Thermal Performance (CW) Calculator
Overall U-factor from frame, glass, and spacer.
About this calculator
A curtain wall's real thermal performance is never just the glass's own U-value -- it is an area-weighted blend of three zones with very different conductivities: the opaque aluminum frame (worst), a 63.5mm edge-of-glass band next to the frame where the spacer bar creates a thermal bridge (middle), and the centre-of-glass zone where the insulated glazing unit performs closest to its rated number (best). This calculator splits every panel into those three zones from Panel Width, Panel Height, and Frame Sightline, then combines their U-values weighted by area share to get Overall U-Factor -- which is why a narrower daylight opening (a wider Frame Sightline relative to panel size) always pulls Overall U-Factor toward the frame's worse number, never the glass's better one.
Total CW Area is a pure output multiplier: it drives Annual Heat Loss up or down but has zero effect on Overall U-Factor, R-Value, the three zone-area percentages, Condensation Resistance, or Inside Surface Temperature -- those are all per-unit-area properties of the assembly, indifferent to how much of it you're building. Condensation Resistance Factor (CRF) works from the same Overall U-Factor: a lower U-factor keeps the inside glass surface warmer relative to the indoor-to- outdoor temperature swing, pushing CRF up in turn.
Inputs
Results
Overall U-factor (W/m²·K)
1.43
Condensation resistance (CRF)
82.8
How to Use This Calculator
- Enter the centre-of-glass U-value and frame U-value in W/m2K.
- Set panel width, height, and frame sightline dimension in millimeters.
- Input outside and inside design temperatures for condensation analysis.
- Enter total curtain wall area for annual heat loss estimation.
- Review the overall U-factor, condensation resistance (CRF), and annual heat loss for energy code compliance.
How the result changes with Glass U-value COG (W/m²·K)
| Glass U-value COG (W/m²·K) | Overall U-factor (W/m²·K) | Condensation resistance (CRF) |
|---|---|---|
| 0.55 | 0.95 | 88.6 |
| 0.83 | 1.19 | 85.7 |
| 1.65 | 1.92 | 76.9 |
| 2.75 | 2.88 | 65.3 |
What each input means
- Glass U-value COG (W/m²·K)
- Centre-of-glass U-value from IGU manufacturer. Double low-E argon ≈ 1.1, triple ≈ 0.6.
- Frame U-value (W/m²·K)
- Aluminum frame U-value. Standard ≈ 5–7, thermally broken ≈ 2.5–4.0.
- Panel width (mm)
- Overall width of one curtain wall panel module.
- Panel height (mm)
- Overall height of one curtain wall panel.
- Frame sightline (mm)
- Visible frame width (sightline) on each edge of the panel.
- Outside design temp (°C)
- Winter design outside air temperature for condensation analysis.
- Inside temp (°C)
- Interior design air temperature.
- Total CW area (m²)
- Total curtain wall area for annual heat loss estimate.
What each result means
- Overall U-factor (W/m²·K)
- Area-weighted U-factor including frame, edge-of-glass, and centre-of-glass zones.
- Overall R-value (m²·K/W)
- Thermal resistance (1/U).
- Centre-of-glass area (%)
- Percentage of panel area that is centre-of-glass zone.
- Edge-of-glass area (%)
- Percentage of panel area that is edge-of-glass zone (63.5 mm band per NFRC).
- Frame area (%)
- Percentage of panel area occupied by frame.
- Condensation resistance (CRF)
- Condensation Resistance Factor per AAMA 1503. Higher is better; ≥50 recommended.
- Inside surface temp (°C)
- Predicted inside glass surface temperature at winter design conditions.
- Annual heat loss (kWh)
- Estimated annual conductive heat loss through the curtain wall (assumes 3,500 HDD climate).
- Glazing infeasible flag
- 1 if Frame sightline consumes the entire panel width or height (no daylight opening left for glass), meaning every other output above is 0 and not a meaningful thermal result. 0 otherwise.
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
What happens if the frame sightline is wide enough to consume the entire panel?
The calculator has no daylight opening left to compute a glass U-value from, so every thermal output -- Overall U-Factor, R-Value, CRF, Inside Surface Temperature, Annual Heat Loss -- reads 0, and Glazing Infeasible Flag switches to 1 to make clear those zeros are not a real (and definitely not a "perfect") thermal result, just a sign the frame sightline and panel dimensions you entered don't leave room for glass.
How does frame U-value affect condensation resistance versus overall U-factor?
Raising Frame U-Value (a worse, less insulating frame) always raises Overall U-Factor and always lowers Condensation Resistance -- a worse frame lets more heat escape (raising U-factor) and lets the inside glass surface run colder relative to the indoor-outdoor temperature swing (dropping CRF), moving in opposite directions on purpose since a colder inside surface is exactly what condensation resistance measures against.
Why doesn't a 100,000 m² tower facade get a different U-factor than a 1 m² test panel of the same construction?
Overall U-Factor, R-Value, the zone-area percentages, CRF, and Inside Surface Temperature are all PER-UNIT-AREA properties -- ratios and averages computed from Panel Width, Panel Height, and Frame Sightline for one representative panel, then assumed constant across the whole facade. Total CW Area never enters that per-panel math at all; it only multiplies Overall U-Factor by area (and by 3,500 heating-degree-days) to produce Annual Heat Loss, a single project-wide total. So a one-square-metre mockup and a 100,000 m² curtain wall built from the same panel design report identical U-factors, R-values, and CRF -- they just generate very different Annual Heat Loss numbers because one is a hundred-thousand times more wall.
Which zone dominates Overall U-Factor: frame, edge-of-glass, or centre-of-glass?
Whichever has the largest area share, since Overall U-Factor is an area-weighted average of all three. At this calculator's default panel proportions, centre-of-glass covers the largest share of the panel and glass U-value (Glass U-Value COG) has the biggest single influence on the result -- but a panel with a proportionally wider Frame Sightline shifts more area (and more weight) onto the frame's typically worse U-value.
Does a lower Overall U-Factor always mean better condensation resistance?
Within the normal design range, yes -- CRF is directly derived from Overall U-Factor and the indoor/outdoor design temperatures, so any input that lowers U-Factor (a better frame, lower Glass U-Value COG, a narrower frame sightline) raises CRF in lockstep. But the relationship has a hidden edge case: if Outside Design Temp and Inside Temp are set to the same value (both ranges overlap between 10-35°C), the temperature swing (ΔT) the CRF formula divides by hits zero, and the calculator reports CRF as a flat 0 regardless of how good Overall U-Factor is -- not because condensation resistance is actually zero, but because the formula's denominator collapses. It's a reminder that CRF measures a RATIO relative to the design temperature swing, not an absolute property of the assembly.
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