U-value (also called thermal transmittance) measures how much heat passes through a building element per unit area, per degree of temperature difference. It is measured in W/m²K. A lower U-value means better insulation. A wall with U-value 0.18 loses less heat than one with U-value 0.35.

How is U-value calculated?

U-value = 1 ÷ (Rsi + R1 + R2 + R3… + Rso). Where Rsi is internal surface resistance, Rso is external surface resistance, and R1, R2 etc are the thermal resistance of each material layer. Each layer's R-value = thickness in metres ÷ thermal conductivity (λ) in W/mK.

What is a good U-value for a wall?

In the UK (Part L 2021), new external walls must achieve U-value ≤ 0.18 W/m²K. In the US, ASHRAE 90.1 requires ≤ 0.124 W/m²K for Climate Zone 5+. A well-insulated passive house wall achieves 0.10–0.15 W/m²K or below.

U-Value Calculator – Calculate Wall & Window Heat Loss (Free Tool)

U-Value Calculator: Calculate Wall & Window Heat Loss (Free Tool)

You are choosing insulation for a wall. You see R-19 on one product and U-0.05 on another. Which one is better? You cannot compare them directly because they measure different things.

R‑value measures resistance to heat flow (bigger is better). U‑value measures the rate of heat transfer (smaller is better). In the United States, building codes often specify U‑values for windows and wall assemblies.

Today, I give you a free U-Value Calculator.

You enter the thickness and thermal conductivity of each layer in a wall, roof, or window assembly. The calculator shows the U‑value and R‑value instantly.

This tool uses the standard formula: U = 1 ÷ (Rsi + Σ(Thickness ÷ Conductivity) + Rse), where Rsi and Rse are surface resistances.

What is U-Value? (Simple Explanation)

U‑value measures how much heat flows through a building element. The lower the U‑value, the better the insulation.

Think of it like a leaky bucket. A low U‑value means a small leak. A high U‑value means a big leak.

The formula:

U = 1 ÷ (Rsi + Σ(Thickness ÷ λ) + Rse)

Where:

Rsi = interior surface resistance
Rse = exterior surface resistance
λ = thermal conductivity (W/m·K)
Σ(Thickness ÷ λ) = sum of all layer R‑values

Key principle: U‑value is the reciprocal of total R‑value. For an assembly, total R = sum of all layer resistances, then U = 1 ÷ total R.

LIVE U-Value Calculator

Enter your wall layers below. The calculator shows the U‑value and R‑value instantly.

U-Value Calculator 2025 — Thermal Transmittance for Walls, Floors & Roofs | Free Tool

U-Value Calculator

// ISO 6946 · ASHRAE · Thermal Transmittance · 2025

ISO 6946

Building Element Type

Quick Presets

Surface Resistances

Internal Surface Resistance (Rsi)

m²K/W

Wall=0.13 · Ceiling=0.10 · Floor=0.17

External Surface Resistance (Rso)

m²K/W

Exposed=0.04 · Sheltered=0.08

Material Layers

Add material layers below ↓

Total R (materials only):0.000 m²K/W

Add a Material Layer

Select Material

Thickness (mm)

Custom λ (W/mK) — optional

W/mK

Custom Name — optional

For Heat Loss Calculation

Element Area

m²

Temperature Difference (ΔT)

Typical: 20K (inside 20°C, outside 0°C)

Thermal Transmittance (U-Value)

— W/m²K

—

Total R-Value

—

m²K/W

Heat Loss Rate

—

Watts

U-Value

—

R-Total

—

Annual Loss

—

Target Pass

—

Thermal Assembly Visualisation (proportional to R-value)

🔴 Warm (inside)🔵 Cold (outside)

U-Value Benchmark Scale (W/m²K)

Passive ≤0.10

—

0.100.150.200.300.501.0+

PassiveExcellentGoodAveragePoor

Results are based on ISO 6946 simplified method assuming homogeneous layers. Thermal bridging, air gaps, fixings, and moisture are not accounted for. For certified building regulation compliance, use accredited software and approved construction details. Consult a qualified building physicist for critical projects.

Thermal conductivity values from ISO 10456:2007 and ASHRAE Handbook of Fundamentals. Click any row to add to your assembly.

Material	λ (W/mK)	Category	Add

Maximum allowable U-values by element type and building regulation / energy standard. Lower is better.

Standard / Country	External Wall	Roof	Floor	Window

💡 All values in W/m²K. Lower values mean better insulation. Always check your local building regulations as these change regularly. The Passive House standard applies to complete certified designs, not individual elements.

Calculate heat loss for multiple elements of a building. Add your U-values, areas, and temperature difference.

Element	U-Value	Area (m²)	ΔT (K)	Heat Loss (W)	Remove

Total Building Heat Loss:

— W

U-Value Quick Guide

≤0.10Passive House standard. Excellent insulation.

0.10–0.15Excellent. Exceeds most modern building regulations.

0.15–0.20Good. Meets UK Part L and EU EPBD requirements.

0.20–0.30Acceptable. Standard new-build performance.

0.30–0.50Average. Some older buildings. Improvement recommended.

0.50–1.0Poor. Typical of uninsulated existing walls.

1.0+Very poor. Single glazing, uninsulated structures.

Common λ Values

Key Tips

Lower U-value = better insulation. A wall at 0.18 loses half the heat of a 0.36 wall.

Surface resistances (Rsi and Rso) account for air film heat transfer. Never leave them at zero.

Air gaps add resistance. A sealed 25mm air cavity = about 0.18 m²K/W.

Thermal bridges (steel ties, window frames) can reduce effective U-value by 20–50%.

Doubling insulation thickness does not halve U-value — the relationship is not linear.

What Is U-Value?

U-value is one of the most important numbers in building physics. It measures how much heat passes through a building element — a wall, roof, floor, or window — for every degree of temperature difference between inside and outside.

A lower U-value means better insulation. A wall with U-value 0.18 W/m²K loses less heat than one with U-value 0.35 W/m²K. You want low U-values to keep heat in during winter and out during summer.

U-value is also called thermal transmittance. The unit is Watts per square metre Kelvin (W/m²K). It is the reciprocal of total thermal resistance (R-value).

U-value calculations follow ISO 6946 (Building components and building elements — Thermal resistance and thermal transmittance). Learn more at Wikipedia: Thermal Transmittance ↗.

The U-Value Formula

The calculation is based on adding up the thermal resistance of each layer in the construction. Then U-value is simply 1 divided by the total.

R = d ÷ λ (for each layer)

d = thickness in metres | λ (lambda) = thermal conductivity in W/mK

U = 1 ÷ (Rsi + R₁ + R₂ + R₃ … + Rso)

Rsi = internal surface resistance | Rso = external surface resistance | R₁, R₂… = each material layer

Heat Loss (W) = U × Area (m²) × Temperature Difference (K)

For annual energy: multiply by 8,760 hours/year ÷ 1000 to get kWh

U-Value Building Regulation Requirements

Standard / Country	External Wall	Roof	Floor	Window
UK Part L (2021)	0.18	0.11	0.13	1.4
EU EPBD (2024)	0.20	0.15	0.20	1.3
Germany EnEV	0.20	0.20	0.30	1.3
ASHRAE 90.1 Zone 5	0.124	0.116	0.284	1.48
Passive House (PHI)	≤0.10	≤0.10	≤0.10	≤0.80
Ireland Part L	0.21	0.16	0.21	1.6

Common Building Materials and Thermal Conductivity

Thermal conductivity (λ, lambda) is the key property of each material. It is measured in W/mK. Lower values mean better insulating materials.

Material	λ (W/mK)	Typical Use	R-value per 100mm
Polyurethane foam (PIR)	0.022	Roof, wall insulation	4.55 m²K/W
Phenolic foam	0.018	Thin high-performance slabs	5.56 m²K/W
Mineral wool (stone)	0.035	Cavity and stud walls	2.86 m²K/W
EPS (expanded polystyrene)	0.036	Underfloor, floor slabs	2.78 m²K/W
XPS (extruded polystyrene)	0.034	Below ground, flat roofs	2.94 m²K/W
Glass wool batt	0.040	Loft, stud walls	2.50 m²K/W
Concrete block (dense)	1.700	Structural walling	0.06 m²K/W
Brick (facing)	0.770	External cladding	0.13 m²K/W
Plasterboard	0.250	Internal lining	0.40 m²K/W
Softwood timber	0.130	Timber frame, joists	0.77 m²K/W

Key things to remember about U-values

U-value = 1 ÷ total R-value. Lower U-value = better insulation.
Each material layer adds resistance: R = thickness ÷ thermal conductivity (λ).
Always include surface resistances (Rsi and Rso) in your calculation.
Rsi values: horizontal flow (wall) = 0.13, upward (ceiling) = 0.10, downward (floor) = 0.17.
Rso values: sheltered location = 0.08, normal = 0.04, exposed = 0.03.
Heat loss per hour = U-value × Area (m²) × Temperature difference (K) in Watts.
Lower thermal conductivity (λ) means better insulation material. PIR at 0.022 outperforms glass wool at 0.040.
Thermal bridging through fixings, studs, and window frames reduces real-world performance — not shown in simple calculations.

Frequently Asked Questions

How to Use This Calculator

Select your building element type. Choose surface resistance values. Add material layers one by one using the library of 40+ materials. Then click Calculate to get your U-value, R-value, and heat loss instantly. The visual assembly diagram shows each layer proportionally to its contribution to total thermal resistance. The further information, see Wikipedia: R-value ↗ and Wikipedia: Building Insulation ↗.

📐 U-Value Calculator

Calculate heat loss through walls, roofs, and windows

🏠 Building Element

Wall (vertical) Roof (horizontal, heat up) Floor (horizontal, heat down)

📐 Surface Type

Exterior (Rse=0.04) Interior (Rsi=0.13)

📊 U-VALUE

0.000 W/m²·K

📊 R-VALUE

0.00 m²·K/W

💡 Lower U-value = better insulation. A U-value below 0.25 is good for walls. ➕ Add Layer

📐 U = 1 ÷ (Rsi + Σ(thickness/λ) + Rse). λ = thermal conductivity (W/m·K).

How to Use This U-Value Calculator

Follow these 3 simple steps.

Step 1: Select the building element (wall, roof, or floor)
Step 2: Select the surface type (exterior or interior)
Step 3: Add layers and enter thickness (meters) and thermal conductivity (W/m·K)

The calculator shows:

U‑value (W/m²·K) – lower is better
R‑value (m²·K/W) – higher is better
Assessment of insulation quality

Common U-Values for Reference

Building Element	U‑value (W/m²·K)	Insulation Quality
Single glazing	5.7	Very poor
Double glazing	2.8-3.3	Poor
Triple glazing	0.8-1.8	Good
Well-insulated wall	0.15-0.25	Excellent
Poorly insulated wall	1.0-1.5	Poor
Well-insulated roof	0.10-0.15	Excellent
Well-insulated floor	0.15-0.20	Excellent

The Formula Explained

Step 1: Calculate each layer’s thermal resistance

R = thickness ÷ λ

Where λ is thermal conductivity (W/m·K).

Step 2: Add all layer resistances and surface resistances

R_total = Rsi + ΣR_layers + Rse

Step 3: Calculate U‑value

U = 1 ÷ R_total

Surface resistances (standard values):

Surface	Rsi (m²·K/W)	Rse (m²·K/W)
Wall (vertical)	0.13	0.04
Roof (horizontal, heat up)	0.10	0.04
Floor (horizontal, heat down)	0.17	0.17

Real Examples

Example 1: Insulated wall

Interior plaster: 0.012m, λ=0.5 → R=0.024
Insulation: 0.1m, λ=0.04 → R=2.5
Exterior brick: 0.1m, λ=0.7 → R=0.143
R_total = 0.13 + (0.024+2.5+0.143) + 0.04 = 2.837
U = 1 ÷ 2.837 = 0.352 W/m²·K

Example 2: Window

Single pane: 0.006m, λ=1.0 → R=0.006
R_total = 0.13 + 0.006 + 0.04 = 0.176
U = 1 ÷ 0.176 = 5.68 W/m²·K

Frequently Asked Questions (FAQs)

1. What is a good U-value?

For walls: below 0.25 W/m²·K. For roofs: below 0.15. For windows: below 1.5 for double glazing.

2. What is the difference between U-value and R-value?

U‑value measures heat transfer (lower is better). R‑value measures resistance to heat flow (higher is better). They are reciprocals.

3. How do I convert R-value to U-value?

U = 1 ÷ R_total.

4. What is thermal conductivity (λ)?

The rate at which heat flows through a material. Lower λ = better insulation.

5. Why do surface resistances matter?

The air layers on each side of a building element add resistance to heat flow. They are included in the calculation.

Common Mistakes

Mistake #1: Using the wrong units

Thickness must be in meters, not millimeters. Conductivity in W/m·K.

Mistake #2: Forgetting surface resistances

Include both interior and exterior surface resistances in the total R‑value.

Mistake #3: Not accounting for thermal bridges

This calculator assumes no thermal bridges. For actual construction, add corrections.

Final Thoughts

U‑value is the standard measure of heat loss through building elements. Lower U‑values mean better insulation and lower energy bills.

My U-Value Calculator gives you:

U‑value from layer thickness and conductivity
R‑value for comparison
Surface resistance options

Bookmark this page. Use it for insulation design and building code compliance.

The next time you need to know how well a wall insulates, you will have the answer.

Disclaimer: This is an educational tool based on ISO 6946. For actual construction, consult an engineer.

External Links (Authority Backlinks):

Wikipedia – Thermal transmittance{:target=”_blank” rel=”noopener noreferrer”}
Wikipedia – R-value (insulation){:target=”_blank” rel=”noopener noreferrer”}
Wikipedia – Thermal conductivity{:target=”_blank” rel=”noopener noreferrer”}
ASHRAE – Fundamentals Handbook{:target=”_blank” rel=”noopener noreferrer”}

Post Views: 195

U-Value Calculator: Calculate Wall & Window Heat Loss (Free Tool)

What is U-Value? (Simple Explanation)

LIVE U-Value Calculator

U-Value Calculator

What Is U-Value?

The U-Value Formula

U-Value Building Regulation Requirements

Common Building Materials and Thermal Conductivity

Key things to remember about U-values

Frequently Asked Questions

How to Use This Calculator

📐 U-Value Calculator

🏠 Building Element

📐 Surface Type

How to Use This U-Value Calculator

Common U-Values for Reference

The Formula Explained

Real Examples

Frequently Asked Questions (FAQs)

1. What is a good U-value?

2. What is the difference between U-value and R-value?

3. How do I convert R-value to U-value?

4. What is thermal conductivity (λ)?

5. Why do surface resistances matter?

Common Mistakes

Final Thoughts

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