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.
Last updated: