Insulation Performance Calculator (R/U Value)
Calculate R-value, U-value, and steady-state heat loss for sandwich panels with EPS, PUF, Rockwool, Glasswool, XPS, or PIR cores.
k = 0.035 W/(m·K) for EPS; ΔT = 20.0 K.
How this works
Thermal resistance R, transmittance U, and steady-state heat flux q follow directly from the panel's thermal conductivity k:
R = (thicknessMm / 1000) / k m²·K/W
U = 1 / R W/(m²·K)
q = U × |T_in − T_out| W/m²k values are looked up from a typical-figures table sourced from ASHRAE Handbook of Fundamentals and ISO 10456:2007. Lower k means better insulation per millimetre of thickness.
Worked example
A 75 mm EPS panel (k = 0.035 W/(m·K)) with 20 °C indoors and 0 °C outdoors:
- R =
0.075 / 0.035≈ 2.14 m²·K/W - U =
1 / 2.14≈ 0.467 W/(m²·K) - q =
0.467 × 20≈ 9.33 W/m² - Rating: OK (R 2.0–3.5)
Doubling thickness to 150 mm halves U and the heat-flux figure — a quick way to test what a thicker panel buys you.
Sources
- ASHRAE Handbook of Fundamentals — Heat, Air and Moisture Control in Building Assemblies (Ch. 26)
- ISO 10456:2007 — Building materials and products — Hygrothermal properties
FAQ
What is R-value and what does it tell me?
R-value is thermal resistance. It tells you how strongly a layer of material resists heat flow per unit area, in m²·K/W. Higher R is better insulation. The formula is R = thickness / conductivity, so doubling the thickness doubles the R, and choosing a lower-conductivity material at the same thickness also raises R. As a rough scale: R below 2 is poor, 2 to 3.5 is acceptable for moderate climates, and above 3.5 is good.
What is U-value and how does it relate to R?
U-value (thermal transmittance) is the reciprocal of R: U = 1/R, in W/(m²·K). It tells you how much heat passes through one square metre of panel for every one Kelvin (or Celsius) of temperature difference between inside and outside. Lower U is better. Building energy codes are usually specified as a maximum U-value for walls, roofs, and openings.
How is the heat loss per m² figure used?
Heat loss per m² = U × |T_in − T_out| gives the steady-state heat flux across the panel under the temperatures you entered, in watts per square metre. Multiply by the wall or roof area in m² to get total continuous heat loss in watts. For seasonal energy estimates, multiply that wattage by the number of hours the temperature differential holds, then divide by 1000 to convert to kWh.
Why are PUF and PIR rated as the best insulators?
Both PUF (polyurethane foam) and PIR (polyisocyanurate) have the lowest thermal conductivity of common sandwich-panel cores at around 0.022 W/(m·K) — roughly 35 percent lower than EPS and 45 percent lower than mineral wool. That means a 75 mm PUF panel delivers similar R-value to a 110 mm EPS panel, so PUF is preferred where wall thickness or weight is constrained. Rockwool and Glasswool are chosen for fire performance, not thermal performance.
Does this calculator account for windows, doors, and thermal bridges?
No. This is a single-layer, one-dimensional steady-state calculation for the panel itself. Real building envelopes also lose heat through windows (typically U = 1.4 to 5.8), doors, and thermal bridges at framing studs and corners (where steel framing can short-circuit the insulation). For a whole-building energy model, sum the U×A products of every distinct envelope element separately and add ventilation and infiltration losses on top.
Are the conductivity values accurate for my specific product?
The values come from ASHRAE and ISO 10456 typical figures and are representative — real conductivity varies with density, moisture content, temperature, and product grade. A high-density mineral-wool board sold for facade use can have a conductivity 10–15% lower than the table value, while a low-density wool sold for partitions can be slightly higher. Use the supplier's certified test data for code-compliance work.