How do you use this tool?
- Pick the window type: single-sash (1.23 × 1.48 m), double-sash (1.48 × 2.18 m), balcony door (1.00 × 2.20 m), or enter custom dimensions.
- Pick frame material and frame width: PVC ≈ 7 cm Uf 1.1, wood ≈ 8 cm Uf 1.3, wood-aluminium ≈ 8 cm Uf 1.2, thermally broken aluminium ≈ 9 cm Uf 1.5.
- Pick glazing: triple-glazed standard (Ug 0.7) for GEG retrofit, triple-glazed premium (Ug 0.5) for passive house, vacuum glass (Ug 0.5) for heritage retrofit.
- Pick the glass-edge spacer class: aluminium (Ψg 0.08) up to top-tier warm edge (Ψg 0.025). Custom datasheet values are editable.
- Output: Uw value, glass-versus-frame area split visualisation, three-tier subsidy check (GEG ≤ 1.30 / KfW EH 55 ≤ 0.95 / passive house ≤ 0.80) — all copy-paste-ready.
What does this window Uw calculator do?
The calculator gives you the Uw value of your window per DIN EN ISO 10077-1 — the unified European standard for whole-window thermal transmittance. Inputs are window dimensions, frame material and frame width, glazing (Ug value) and glass-edge spacer (Ψg value). Outputs are the Uw value, the glass-versus-frame area split, plus a three-tier compliance card stack: GEG 2024 (German Building Energy Act), KfW EH 55, and passive-house standard.
Three differentiators no other online calculator integrates this cleanly: live area-split visualisation showing the actual glass-to-frame ratio at your dimensions — addressing the common pain that smaller windows hit worse Uw values with the same components; vendor-neutral spacer-edge classification (aluminium / stainless hybrid / premium warm edge / top tier) — datasheet values without brand-name lock-in; three-tier subsidy classification (GEG / KfW / passive house) with clear thresholds — no blurring between regulatory and certification levels.
All math runs locally in your browser. No upload, no tracking, no data leaves your device. Pure-client matches the sensitive nature of retrofit and subsidy-application data.
How is the Uw value calculated?
DIN EN ISO 10077-1 combines three thermal components into a single whole-window value. The master formula:
Uw = (Ag · Ug + Af · Uf + lg · Ψg) / (Ag + Af)Where:
- Ag = visible glazing area in m² — (b − 2·f) × (h − 2·f) for single-sash windows
- Ug = centre-pane U-value of the glazing (W/m²K) — from the manufacturer datasheet
- Af = frame area in m² (whole-window area minus glazing area)
- Uf = U-value of the frame (W/m²K) — datasheet per DIN EN ISO 10077-2
- lg = visible perimeter of the glazing in m
- Ψg = linear thermal-bridge coefficient at the glass edge (W/(m·K))
Worked example: living-room window 1.23 × 1.48 m (Aw = 1.82 m²), 7 cm PVC frame, triple glazing Ug 0.7, Uf 1.1, premium warm-edge Ψg 0.033:
- Ag = (1.23 − 0.14) × (1.48 − 0.14) = 1.09 × 1.34 = 1.46 m²
- Af = 1.82 − 1.46 = 0.36 m²
- lg = 2·(1.09 + 1.34) = 4.86 m
- Uw = (1.46 · 0.7 + 0.36 · 1.1 + 4.86 · 0.033) / (1.46 + 0.36)
- Uw = (1.022 + 0.396 + 0.160) / 1.82 = 1.578 / 1.82 ≈ 0.87 W/m²K
The window lands between KfW EH 55 (≤ 0.95) and passive house (≤ 0.80) — so KfW EH 55 eligible.
What are the GEG, KfW and passive-house thresholds?
Three clearly separated tiers with different consequences:
| Tier | Threshold | Legal basis | Typical configuration |
|---|---|---|---|
| GEG 2024 §48 | Uw ≤ 1.30 W/m²K | Retrofit minimum | Double glazing + PVC frame + stainless spacer |
| KfW EH 55 / BAFA | Uw ≤ 0.95 W/m²K | BEG-EM subsidy | Triple glazing + PVC frame + premium warm edge |
| KfW EH 40 / passive house | Uw ≤ 0.80 W/m²K | Passive-house certificate | Triple glazing + wood-alu / passive-PVC + top-tier warm edge |
Important: GEG ≤ 1.30 is a MINIMUM requirement — mandatory for every new window in an existing German building. The KfW EH 55 threshold at ≤ 0.95 is a SUBSIDY threshold — clearing it unlocks grants or amortisation bonuses. Passive-house at ≤ 0.80 is a CERTIFICATION standard — relevant for full buildings, not single windows.
The calculator shows all three tiers as stacked cards and highlights the achieved one. Concrete subsidy amounts are deliberately NOT shown — those change quarterly and a “subsidy advisor” tool would create liability risk.
Which glazing reaches which Ug value?
| Glazing | Ug value | 2024 market share | Notes |
|---|---|---|---|
| Single pane (legacy) | 5.0–6.0 | < 5 % | Only GEG-compliant in unheated buffer rooms |
| Double Low-E + argon | 1.0–1.3 | ~ 10 % | Pre-2024 baseline, barely passes GEG retrofit |
| Triple Low-E + argon (standard) | 0.5–0.8 | ~ 90 % | Market default since 2024 |
| Triple Low-E + krypton (premium) | 0.4–0.6 | ~ 5 % | Passive house, slimmer build depth |
| Vacuum glass | 0.4–0.7 | < 1 % | Heritage / old buildings, market-ready since Glasstec 2026 |
2026 trend: Vacuum glass reaches triple-glazing Ug values at only 8 mm build depth, versus 36 mm for triple glazing — critical for heritage windows where the profile depth is strictly limited. Market prices are slowly falling below 300 €/m². The calculator surfaces vacuum glass as its own glazing option.
Which frame materials are available?
| Material | Uf value | Frame width | Strengths / weaknesses |
|---|---|---|---|
| PVC (6+ chamber) | 1.0–1.4 | 6–8 cm | Market default, good price-performance, low maintenance |
| Wood (larch, oak) | 1.1–1.5 | 7–9 cm | Natural material, requires paint every 5–10 years |
| Wood-aluminium | 1.0–1.3 | 7–9 cm | Wood interior, aluminium weather shell, low maintenance |
| Aluminium (thermally broken) | 1.2–1.8 | 8–12 cm | Slim, mostly for curtain walls and large formats |
PVC remains the market default for residential buildings — over 60 % of all new windows in Germany 2024. Wood-aluminium is growing in the premium segment. Pure aluminium is mostly for commercial mullion-transom facades. The calculator covers all four classes with typical Uf values and default frame widths — manufacturer datasheet values override the defaults.
Why does the glass-edge spacer matter so much?
The spacer (Abstandhalter) holds the glass panes in insulating glass at a constant distance and seals the gas cavity (argon, krypton, vacuum). Its thermal conductivity controls the linear thermal bridge Ψg at the glass edge — and can shift the Uw value by up to 0.1 W/m²K.
| Class | Ψg value (W/m·K) | Material | Subsidy eligibility |
|---|---|---|---|
| Aluminium spacer | 0.07–0.10 | Aluminium | GEG borderline, NOT KfW |
| Stainless hybrid | 0.038–0.047 | Stainless + plastic | KfW EH 55 |
| Premium warm edge | 0.029–0.036 | Optimised plastic hybrid | KfW EH 40 typical |
| Top-tier warm edge | 0.022–0.028 | Highest grade | Passive house |
Common pain point: “The edge spacer is the spec that gets forgotten in the quote.” If a manufacturer offer lists no Ψg value, you’re probably getting standard stainless hybrid (Ψg ≈ 0.042) — which becomes a problem when you later apply for KfW funding. The calculator defaults to premium warm edge as the 2026 default for KfW EH 55 orders.
We deliberately don’t name spacer brands — that would create vendor bias. Manufacturer datasheet values are more informative than brand labels and can be entered as custom values in the tool.
How does window size affect the Uw value?
Smaller windows have a higher frame fraction — and the frame usually insulates worse than the glass. This is the most common pain point in DIY forums: “On larger windows I get Uw 1.1, on small ones Uw 1.4 — what’s going on?”
Physics. A 1.23 × 1.48 m window with 7 cm frame has about a 20 % frame share. The same components in a 0.80 × 0.80 m bathroom window push the frame share to about 32 %. The consequence: Uw rises by 0.05 to 0.15 W/m²K — same manufacturer, same components, different size.
The live visualisation in the calculator shows this directly as a glass-versus-frame split. As you change dimensions, the ratio shifts. That’s why DIN EN 14351-1 Annex E mandates two standard test sizes — 1.23 × 1.48 m for single-sash and 1.48 × 2.18 m for double-sash — so that manufacturers report comparable Uw values.
What does the tool NOT calculate?
Deliberately out of scope to keep scope and liability clean:
- No heating-cost or amortisation calculation. Site data plus heating-energy assumptions would trigger liability risk — “you save 480 €/year” is paid energy advice.
- No paragraph-precise subsidy application help. Rules change quarterly. Only classification, no concrete grant amounts.
- No installation thermal bridge (Ψ-install). The detail between window and wall requires DIN EN ISO 10077-2 plus CAD detail — that belongs in professional software.
- No g-value / solar-gain trade-off. Would turn the tool into an energy advisor.
- No 3D visualisation. Refined-minimalism cap — flat bento cards do the job.
- No manufacturer model database. Vendor-name lint plus data-maintenance burden would turn the tool into a marketing platform.
Which construction tools relate to this one?
For full retrofit packages: Insulation calculator for wall and roof U-values as a sanity check against the energy balance, Screed calculator for layer thicknesses during heating modernisation, Masonry calculator for wall material quantities, Concrete calculator for floor slabs and strip footings, Wallpaper calculator for interior finishing after the window swap.
A separate window-subsidy calculator (BAFA + KfW tier comparison + application checklist) is a Phase-B candidate — depending on search-volume data. If GSC shows enough subsidy queries over 6 weeks, we’ll ship it. Until then, the Uw calculator surfaces the subsidy thresholds as plausibility cards.
What do people ask about window Uw values?
The frontmatter of this page contains eight FAQ entries with schema.org/FAQPage markup, sourced from top Google “People also ask” patterns for window Uw topics. The answers address the question in the first 10 words — voice-search optimised for smart speakers and AI search assistants.
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