Free Window U-Value Calculator
Calculate whole-window U-value (U_w) per EN ISO 10077-1 — instantly, free, no login. Compare FRP, aluminum, PVC, and timber frames against Passive House (U_w ≤ 0.80) targets.
Published
Apr 5, 2026
Updated
Jul 8, 2026
Technical Review
Haifeng Gong, Ph.D.
Standards and application check
Standards and References
Whole-Window U-Value Calculator
Calculate the overall thermal transmittance (Uw) of a window or door unit per EN ISO 10077-1. Select frame material, glass configuration, spacer type, and window dimensions.
Compliance targets compared: IECC / ENERGY STAR (US) · NRCan ENERGY STAR (Canada) · EPBD (EU) · GB 55015 / GB 50189 (China)
Breakdown
- Glass area (Ag)
- 1.13 m²
- Frame area (Af)
- 0.55 m²
- Glass ratio
- 67%
- Glass perimeter (lg)
- 4.27 m
- Frame Uf
- 1.2 W/m²K
- Glass Ug
- 0.6 W/m²K
- Spacer Ψg
- 0.04 W/mK
vs Aluminum (no break)
44% better thermal performance compared to an aluminum frame without thermal break (Uw = 1.62 W/m²K).
Standards Compliance
How this U-value is built
The whole-window U-value is an area-weighted average of three parallel heat-loss paths — through the glass, through the frame, and the extra leak at the glass edge where the spacer bridges the seal. This model updates live with your selection above.
1 · Window geometry
- Aᵍ — glass area = 0.97 × 1.17 = 1.13 m²
- Aᶠ — frame + sash area = 0.55 m²
- lᵍ — glass edge (spacer bridge) = 4.27 m
2 · Where the heat escapes
Each path’s loss coefficient (W/K) = area (or length) × its U (or Ψ) value. Together they make the total window loss L = 1.51 W/K.
- Glazing(Aᵍ·Uᵍ)
- 0.68 W/K45%
- Frame + sash(Aᶠ·Uᶠ)
- 0.66 W/K44%
- Glass-edge bridge(lᵍ·Ψᵍ)
- 0.17 W/K11%
3 · The formula, with your numbers
Inputs — frame F1 FRP 70-Series (70 mm, 3-chamber) (Uᶠ 1.20), glass Triple — 4/14Ar/4/14Ar/4 2×Low-E (Uᵍ 0.60), spacer Warm-edge (standard) (Ψᵍ 0.04). Dimensions in m; areas m², perimeter m. Per EN ISO 10077-1 §5.2 the window U-value is this area-weighted mean of the frame and glazing, plus the linear edge term.
Frame Material Uf Comparison
Frame thermal transmittance values used in this calculator. FRP frames achieve low Uf values inherently — no thermal break inserts required.
| Frame Material | Uf (W/m²K) | Profile Depth (mm) | Face Width (mm) | Thermal Break Required |
|---|---|---|---|---|
| F1 FRP 65-Series (65 mm, 2-chamber) | 1.4 | 65 | 54 | No — inherently insulating |
| F1 FRP 70-Series (70 mm, 3-chamber) | 1.2 | 70 | 58 | No — inherently insulating |
| F1 FRP 80-Series (80 mm, 3-chamber) | 1 | 80 | 65 | No — inherently insulating |
| F1 FRP 90-Series (90 mm, 3-chamber) | 0.85 | 90 | 72 | No — inherently insulating |
| Aluminum (no thermal break) | 5.9 | 65 | 50 | No — but very high conductivity |
| Aluminum (polyamide break) | 3.2 | 70 | 55 | Yes — polyamide strip |
| PVC Multi-chamber | 1.5 | 70 | 62 | No — but lower stiffness |
| PVC Steel-reinforced | 1.8 | 70 | 65 | Steel core creates thermal bridge |
| Timber (softwood, 68 mm) | 1.4 | 75 | 65 | No — inherently insulating |
Window U-Value Requirements by Standard
Maximum allowable whole-window thermal transmittance (Uw) under major international building energy codes. Values shown are for residential windows unless noted.
| Region | Standard | Climate Zone / Tier | Max Uw (W/m²K) | Notes |
|---|---|---|---|---|
| Europe | EN ISO 10077-1 / PHI | Passive House window (cool-temperate) | ≤ 0.80 | PHI component criterion; U_w,installed ≤ 0.85 |
| Europe | EPBD / national codes | nZEB (Central Europe, typical) | ≤ 1.30 | Member-state specific — always check the national annex |
| Europe | EN 14351-1 | CE Marking baseline | Declared | No max limit — declared value for CE marking |
| USA | IECC 2024 / IRC N1102 | Zone 4 (New York) | ≤ 1.70 | U ≤ 0.30 Btu/h·ft²·°F; mixed humid |
| USA | IECC 2024 / IRC N1102 | Zones 5–6 (Chicago) | ≤ 1.53 | U ≤ 0.27 — tightened from 0.30 in IECC 2021 |
| USA | IECC 2024 / IRC N1102 | Zones 7–8 (Alaska) | ≤ 1.25 | U ≤ 0.22 — tightened from 0.30 in IECC 2021 |
| USA | ENERGY STAR v7.0 (2023) | Northern Zone | ≤ 1.25 | U ≤ 0.22; SHGC ≥ 0.17 prescriptive path; most stringent US program |
| USA | ENERGY STAR v7.0 (2023) | North-Central Zone | ≤ 1.42 | U ≤ 0.25; SHGC ≤ 0.40 |
| USA | ENERGY STAR v7.0 (2023) | South-Central Zone | ≤ 1.59 | U ≤ 0.28; SHGC ≤ 0.23 |
| USA | ENERGY STAR v7.0 (2023) | Southern Zone | ≤ 1.82 | U ≤ 0.32; SHGC ≤ 0.23 primary |
| Canada | NRCan ENERGY STAR v5.0 | All Canada (single zone since 2020) | ≤ 1.22 | Or Energy Rating ER ≥ 34; former A/B/C climate zones retired |
| Canada | NBC 2020 / NECB | Prescriptive, climate-zone dependent | Varies | See NBC 9.36 / NECB tables for the project climate zone |
| China | GB 55015-2021 | All zones — mandatory since Apr 2022 | Zone / WWR dependent | General code; supersedes the thermal provisions of GB 50189-2015 |
| China | GB 50189-2015 | Severe Cold A/B, public, WWR ≤ 0.2 | ≤ 2.70 | Limits tighten with window-to-wall ratio (2.5 / 2.2 …) |
| China | GB 50189-2015 | Severe Cold C, public, WWR ≤ 0.2 | ≤ 2.90 | Shape factor ≤ 0.3 column; tightens with WWR |
| China | GB/T 8484-2020 | Test method | — | Hot-box test method for window thermal performance |
How to use the U-value calculator
Whole-window U-value (Uw) quantifies heat loss through a window assembly in W/m²·K. It combines the frame Uf, glazing Ug, and linear thermal bridging ψ at the frame-glass junction, weighted by the respective surface areas. This calculator follows EN ISO 10077-1, the international reference used for CE marking, Passive House certification, and most national energy codes. Lower Uw means better insulation — passive house targets are typically Uw ≤ 0.80 W/m²·K.
Input example — passive house window
A passive house project specifies a 1230 × 1480 mm fixed window. The designer compares three frame options: a 70 mm aluminum frame with thermal break (Uf ≈ 1.8 W/m²·K), a 70 mm PVC frame (Uf ≈ 1.3 W/m²·K), and an F1 Composite 90-series FRP frame (Uf = 0.85 W/m²·K). All three are paired with Ug = 0.6 triple glazing, warm-edge spacer (ψ = 0.035 W/m·K), and a frame-area ratio typical of fenestration profiles.
The calculator returns Uw ≈ 1.05 W/m²·K (aluminum), 0.85 W/m²·K (PVC), and 0.72 W/m²·K (FRP 90-series). Only the FRP frame meets the PHI passive house limit. The result also shows that on this window size roughly 25% of Uw comes from the frame — which is why frame choice dominates above Ug = 0.7.
How to interpret the results
- Frame dominates on small windows. A 600 × 900 mm sash has 35–40% of its area covered by the frame, so frame Uf drives Uw more than glazing. On a 2400 × 2400 mm picture window, frame area is under 15% and Ug dominates. Always compute with realistic dimensions.
- Warm-edge spacer matters more than people think. Switching from an aluminum spacer (ψ ≈ 0.065) to a warm-edge stainless or composite spacer (ψ ≈ 0.035) reduces Uw by roughly 0.05–0.12 W/m²·K depending on window size — often the cheapest single improvement.
- Triple glazing without a good frame is wasteful. Upgrading from Ug = 1.0 double to Ug = 0.6 triple only delivers its full benefit if the frame Uf is below about 1.2 W/m²·K. Pairing Ug = 0.6 glass with an aluminum frame at Uf = 2.0 wastes most of the glass upgrade.
Common specification mistakes
- Quoting Uf instead of Uw. Manufacturer datasheets often advertise frame-only Uf or center-of-glass Ug. Energy codes and PHI certification are based on whole-window Uw. Always ask for the Uw at the specific size being installed.
- Ignoring installation psi (ψinst). The window-to-wall joint adds another linear thermal bridge typically worth 0.02–0.10 W/m·K. Under strict certification schemes this must be included separately — this calculator gives the assembly Uw, not the installed Uw,inst.
- Assuming aluminum with thermal break is "good enough." Even premium thermally-broken aluminum frames rarely reach Uf below 1.4 W/m²·K. For passive house or net-zero buildings, aluminum cannot meet the target without resorting to oversized glazing cavities — FRP, fiberglass, or triple-chamber PVC become the only viable frame choices.
Referenced standards
- EN ISO 10077-1: Thermal performance of windows, doors and shutters — Calculation of thermal transmittance — Part 1: General
- EN ISO 10077-2: Numerical method for frames
- ISO 15099: Thermal performance of windows, doors and shading devices — Detailed calculations
- NFRC 100: Procedure for Determining Fenestration Product U-factors (North America)
- Passive House Institute (PHI) certified components database — Uw ≤ 0.80 W/m²·K target
Frequently Asked Questions
Is this U-value calculator free, and does it follow PHI / Passive House standards?
Yes — the calculator is fully free, runs in your browser, and follows EN ISO 10077-1 for whole-window U-value (U_w). It supports the Passive House U_w ≤ 0.80 W/m²·K target check used by Passivhaus Institut (PHI) climate-class certification. F1 Composite's PHI-certified 90 Series fenestration achieves U_w 0.78 W/m²·K — verifiable in the calculator using a typical 90 mm FRP frame and triple-glazing input.
Why do FRP frames out-perform thermally broken aluminum on U-value?
FRP thermal conductivity is approximately 0.3–0.4 W/m·K — hundreds of times lower than aluminum (~160 W/m·K) and of the same order as PVC (~0.17 W/m·K). FRP frames reach low U_f not through wall conductivity alone: slim, stiff pultruded walls allow deep multi-chamber (and foam-fillable) profiles with no metal reinforcement bridging the section, whereas PVC of equal stiffness needs a steel core that short-circuits its chambers. Thermally broken aluminum retains a continuous metallic path and typically lands at U_f ≈ 2.8–3.4 W/m²·K for standard polyamide-break systems (premium multi-break systems ≈ 1.6–2.5). Pultruded FRP frames achieve U_f of 0.85–1.4 W/m²·K with no thermal break required.
What inputs does the calculator need?
Frame system (FRP 65/70/80/90 mm series, aluminum, PVC, timber), glazing configuration (double / triple / quadruple with gas fill), spacer type (aluminum, steel, or warm-edge), window type (fixed, casement, sliding, or entrance door), and unit dimensions. The calculator resolves frame and glass areas automatically and returns the whole-window U_w, the area and component breakdown, and compliance checks including the Passive House target.
Can the calculator help me select a window for my climate zone?
Yes — as a screening check. The compliance panel compares your U_w against the PHI cool-temperate Passive House target (U_w ≤ 0.80), EU nZEB levels, US ENERGY STAR v7.0 and IECC 2024 zones, the Canadian NRCan ENERGY STAR requirement (U ≤ 1.22 since 2020), and Chinese GB targets. Note that PHI's other climate classes (Arctic through Warm) carry different targets. For Passive House certification specifically, F1 Composite's 90 Series is PHI Component-ID 2491wi03 at U_w 0.78 — verifiable in the calculator with EN ISO 10077-1 inputs.
How does this compare with NFRC simulation in the US/Canada?
NFRC 100 (US) and CSA A440.2 (Canada) use 2D thermal simulation with WINDOW/THERM at fixed model sizes and different boundary conditions, rather than the simplified EN ISO 10077-1 approach. For symmetric frame profiles results typically agree within ±0.1–0.2 W/m²·K, but the ratings are not interchangeable — an EN ISO 10077-1 value cannot be quoted as an NFRC U-factor. For NFRC certification, F1 Composite supplies frames with NFRC-compliant simulations on request.
Explore our complete range of pultruded FRP window and door frame systems — 65/70/80/90-series with Uf values from 0.85 to 1.5 W/m²K.
View Fenestration Systems →Want help spec'ing a passive house window?
Open the FRP Engineering Advisor with your climate zone and U-value target — it will match an F1 Composite series, suggest glazing, and outline the certification path.
Pre-filled question: “I'm using the U-Value Calculator on F1 Composite (/technology/u-value-calculator). Walk me through how to choose realistic inputs and how to interpret the result for an FRP project.”
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