Window U-value calculator
Calculate the whole-window U-value (Uw) from the frame, the double or triple glazing (Ug) and the edge seal, with the EN ISO 10077-1 simplified method. Compare FRP, aluminum, PVC and timber frames against clearly labeled numeric targets, or load the locked PHI certificate reference.
- Method
- EN ISO 10077-1
- Frames compared
- 10
- Numeric targets
- 14
Numeric targets compared for screening only: PHI (EU) · England · Germany · IECC / ENERGY STAR (US) · ENERGY STAR Canada / NBC · New Zealand · GB (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/m²·K
- Glass Ug
- 0.6 W/m²·K
- Spacer Ψg
- 0.04 W/m·K
vs aluminum (no break)
44% better thermal performance compared to an aluminum frame without thermal break (Uw = 1.62 W/m²·K).
Numeric target comparison, not compliance
This compares numbers only. EN ISO 10077-1 output is not an NFRC, CSA A440.2 or AFRC rating; ENERGY STAR also requires SHGC and certified product data; England accepts a window energy rating instead of the U-value for replacements; Chinese acceptance requires the applicable project limit and GB/T 8484 test evidence.
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
Uw = (Ag·Ug + Af·Uf + lg·Ψg) / (Ag + Af)
= (1.13·0.60 + 0.55·1.20 + 4.27·0.04) / (1.13 + 0.55)
= (0.68 + 0.66 + 0.17) / 1.68
= 1.51 / 1.68
Uw = 0.90 W/m²·K
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.
Choose your window supply route
Use this simplified U-value result to discuss your target and assembly. It is not finished-window certification; confirm the required rating or test evidence for your project.
Profiles for window fabricators
Review FRP profiles for local fabrication and discuss frame series, glazing and assembly requirements.
Finished fiberglass windows and doors
Define opening sizes, quantities, project location and required evidence for a finished-unit quotation.
FRP windows buyer guide
Compare supply options and the specification checks to complete before requesting a quote.
How to calculate the U-value of a window
Worked example
A passive house window
A project specifies a 1230 × 1480 mm fixed window with triple glazing (Ug 0.6) and a standard warm-edge spacer (Ψg 0.04). With the calculator’s frame values, aluminum with a polyamide thermal break (Uf 3.2 W/m²·K) gives Uw ≈ 1.12, multi-chamber PVC (Uf 1.5) gives ≈ 0.87 and the F1 FRP 90-series (Uf 0.85) gives ≈ 0.76 W/m²·K.
The FRP result is numerically below 0.80, but that alone does not make the assembly PHI-certified; the certified reference is the locked PHI preset. At this size the frame covers 16–20% of the window, which is why realistic dimensions matter.
Reading the result
- The frame dominates on small windows. On a 600 × 900 mm fixed light the frame covers 35–40% of the area, and more with an opening sash, so the frame Uf drives Uw more than the glazing does. On a 2400 × 2400 mm picture window the frame is under 15% of the area and Ug dominates. Calculate with the real dimensions.
- The spacer matters more than it looks. Switching from an aluminum spacer (Ψg 0.08) to a warm-edge spacer (Ψg 0.03–0.04) lowers Uw by about 0.05–0.2 W/m²·K, more on small windows. It is often the cheapest single improvement.
- Triple glazing needs a frame to match. With Ug 0.6 glass in an aluminum frame (Uf 3.2), the frame of a 1230 × 1480 mm fixed light loses as much heat as all of the glass. An insulating frame keeps the glazing upgrade visible in Uw.
Common specification mistakes
Quoting Uf instead of Uw
Datasheets often give the frame-only Uf or the center-of-glass Ug. Energy codes and PHI certification use the whole-window Uw, so ask for Uw at the size being installed.
Leaving out the installation Ψ
The joint between window and wall adds another linear thermal bridge, typically 0.02–0.10 W/m·K. Strict certification schemes count it separately. This calculator gives the assembly Uw, not the installed Uw,inst.
Assuming thermally broken aluminum is good enough
Even premium multi-break aluminum frames rarely reach Uf below 1.4 W/m²·K, so passive house targets need deep, insulated aluminum systems. FRP, timber and multi-chamber PVC reach them with simpler sections.
Referenced standards
- EN ISO 10077-1: thermal transmittance of windows, doors and shutters, general method
- EN ISO 10077-2: numerical method for frames
- ISO 15099: detailed calculation of windows, doors and shading devices
- NFRC 100: fenestration product U-factors (North America)
- Passive House Institute certified components: Uw ≤ 0.80 W/m²·K, cool-temperate
Frame values used
| Frame | Uf (W/m²·K) | Depth (mm) | Face width (mm) | Thermal break |
|---|---|---|---|---|
| F1 FRP 65-Series (65 mm, 2-chamber) | 1.40 | 65 | 54 | No, the material insulates |
| F1 FRP 70-Series (70 mm, 3-chamber) | 1.20 | 70 | 58 | No, the material insulates |
| F1 FRP 80-Series (80 mm, 3-chamber) | 1.00 | 80 | 65 | No, the material insulates |
| F1 FRP 90-Series (90 mm, 3-chamber) | 0.85 | 90 | 72 | No, the material insulates |
| PHI certificate 2491wi03 — Fengdu Passive GFRP 90 | 0.78 | 90 | 109 | No, the material insulates |
| Aluminum (no thermal break) | 5.90 | 65 | 50 | No, but aluminum conducts heat freely |
| Aluminum (polyamide break) | 3.20 | 70 | 55 | Yes, a polyamide strip |
| PVC, multi-chamber | 1.50 | 70 | 62 | No, but the section is less stiff |
| PVC, steel-reinforced | 1.80 | 70 | 65 | The steel core bridges the chambers |
| Timber (softwood, 68 mm) | 1.40 | 75 | 65 | No, the material insulates |
Window U-value reference targets
| Region | Standard | Climate zone or tier | Max Uw (W/m²·K) | Notes |
|---|---|---|---|---|
| Europe | EN ISO 10077-1 / PHI | Passive House window, cool-temperate | ≤ 0.80 | PHI component criterion; Uw,installed ≤ 0.85. Cold climate 0.60 (0.65 installed), warm-temperate 1.00 (1.05) |
| Europe | EPBD (EU) 2024/1275 | All member states | National | No EU-wide window limit; member states set element limits in national law |
| Europe | EN 14351-1 | CE marking | Declared | No maximum; the declared Uw goes on the declaration of performance |
| Germany | Gebäudemodernisierungsgesetz (GModG, formerly GEG), Anlage 7 | Windows replaced in existing buildings | ≤ 1.30 | Roof windows 1.40; applies when windows in an existing building are replaced or renewed. The GEG was renamed GModG from 29 July 2026 (BGBl. 2026 I Nr. 226); these window values did not change |
| UK (England) | Approved Document L 2021 | New dwellings, limiting value | ≤ 1.60 | Notional dwelling uses 1.2; compliance is by whole-dwelling targets. The 2026 edition (Future Homes Standard, published 24 March 2026) applies from 24 March 2027, keeps the 1.2 notional value and rates each window at its actual size |
| UK (England) | Approved Document L 2021 | Replacement windows, existing dwellings | ≤ 1.40 | Or window energy rating band B; industry guidance expects the replacement-window rules to stay essentially unchanged in the 2026 edition |
| USA | IECC 2024 / IRC N1102 | Zones 3 and 4 (except Marine) | ≤ 1.70 | U ≤ 0.30 Btu/h·ft²·°F |
| USA | IECC 2024 / IRC N1102 | Zones 5–6 and Marine 4 | ≤ 1.59 | U ≤ 0.28; 0.30 allowed above 4,000 ft elevation or in windborne-debris regions |
| USA | IECC 2024 / IRC N1102 | Zones 7–8 | ≤ 1.53 | U ≤ 0.27 (0.30 in the 2021 edition) |
| USA | ENERGY STAR v7.0 (2023) | Northern | ≤ 1.25 | U ≤ 0.22; SHGC ≥ 0.17 on the prescriptive path |
| USA | ENERGY STAR v7.0 (2023) | North-Central | ≤ 1.42 | U ≤ 0.25; SHGC ≤ 0.40 |
| USA | ENERGY STAR v7.0 (2023) | South-Central | ≤ 1.59 | U ≤ 0.28; SHGC ≤ 0.23 |
| USA | ENERGY STAR v7.0 (2023) | Southern | ≤ 1.82 | U ≤ 0.32; SHGC ≤ 0.23 |
| Canada | ENERGY STAR Canada v5.0 | All of Canada (one zone since 2020) | ≤ 1.22 | Or energy rating ER ≥ 34; check NRCan for the version in force |
| Canada | NBC 2020, 9.36.2.7 | Zones 4–5 / 6–7A / 7B–8 | 1.80 / 1.60 / 1.40 | Houses and small buildings, prescriptive path; NBC 2025 (December 2025) is adopted province by province, so check the edition in force |
| Australia | NCC (Section J, Part H6) | Climate-zone dependent | No single limit | Glazing is assessed with the wall or through NatHERS; product ratings come from AFRC |
| New Zealand | H1/AS1 6th edition (Nov 2025) | Zones 1–4 / 5–6 (housing) | ≈ 2.17 / 2.00 | Reference-building window R-values R0.46 / R0.50 in the calculation method (the schedule method was removed). The 5th edition can be used until 26 November 2026 |
| China | GB 55015-2021 | All zones, mandatory since April 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 |
This calculator uses the EN ISO 10077-1:2017 simplified method. US ratings follow NFRC 100 and Canadian ratings CSA A440.2, both by simulation (THERM and WINDOW) at fixed model sizes and boundary conditions; Australian ratings come from AFRC, which uses NFRC-based methods. Their values are not interchangeable with EN ISO 10077-1 results. From March 2027 the Future Homes Standard in England asks for U-values calculated at the actual window size. For Chinese compliance, U-values are verified by the GB/T 8484-2020 hot-box test against the limits of the mandatory GB 55015-2021. The values are indicative; confirm the edition in force with the local authority.
Frequently asked questions
Is this U-value calculator free, and does it follow PHI / Passive House standards?
The calculator is free and uses the EN ISO 10077-1 simplified whole-window formula. It can compare a result numerically with the PHI cool-temperate Uw ≤ 0.80 W/m²·K criterion, but that comparison is not certification. A separate locked preset reproduces the published inputs for PHI Component-ID 2491wi03: Fengdu Passive GFRP 90 Series, 1.23 × 1.48 m, Uf 0.78, Ug 0.70 and Ψg 0.023, giving the certified Uw 0.78 after rounding.
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 Uf 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 Uf ≈ 2.8–3.4 W/m²·K for standard polyamide-break systems (premium multi-break systems ≈ 1.6–2.5). Pultruded FRP frames achieve Uf of about 0.78–1.4 W/m²·K (65- to 90-series, 0.78 on the PHI-certified 90 series) with no metal 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 Uw, the area and component breakdown, and clearly labeled numeric target comparisons.
Can the calculator help me select a window for my climate zone?
Yes, for numerical screening. The target panel compares Uw with the PHI criteria, the England Approved Document L 2021 limits, the German limit for replaced windows, US ENERGY STAR v7.0 and IECC 2024 U-factor limits, ENERGY STAR Canada and the NBC 2020 prescriptive values, the New Zealand H1/AS1 values and selected Chinese GB limits. The EU itself sets no window U-value; member states do. The panel does not determine compliance: NFRC, CSA A440.2 and AFRC use different rating procedures, ENERGY STAR also requires SHGC, and Chinese acceptance depends on the project zone, window-to-wall ratio, and test evidence.
How do I convert a U-value from metric to imperial?
Multiply the metric value by 0.1761: 1 W/m²·K equals 0.1761 BTU/(h·ft²·°F). A Uw of 0.78 W/m²·K is therefore about 0.14 BTU/(h·ft²·°F). The conversion changes the units only; an EN ISO 10077-1 result is still not an NFRC U-factor.
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 their own boundary conditions, rather than the simplified EN ISO 10077-1 approach. The two results can differ by more than rounding and 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.
Send the window sizes and U-value target with your RFQ
Send the project climate, window types and sizes, the target U-value and whether you need finished units or profiles for local fabrication. We reply within one business day.