U-value & Temperature Profile

Try a real assembly

Click any card to load the layers into the diagram. Each row shows the resulting U-value and total wall thickness so you can see at a glance how performance scales with construction.

Annual heat loss through this wall

How much heating energy escapes per year through the assembly currently loaded in the diagram. Heating degree-days (HDD) summarise a climate: London ≈ 2200 K·d, Boston ≈ 3100 K·d, Helsinki ≈ 4900 K·d.

Wall area m² Heating degree-days K·d/yr Energy price (optional) per kWh

Current loss—W

Annual energy—kWh/yr

Annual cost—

Q_annual = U · A · HDD · 24 / 1000 kWh. Steady-state only — real losses also include thermal bridges, ventilation, and air leakage.

What you're looking at

This is a steady-state model of 1-D heat flow through a layered wall. Heat moves from the warmer side to the colder side, dropping in temperature as it crosses each layer. The red curve is the temperature at every position; the colored bars are the materials, drawn to scale.

Where the curve bends sharply through a layer, that layer is offering a lot of resistance — a good insulator. Where it slopes gently, the layer barely slows the heat down.

Key concepts

λ — Thermal conductivity: How easily heat passes through a unit thickness of a material. Units: W/(m·K). Mineral wool ≈ 0.04 (great insulator), brick ≈ 0.77, concrete ≈ 1.65, steel ≈ 50.
R — Thermal resistance of a layer: R = d / λ, where d is thickness in metres. Bigger R = harder for heat to pass. Units: m²·K/W. Resistances of layers add up in series, like electrical resistors.
R_si, R_se — Surface (air-film) resistances: Even at the wall surface, a thin layer of air resists heat transfer (via convection + radiation). EN ISO 6946 standard values for a vertical wall with horizontal heat flow: R_si = 0.13, R_se = 0.04 m²·K/W. They appear as the hatched zones in the diagram.
R_total — Total resistance of the assembly: R_total = R_si + Σ (d_i / λ_i) + R_se. Just a sum.
U — Thermal transmittance ("U-value"): U = 1 / R_total. Units: W/(m²·K). The watts that leak through 1 m² of wall for every 1 K of temperature difference. Lower U = better insulation.
q — Heat flux density: q = U · ΔT = U · (T_i − T_e). Watts per square meter of wall. In steady state, q is the same at every point across the wall (energy conservation).

How the temperature curve is built

The model assumes:

Heat flow is steady — nothing changes with time, no thermal mass effects.
Heat flows in one dimension, perpendicular to the wall.
Each layer is uniform (no studs, no voids).

Because the same q flows through every part of the assembly, the temperature drop across any segment is proportional to that segment's resistance:

ΔT_segment = q · R_segment

Start from T_i (inside air). Subtract q · R_si to get the inside surface temperature. Subtract q · R for each layer in turn. Subtract q · R_se at the end — you should land exactly on T_e.

How to read the diagram

Slope of the curve inside a layer — steep = small R = poor insulator; almost flat = big R = good insulator.
The two slopes in the hatched air zones — those are the surface films. They aren't zero, but for most walls they're small compared with the insulation.
Total drop (T_i − T_e) — gets split between all the resistances in proportion to their share of R_total.
Drag an interior handle — the wall thickness stays fixed; the two adjacent layers redistribute. Watch which kink shifts.
Drag a red endpoint — ΔT changes, so q scales linearly. The shape of the curve stays the same; it just shifts.

Typical U-value targets (rough guide)

Element	UK Part L (2022)	Passivhaus
External wall	≤ 0.18 W/m²K	≤ 0.15
Pitched roof	≤ 0.11	≤ 0.15
Ground floor	≤ 0.13	≤ 0.15
Window (whole)	≤ 1.4	≤ 0.80

Numbers vary by jurisdiction and update; use as orientation, not as a spec.

Try this in the tool

Drag the gypsum ↔ mineral-wool handle back and forth. Notice how the curve barely changes in the gypsum but bends a lot in the wool — gypsum's R is tiny compared with mineral wool.
Swap the mineral-wool layer for brick at the same thickness. U-value rockets up because λ_brick ≈ 19 × λ_wool.
Set T_inside to 21°C and T_outside to −10°C, then watch q. Now flip them — q reverses sign (heat flows the other way).
Add a 50 mm PUR layer to the assembly and remove 50 mm of brick. Compare U before and after — a thin foam outperforms a lot of masonry.
Replace the brick with steel. Watch the curve become almost flat through the steel: very low R means the metal does almost nothing for insulation.

Material library used by this tool

What this model does not capture

Thermal mass. Real walls store and release heat over time. This is steady-state only — the long-term average.
Thermal bridges. Real assemblies have shortcuts at junctions, studs, ties, balconies. Measured U is typically 10–30% worse than the 1-D calculation.
Moisture & condensation. No vapor diffusion, no dew-point analysis. (A Glaser-method overlay is a natural future addition.)
Air leakage. The U-value only describes conductive losses through the fabric; ventilation and infiltration losses are separate.