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.
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.
Referenced by: IECC / ASHRAE 90.1 (US) · CSA A440 / NEB (Canada) · EPBD (EU) · GB 50189 / GB 50176 (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
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 | Passive House Premium | ≤ 0.80 | PHI certified; whole-window including installation |
| Europe | EN ISO 10077-1 | Passive House Classic | ≤ 0.85 | PHI certified; frame + glass + spacer |
| Europe | EPBD 2024 / EN 15603 | nZEB (Central Europe) | ≤ 1.30 | Near-zero energy building directive; varies by member state |
| Europe | EN 14351-1 | CE Marking baseline | Declared | No max limit — declared value for CE marking |
| USA | IECC 2024 / ASHRAE 90.1 | Zone 1 (Miami) | ≤ 3.69 | Climate Zone 1; very hot humid |
| USA | IECC 2024 / ASHRAE 90.1 | Zone 4 (New York) | ≤ 1.99 | Climate Zone 4; mixed humid |
| USA | IECC 2024 / ASHRAE 90.1 | Zone 5 (Chicago) | ≤ 1.99 | Climate Zone 5; cold |
| USA | IECC 2024 / ASHRAE 90.1 | Zone 7-8 (Alaska) | ≤ 1.70 | Climate Zone 7–8; very cold / subarctic |
| USA | ENERGY STAR v7.0 | Northern Zone | ≤ 1.70 | NFRC-rated; most stringent US program |
| USA | ENERGY STAR v7.0 | North-Central Zone | ≤ 1.82 | NFRC-rated |
| USA | ENERGY STAR v7.0 | South-Central Zone | ≤ 2.27 | NFRC-rated; SHGC also applies |
| USA | ENERGY STAR v7.0 | Southern Zone | ≤ 2.27 | NFRC-rated; SHGC ≤ 0.25 primary |
| Canada | CSA A440 / NEB 2024 | Zone A (Coldest) | ≤ 1.20 | NRCan ENERGY STAR; triple-glazed typical |
| Canada | CSA A440 / NEB 2024 | Zone B | ≤ 1.40 | NRCan ENERGY STAR |
| Canada | CSA A440 / NEB 2024 | Zone C (Mildest) | ≤ 1.60 | NRCan ENERGY STAR; Southern BC / Southern ON |
| Canada | NBC 2020 / NECB | Zone 7A (most of Canada) | ≤ 1.60 | National Building Code prescriptive |
| China | GB 50189-2015 | Severe Cold (哈尔滨) | ≤ 1.50 | Public buildings; residential may differ per GB 50176 |
| China | GB 50189-2015 | Cold (北京) | ≤ 2.00 | Public buildings |
| China | GB 50189-2015 | Hot Summer Cold Winter (上海) | ≤ 2.30 | Public buildings; SHGC also regulated |
| China | GB 50189-2015 | Hot Summer Warm Winter (广州) | ≤ 3.00 | Public buildings; SHGC primary concern |
| China | GB/T 8484-2020 | Test method | — | Standard test method for window thermal performance |
| China | GB 50176-2016 | Residential (Severe Cold) | ≤ 1.50 | Residential building thermal design code |
| China | GB 50176-2016 | Residential (Cold) | ≤ 2.00 | Residential building thermal design code |
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 0.10–0.15 W/m²·K — 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 W/m·K — roughly 1/170 of aluminum (160 W/m·K) and 1/3 of PVC. Even thermally broken aluminum with polyamide isolators retains a metallic continuous path that limits frame U-value to roughly 1.6–2.5 W/m²·K. Pultruded FRP frames achieve frame U-value of 0.9–1.4 W/m²·K with no thermal break required, because the entire frame is intrinsically insulating.
What inputs does the calculator need?
Frame type (FRP, aluminum, PVC, timber), frame depth (65/70/80/90/140 mm series), glazing configuration (double / triple / quadruple, glass thickness, gas fill), spacer type (aluminum or warm-edge), frame/glass area ratio, and unit dimensions. The calculator returns U_w whole-window, frame and glass component U-values, and a Passive House compliance check.
Can the calculator help me select a window for my climate zone?
Yes. PHI climate classes (Arctic, Cold, Cool-Temperate, Warm-Temperate, Warm) have different U_w requirements. The calculator flags whether the configuration meets the standard target for each PHI class. For Passive House certification specifically, F1 Composite's 90 Series is PHI Component-ID 2491wi03 at U_w 0.78 — the calculator confirms this against 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 rather than the simplified EN ISO 10077-1 approach. Results typically agree within ±5% for symmetric frame profiles. 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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