Insulation R/U-value Calculator
Region (Central1·Central2·South·Jeju) × element (wall/roof/floor) × layered build-up → composite R and U value with auto code compliance check.
Result
Set region, element, and layers then click Calculate.
What this tool does
The insulation R-value/U-value calculator computes thermal resistance (R) and thermal transmittance (U) from a wall assembly and judges pass/fail against Korean Building Energy Conservation Code (Appendix 1) zonal and component limits. Use it for new-build insulation thickness, passive house design, and energy efficiency grading.
Who uses this
- New-build design: thickness to pass Appendix 1 U-value limits
- Renovation: R-value improvement when adding insulation to existing walls
- Passive house: back-calculate ultra-low target U-value
- Condensation check: surface temperature and risk for under-insulated areas
- Energy grading: baseline data for building energy efficiency certification
How to use (3 steps)
- 1Enter insulation type and thickness. Material thermal conductivity (λ) for glasswool, XPS, EPS, urethane, etc. is applied automatically.
- 2Add the wall assembly (drywall, plywood, cladding) layer by layer. R-values sum across each layer's thickness and conductivity.
- 3Select zone (Central-1/Central-2/Southern/Jeju) and component (wall/roof/floor) to compare against Appendix 1 limits and see pass/fail.
R-value / U-value formula (Energy code)
Layer resistance R = thickness(m) / conductivity λ(W/m·K) Example: glasswool 100mm, λ=0.037 → R = 0.1/0.037 = 2.70 ㎡·K/W Total resistance R_total = R_inside + ΣR_layers + R_outside (Inside surface 0.11, outside 0.043 ㎡·K/W standard) Thermal transmittance U = 1 / R_total (W/㎡·K) → lower U = better insulation Appendix 1 wall U-value limits (2026): Central-1: 0.150 / Central-2: 0.170 / Southern: 0.220 / Jeju: 0.290 W/㎡·K (Roof is stricter, floor varies by location)
Real examples
Example 1: Central-2 wall — glasswool 100mm
R = inside 0.11 + glasswool (0.1/0.037=2.70) + drywall (0.0125/0.18=0.07) + outside 0.043 = 2.92. U = 1/2.92 = 0.342. Central-2 limit 0.170 → FAIL! Need thicker.
Example 2: Same wall — glasswool upgraded to 220mm
R = 0.11 + (0.22/0.037=5.95) + 0.07 + 0.043 = 6.17. U = 1/6.17 = 0.162. Central-2 limit 0.170 PASS ✓. About 220mm insulation needed.
Example 3: Passive house target U=0.10
U=0.10 → R_total=10 needed. Glasswool alone ~360mm, or high-performance urethane (λ=0.023) 230mm. Combine exterior+interior insulation to minimize thermal bridging.
Frequently asked questions
R-value or U-value — which higher is better?+
Higher R (resistance) is better; lower U (transmittance) is better. They're reciprocals (U=1/R). Korea regulates by U-value limit; the US often labels by R-value.
How are Appendix 1 zones determined?+
Central-1 (parts of Gangwon, northern Gyeonggi), Central-2 (Seoul, most Gyeonggi, Chungcheong), Southern (Jeolla, Gyeongsang, Busan), Jeju. Colder zones have stricter (lower) U-value limits. Check the city/county table for exact zones.
Are thermal bridges accounted for?+
No — this calculator uses 1D R-value summation and ignores thermal bridges through framing (studs, concrete beams). Actual U-value can be 10-30% worse, so precise design needs thermal bridge analysis (e.g., THERM).
Why add surface resistances?+
Air boundary layers at wall surfaces add insulation. Inside 0.11, outside 0.043 ㎡·K/W are standard. Small but must be included for an accurate U-value.
Do insulation materials differ much?+
Yes. Lower λ means the same performance in less thickness. Glasswool 0.037, EPS 0.034, XPS 0.028, urethane 0.023, phenolic foam 0.020 W/m·K. Urethane achieves the same as glasswool at ~60% thickness.
How do I judge condensation?+
Insufficient insulation drops the interior surface below dew point, causing condensation. Surface temp = indoor temp − (indoor-outdoor diff × inside resistance / R_total). It must stay above dew point. Passing Appendix 1 usually prevents condensation too.
Cautions
- •Uses 1D R-value summation. Thermal bridges (studs, beams) ignored — actual U-value may be worse.
- •Appendix 1 limits vary by zone, component, and year. Current as of 2026.
- •Surface and cavity resistances use standard values; precise design uses measured values.
- •Condensation judgment is simplified. Precise review needs steady-state heat/moisture analysis.
- •Final design requires review by an architect or energy assessor.
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Last reviewed: 2026-05-30