climates you want higher SHGC to capture winter sun.