Spandrel Panel Calculator
Opaque panel insulation and finish for spandrel areas.
About this calculator
Spandrel zones are the curtain wall's opaque panels -- hiding floor slabs and mechanical equipment behind painted or fritted glass -- and they carry their own energy-code insulation requirement even though nobody ever looks through them. This calculator works backward from Target R-Value to a buildable Insulation Thickness: it divides the target by the chosen Insulation Type's conductivity, then rounds UP to the nearest 25mm standard increment, which is why Actual R-Value always comes out slightly ABOVE what you asked for -- you cannot buy insulation in the exact fractional thickness the math wants, only in 25mm steps, so the achieved performance always overshoots the target by whatever that rounding leaves on the table.
Insulation Type matters here: mineral wool's higher conductivity needs more thickness for the same R-value than polyiso (this calculator's most efficient standard option) or XPS, so the same Target R-Value produces a different Insulation Thickness, Build- Up Depth, and panel weight depending which material is selected -- but it has zero effect on Glass Thermal Expansion, which only depends on panel size and Temperature Swing. Glass Thermal Expansion itself tracks whichever panel dimension is LARGER (width or height), since that is the one imposing the bigger absolute stretch under the same coefficient of thermal expansion and temperature swing -- widening the shorter dimension never moves it.
Inputs
Results
Insulation thickness (mm)
100
Spandrel U-value (W/m²·K)
0.2
How to Use This Calculator
- Enter the spandrel panel width and height in millimeters.
- Set the framing system (stick-built or unitized) and thermal insulation R-value.
- Input the panel weight for structural dead load calculations.
- Review the overall spandrel U-value and estimated annual heat loss.
- Use the panel weight to specify anchor design loads in structural drawings.
How the result changes with Target R-value (m²·K/W)
| Target R-value (m²·K/W) | Insulation thickness (mm) | Spandrel U-value (W/m²·K) |
|---|---|---|
| 1.75 | 50 | 0.38 |
| 2.63 | 75 | 0.27 |
| 5.25 | 125 | 0.17 |
| 8.75 | 200 | 0.11 |
What each input means
- Panel width (mm)
- Width of the spandrel panel module.
- Panel height (mm)
- Height of the spandrel zone (floor slab depth + fire-safing).
- Target R-value (m²·K/W)
- Required insulation R-value per energy code (ASHRAE 90.1). Typical: R-3.0 to R-4.5.
- Insulation type (1-3)
- 1 = Mineral wool, 2 = Polyisocyanurate, 3 = Extruded polystyrene (XPS).
- Spandrel glass (mm)
- Thickness of opacified spandrel glass (ceramic frit or painted back).
- Temperature swing (°C)
- Total temperature range the spandrel glass experiences (dark colors can reach 80–100°C swing).
What each result means
- Insulation thickness (mm)
- Required insulation thickness rounded up to standard 25 mm increments.
- Actual R-value (m²·K/W)
- Achieved R-value with standardized insulation thickness.
- Spandrel U-value (W/m²·K)
- Overall spandrel panel U-value including all layers and air films.
- Total build-up depth (mm)
- Total depth from outside face of glass to inside face of back pan.
- Glass thermal expansion (mm)
- Maximum thermal expansion of spandrel glass across the temperature swing.
- Total panel weight (kg)
- Combined weight of glass, insulation, and aluminum back pan.
- Glass weight (kg)
- Weight of the spandrel glass alone.
- Insulation weight (kg)
- Weight of the insulation layer.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPanel width (mm) = 1500, Panel height (mm) = 1200, Target R-value (m²·K/W) = 3.5, Insulation type (1-3) = 2 = 6 input(s) provided
- Calculate Insulation thicknessInsulation thickness = ceil(insulThickMm / 25) * 25100 = 100
- Calculate Spandrel U-valueSpandrel U-value = 1 / rTotal0.204 = 0.204
- Calculate Actual R-valueActual R-value = (standardThickMm / 1000) / conductivity4.55 = 4.55
- Calculate Total build-up depthTotal build-up depth = glassThickness + shadowBoxDepth + standardThickMm + 1.5133 = 133
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does Actual R-Value always come out higher than my Target R-Value?
Because insulation is only manufactured in standard thicknesses, and this calculator rounds the calculated required thickness UP to the nearest 25mm increment before reporting Actual R-Value -- so the achieved R-value always meets or exceeds the target, never falls short of it, at the cost of some "wasted" thickness beyond what the target strictly required.
Insulation Type changes three outputs (thickness, R-value, weight) but not Glass Thermal Expansion -- why doesn't the glass layer care what's behind it?
Because Glass Thermal Expansion is purely a property of the GLASS -- its own coefficient of thermal expansion, Panel Width, Panel Height, and Temperature Swing -- computed independently of whatever sits in the shadow-box cavity behind it. Insulation Type instead governs a completely separate calculation: how thick a layer of mineral wool, polyiso, or XPS is needed to hit Target R-Value, and how much that layer weighs. The two calculations share the same panel but never share a variable, which is why swapping insulation materials can move Insulation Thickness by tens of millimetres and Panel Weight by several kilograms while leaving Glass Thermal Expansion exactly where it started, decimal point for decimal point.
Which panel dimension drives Glass Thermal Expansion -- width, height, or both?
Whichever is larger. The calculator applies the glass's thermal expansion coefficient to the larger of Panel Width or Panel Height, since that dimension experiences the greater absolute stretch under the same temperature swing -- so for a panel wider than it is tall, widening it further increases Glass Thermal Expansion, but making it taller (while still shorter than the width) does not move the number at all.
How much does insulation type change the required thickness for the same R-value target?
Meaningfully -- at this calculator's default 3.5 m²·K/W target, mineral wool (the least efficient of the three standard options here) needs 125mm to polyiso's 100mm, because mineral wool's higher thermal conductivity means more material is needed to reach the same resistance. XPS sits between the two. The gap widens further as Target R-Value increases.
What actually goes into Spandrel U-Value -- is panel size part of the formula?
Spandrel U-Value and Actual R-Value are built entirely from the layer stack-up: outside air film, glass conductivity, the enclosed air space, insulation R-value, the aluminum back pan, and inside air film, added as resistances and inverted. Panel Width and Panel Height never enter that stack-up at all, so U-Value and R-Value hold steady no matter how big or small the panel is -- panel size shows up elsewhere instead, in Glass Thermal Expansion and the panel/component weights.
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