Engineering Tool

Window U-Value Calculator

Calculate the whole-window thermal transmittance (Uw) per EN ISO 10077-1. Select frame material, glazing configuration, spacer type, and window dimensions to compare thermal performance across materials.

Calculator

Whole-Window U-Value Calculator

Calculate the overall thermal transmittance (U_w) of a window or door unit per EN ISO 10077-1. Select frame material, glass configuration, spacer type, and window dimensions.

Frame SystemU_f = 1.2 W/m²K

Glass ConfigurationU_g = 0.6 W/m²K · 40 mm thick

Edge SpacerΨ_g = 0.04 W/mK

Window TypeSash width: 30 mm

Window Dimensions (mm)

Width

Height

EN ISO 10077-1: U_w = (A_g·U_g + A_f·U_f + l_g·Ψ_g) / (A_g + A_f)
Referenced by: IECC / ASHRAE 90.1 (US) · CSA A440 / NEB (Canada) · EPBD (EU) · GB 50189 / GB 50176 (China)

Whole-Window U_w0.88W/m²KPassive House / nZEB

Breakdown

Glass area (A_g): 1.20 m²
Frame area (A_f): 0.48 m²
Glass ratio: 71%
Glass perimeter (l_g): 4.40 m
Frame U_f: 1.2 W/m²K
Glass U_g: 0.6 W/m²K
Spacer Ψ_g: 0.04 W/mK

vs Aluminium (no break)

52% better thermal performance compared to an aluminium frame without thermal break (U_w = 1.84 W/m²K).

Standards Compliance

🇪🇺 EU Passive House≤ 0.80

🇪🇺 EU nZEB Directive≤ 1.30 ✓

🇺🇸 US ENERGY STAR North≤ 1.70 ✓

🇺🇸 US ENERGY STAR South≤ 2.27 ✓

🇨🇦 CA Zone A (Coldest)≤ 1.20 ✓

🇨🇦 CA Zone C≤ 1.60 ✓

🇨🇳 CN Severe Cold Zone≤ 1.50 ✓

🇨🇳 CN Cold Zone≤ 2.00 ✓

🇨🇳 CN Hot Summer Cold Winter≤ 2.30 ✓

Frame Material U_f Comparison

Frame thermal transmittance values used in this calculator. FRP frames achieve low U_f values inherently — no thermal break inserts required.

Frame Material	U_f (W/m²K)	Frame Width (mm)	Thermal Break Required
F1 FRP 60-Series (60 mm, 2-chamber)	1.5	60	No — inherently insulating
F1 FRP 70-Series (70 mm, 3-chamber)	1.2	70	No — inherently insulating
F1 FRP 80-Series (80 mm, 3-chamber)	1	80	No — inherently insulating
F1 FRP 90-Series (90 mm, 3-chamber)	0.85	90	No — inherently insulating
Aluminium (no thermal break)	5.9	65	No — but very high conductivity
Aluminium (polyamide break)	3.2	70	Yes — polyamide strip
PVC Multi-chamber	1.5	70	No — but lower stiffness
PVC Steel-reinforced	1.8	70	Steel core creates thermal bridge
Timber (softwood, 68 mm)	1.4	75	No — inherently insulating

Window U-Value Requirements by Standard

Maximum allowable whole-window thermal transmittance (U_w) under major international building energy codes. Values shown are for residential windows unless noted.

Region	Standard	Climate Zone / Tier	Max U_w (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

Calculation standard: This calculator uses the EN ISO 10077-1 simplified method. For NFRC (US/Canada) ratings, use NFRC 100 simulation software (THERM + WINDOW). For Chinese GB compliance, U-values should be verified per GB/T 8484-2020 hot-box test. Values above are indicative — always confirm with the applicable edition and local authority.

Explore our complete range of pultruded FRP window and door frame systems — 60/70/80/90-series with U_f values from 0.85 to 1.5 W/m²K.

View Fenestration Systems →

Need help selecting the right frame and glass for your project?

Our engineering team is ready to help you find the right FRP solution. Get in touch for technical consultation or a detailed quotation.

Request a Quote Browse Products