How do you use this tool?
- Pick the region: US (IECC 2021, R-Value in ft²·°F·hr/BTU — higher is better) or DACH (GEG 2024, U-value in W/m²K — lower is better).
- Pick the building element from ten options (exterior wall EWI / rainscreen / interior wall / pitched roof between or above rafter / flat roof / top-floor ceiling / floor slab / basement wall / perimeter).
- Pick the material from ten standard insulators (fiberglass / mineral wool / EPS / XPS / PUR/PIR / wood fibre / aerogel / hemp / flax / cellulose) or let smart-suggest pick by element.
- Set the insulation thickness in mm — or click „Auto thickness“ and the calculator returns the minimum thickness for IECC compliance in your climate zone (or GEG conformity).
- Output: U-value + R-SI + R-Value Imperial in parallel, plus a traffic-light compliance card (green / yellow / red). Optional: multi-element builder with material volume for the shopping list.
What does this calculator do?
The insulation calculator gives you everything you need for an insulation decision: R-Value (US convention) and U-value (DACH convention) in parallel, traffic-light compliance against IECC 2021 or GEG 2024, smart-suggest for material choice by element, and a multi-element builder with material volume for the shopping list. Plus the one reminder that saves most retrofit projects: IECC compliance is the minimum, not the optimum — a licensed energy professional should sign off on a real building envelope design. The calculator gives the upfront estimate, not the engineering certificate.
Five differentiating features no other insulation calculator integrates this cleanly: bidirectional R-Value ↔ U-value as headline feature (US and DACH in one tool, conversion formula visible), IECC 2021 + GEG 2024 traffic-light compliance (green / yellow / red — automatic by element and climate zone), layered assembly with existing R-Value (additive R math, retrofit-realistic), material smart-suggest per element (vendor-neutral, material classes only) and a multi-element builder with material volume (shopping list in m³ for any layered assembly). All math runs locally in your browser — no upload, no tracking, no data leaves your device.
The calculator follows ASTM C518 (Heat Flow Meter Method) for material thermal conductivity, IECC 2021 for US residential R-Value requirements by climate zone, and on the DACH side DIN EN ISO 6946 for U-value calculation and the German GEG 2024 building energy law for minimum requirements.
R-Value or U-value — which means what?
The two worlds of insulation rating use different quantities, which routinely drives US installers crazy when they read European datasheets and vice versa.
R-Value (Thermal Resistance, US convention, also SI variant): Imperial unit ft²·°F·hr/BTU, SI unit m²·K/W. Measures the thermal resistance of the assembly. Higher is better — more resistance to heat flow. Range from R-3 (1 inch EPS) to R-60 (polar wall). US standard per ASTM C518 (material) and IECC (assembly requirement).
U-value (Thermal Transmittance, DACH/EU convention): Unit W/(m²·K) — Watts per square meter of assembly area per Kelvin temperature difference. Measures heat flow through the assembly. Lower is better — less heat loss. Range from 0.10 (passive-house wall) to 5.8 (single-pane window). Standard per DIN EN ISO 6946.
Conversion formulas:
R_SI (m²K/W) = R_Imperial (ft²·°F·hr/BTU) × 0.1761
R_Imperial = R_SI × 5.678
U (W/m²K) = 1 / R_SIExample: A 4-inch fiberglass batt (λ ≈ 0.044 W/mK) gives R_SI = 0.1016 ÷ 0.044 = 2.31 m²K/W. That’s R-13 in US notation (2.31 × 5.678) and U-value 0.43 W/m²K — typical for a 2x4 stud wall cavity, well short of IECC Zone 5’s R-30 wall requirement.
What insulation materials are there?
Ten vendor-neutral standard insulators with convergent λ values from multiple sources (DIN 4108-4, BAUNETZ-WISSEN, baustoffkatalog.eu, BPIE database). Where sources disagree, the median value is used.
| Material | λ (W/mK) | R/inch (Imperial) | Typical use |
|---|---|---|---|
| Fiberglass batt | 0.035 | R-3.7 | US/DACH residential standard, A1 non-combustible |
| Mineral wool (stone) | 0.036 | R-3.6 | Fire / sound protection, A1, façade and ceiling |
| EPS (polystyrene) | 0.034 | R-3.8 | EWI standard, B-s1,d0, NOT for floors (moisture) |
| XPS (extruded) | 0.032 | R-4.0 | Perimeter / slab / below-grade, compressive, moisture-resistant |
| PUR / PIR | 0.025 | R-5.1 | High-performance, thinnest assembly, B-s1,d0 |
| Wood fibre | 0.042 | R-3.1 | Eco / summer thermal mass |
| Aerogel | 0.020 | R-6.4 | Premium, thinnest interior retrofit, very expensive |
| Hemp | 0.042 | R-3.1 | Bio, capillary-active, regional |
| Flax | 0.043 | R-3.0 | Bio, odor-neutralizing |
| Cellulose loose-fill | 0.040 | R-3.2 | Recycled paper, blown into cavities |
The calculator shows λ directly in the material picker. With smart-suggest enabled, switching elements auto-selects the standard material — manual override is always available.
What does IECC 2021 require by climate zone?
The International Energy Conservation Code 2021 (Table R402.1.3) defines R-Value requirements for residential construction by climate zone. Eight zones, from 1 (hot-humid, Miami) to 8 (subarctic, Alaska).
| Zone | Example cities | Wall R | Ceiling R | Floor R |
|---|---|---|---|---|
| 1 | Miami, Honolulu, Puerto Rico | R-13 | R-30 | R-13 |
| 2 | Houston, So. California, Vegas | R-13 | R-49 | R-13 |
| 3 | Atlanta, Phoenix, Dallas, San Francisco | R-20 | R-49 | R-19 |
| 4 | NYC, DC, Seattle, Kansas City | R-30 | R-60 | R-19 |
| 5 | Boston, Chicago, Denver, Pittsburgh | R-30 | R-60 | R-30 |
| 6 | Vermont, Minneapolis, Bismarck | R-30 | R-60 | R-30 |
| 7 | Northern MN, Northern ND | R-30 | R-60 | R-38 |
| 8 | Alaska Interior | R-30 | R-60 | R-38 |
Pick „US” region in the calculator and your zone — the compliance card auto-checks against IECC 2021. Example: Zone 5 wall R-30 needs at least 7.3 inches of fiberglass (R-3.7/inch) or 5.2 inches of XPS (R-5.0/inch). UK construction follows ADL-2A approved-document U-values rather than IECC; both are conceptually equivalent (UK uses U-values like DACH). AU follows BCA/NCC R-Values like the US.
What does GEG 2024 require for U-values?
The German Building Energy Law (GEG 2024) sets these maximum U-values for residential new builds (Annex 1 Table 1):
| Element | U_max GEG (W/m²K) | KfW EH 55 | KfW EH 40 | Min. thickness at λ = 0.035 |
|---|---|---|---|---|
| Exterior wall | 0.24 | 0.20 | 0.15 | 146 / 175 / 233 mm |
| Pitched roof | 0.20 | 0.14 | 0.12 | 175 / 250 / 292 mm |
| Flat roof | 0.20 | 0.14 | 0.12 | 175 / 250 / 292 mm |
| Floor slab / basement ceiling | 0.30 | 0.25 | 0.18 | 117 / 140 / 194 mm |
| Top-floor ceiling | 0.24 | 0.20 | 0.14 | 146 / 175 / 250 mm |
For retrofit (GEG §48) the same wall U ≤ 0.24, roof U ≤ 0.20 and top-floor ceiling U ≤ 0.24 apply. KfW funding tiers (EH-55, EH-40) tighten the bar by 30 % and 50 % respectively — the calculator surfaces these as hint cards when your design is below the basic GEG threshold.
How does layered assembly add up R-value?
A real assembly has multiple layers — siding, sheathing, cavity insulation, drywall. The R-Value of the whole assembly is the sum of layer R-Values: R_total = ΣR_layer. Then U = 1 ÷ R_total.
Retrofit example: existing solid brick wall (36 cm at λ ≈ 0.81) gives R_existing = 0.36 ÷ 0.81 = 0.44 m²K/W. To meet GEG outer wall U ≤ 0.24, you need R_total ≥ 4.17 m²K/W. The new insulation must contribute R_new ≥ 3.7 m²K/W → at fiberglass λ = 0.035 that’s 130 mm instead of the bare-new-build 146 mm. The calculator subtracts existing R automatically once you enter it in the „existing layer” accordion.
This additive logic is why flat „rule-of-thumb” calculators often recommend 5–15 cm too much insulation, and you end up buying more material than needed. US R-X+Yci notation (cavity + continuous insulation) is the same idea expressed differently — total R, contributed by stacked layers.
What are typical worked examples?
Example 1 — GEG-conforming exterior wall with fiberglass: Region DACH, element exterior wall EWI, material fiberglass (λ = 0.035), thickness 150 mm. R_new = 0.15 ÷ 0.035 = 4.29 m²K/W → U = 0.233 W/m²K. GEG requires U ≤ 0.24 → green compliance, 3 % below limit. At 100 m² façade: 15 m³ of fiberglass.
Example 2 — IECC Zone 5 wood-frame wall: Region US, element exterior wall, climate zone 5 (Boston/Chicago), material fiberglass batt, thickness 185 mm (≈ 7.3 inches). R_SI = 0.185 ÷ 0.035 = 5.29 → R_Imperial = 30.0. IECC Zone 5 wall requires R-30 → green compliance, exactly at limit. Sufficient for basic IECC; local codes may exceed.
Example 3 — Above-deck retrofit with PUR: Region DACH, element pitched roof above-rafter, material PUR (λ = 0.025), thickness 200 mm. R_new = 0.20 ÷ 0.025 = 8.0 m²K/W → U = 0.125 W/m²K. GEG roof U ≤ 0.20 → green compliance, 38 % below limit, KfW-EH-55 eligible. At 150 m² roof area: 30 m³ of PUR (over double the fiberglass volume thanks to lower λ).
Which other construction tools are related?
For supporting tasks around a construction or retrofit project: tile calculator for bath and kitchen, wallpaper calculator for wall coverings, laminate calculator for floor covering. Specialty calculators for heat-load (DIN EN 12831), passive-house PHPP design and federal/state rebate filing will come in Phase B once first-week search-console data confirms real demand.
Where are the frequently asked questions?
The page header includes eight FAQ entries with schema.org/FAQPage markup, sourced from Google „people also ask” patterns for insulation queries. Each answer responds to its question in the first ten words — voice-search-optimized for smart speakers and AI search assistants.
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