U-Value Calculator - Layer R-Values, ISO 6946, Heat Loss

Q: What if my product is not in the material list?

Pick Custom Material and type the lambda (λ) from the data sheet or the product label. Then set the thickness as normal. Do not guess a lambda. A small error in λ moves the U-value a lot.

Q: Why is my air cavity showing an R-value of zero?

You have it set to well ventilated . Outside air flows straight through, so it adds nothing. Use unventilated for a sealed cavity and slightly ventilated for small weep holes or gaps. A sealed cavity 25 mm or wider gives about 0.18 m²·K/W.

Q: How thick does insulation need to be to hit 0.18 W/(m²·K)?

Switch to Goal Mode , type 0.18, pick your insulation layer, and the tool tells you exactly. Rough guide for a typical cavity wall: Mineral wool (λ 0.035): about 160–170 mm EPS (λ 0.038): about 175–185 mm PIR board (λ 0.022): about 100–110 mm

U Value Calculator

Updated Aug 31, 2026 By Jehan Wadia

Introduction

A U-value tells you how fast heat escapes through a wall, roof, floor, or window. The lower the number, the less heat you lose and the less you pay to heat the building. This U-value calculator works out that number for you in W/(m²·K) or BTU/(hr·ft²·°F).

You build your wall or roof layer by layer, just like on site. Pick a material, type in the thickness, and the tool finds the R-value of each layer, adds them up with the inside and outside surface resistances, and gives you the final U-value.

With this calculator you can:

Start from a preset build-up, like a cavity wall, timber frame, flat roof, or concrete floor
Choose from common materials such as brick, block, mineral wool, PIR, EPS, timber, and plasterboard
Add air cavities using ISO 6946 values
Switch between metric and imperial units at any time
Use Goal Mode to find the insulation thickness needed to hit a target U-value
Estimate heat loss from the area and the temperature difference
Compare your result with UK Part L, EU, and US IECC targets

You also get a full layer breakdown, a cross-section view, charts, and the step-by-step math. That makes it easy to see which layer is doing the work and where more insulation will help most.

How to use our U Value Calculator

Enter your building element, its material layers, and each layer's thickness. The calculator gives you the U-value, the total thermal resistance (R-value), a layer-by-layer breakdown, and the heat loss through the element.

Building Element Type: Pick what you are building, like an external wall, flat roof, pitched roof, ground floor, exposed floor, or window. This sets the correct surface resistances for you.

Unit System: Choose SI/Metric or Imperial/US. All values change over for you, and you can still mix units in each drop-down. For one-off conversions, the unit converter and mm to inches calculator can help.

Calculation Mode: Use Standard to find the U-value of your build-up. Use Goal to find how thick one layer must be to hit a target U-value.

Internal Surface Resistance (Rsi): Leave the default for your element type, or type your own value. Press the reset arrow to go back to the default.

External Surface Resistance (Rse): Same as above, but for the outside face of the element.

Whole-Unit U-Value (windows only): If you picked Window/Glazing, type the U-value from the maker's data sheet for the full window unit.

Preset Construction Template: Pick a ready-made wall, roof, or floor build-up and click Load Preset. This replaces all your current layers.

Add Layer: Click to add a new layer. You can have up to 20 layers. The top layer is the outside face, and the bottom layer is the inside face.

Layer name: Type a short name so you can spot each layer in the results table and cross-section.

Material: Choose from the list, such as brick, block, mineral wool, PIR, timber, plasterboard, or air gap. The thermal conductivity (λ) fills in for you. Pick Custom Material to type your own λ.

Thickness (d): Type how thick the layer is and pick the unit (mm, cm, m, in, or ft).

Conductivity (λ): Change this if your product data sheet lists a different value than the default.

Cavity ventilation: For air gap layers, choose unventilated, slightly ventilated, or well ventilated. The R-value comes from the ISO 6946 cavity table.

Layer actions: Use the arrows to move a layer up or down, the copy button to duplicate it, and the bin to delete it. You can undo a delete.

Target U-Value (Goal mode): Type the U-value you must meet, such as a Part L limit.

Layer to Solve For: Pick which layer, usually the insulation, should get thicker to reach your target.

Show Required Thickness In: Choose the unit for the answer, then click Apply to Layer to put that thickness into your build-up.

Element Area: Type the area of the wall, roof, or floor to work out heat loss. Work it out first with the square footage calculator or, for a pitched roof, the roof area calculator. Leave it if you only want the U-value.

Internal Temperature: Type the room temperature inside, in °C, °F, or K. The Celsius to Fahrenheit calculator handles any conversions you need.

External Temperature: Type the outside design temperature in the same or a different unit.

Heat Loss Output Unit: Choose W, kW, or BTU/hr for the heat loss result. See the BTU calculator if you are sizing heating output.

Result units: In the results, switch the U-value between W/(m²·K) and BTU/(hr·ft²·°F), and the R-value between m²·K/W and hr·ft²·°F/BTU.

Click Calculate to update the results, or Reset to start again with the default wall.

What Is a U-Value?

A U-value tells you how fast heat escapes through a wall, roof, floor, or window. It is measured in W/(m²·K) — watts lost per square metre for each degree of temperature difference between inside and outside. A low U-value means good insulation. A high U-value means the building leaks heat and costs more to warm up.

U-Value and R-Value

R-value is the opposite of U-value. R measures how well a material stops heat. U measures how much heat gets through. They are linked by one simple rule:

U = 1 ÷ R_T and R_T = 1 ÷ U

R_T is the total resistance of the whole build-up. To find it, you add up the R-value of every layer, plus the two thin films of still air on each face of the wall.

How Each Layer Is Worked Out

Every solid layer has its own R-value, found with:

R = d ÷ λ

d = thickness of the layer, in metres
λ (lambda) = thermal conductivity, in W/(m·K)

A low lambda means the material blocks heat well. Mineral wool is about 0.035, PIR board about 0.022, and brick is about 0.77. That is why 100 mm of insulation beats 300 mm of brick.

Surface Resistances (R_si and R_se)

Thin layers of still air cling to both faces of a building element and slow heat down a little. ISO 6946 gives standard values for these:

Walls: R_si 0.13, R_se 0.04
Roofs (heat flowing up): R_si 0.10, R_se 0.04
Floors (heat flowing down): R_si 0.17, R_se 0.04

Air Cavities

An air gap adds resistance, but only if it is sealed. An unventilated cavity 25 mm or wider gives about 0.18 m²·K/W. A slightly ventilated cavity gives roughly half of that. A well-ventilated cavity gives nothing at all, because outside air moves right through it.

Heat Loss Through the Element

Once you know the U-value, you can work out heat loss with:

Q = U × A × ΔT

Q is the heat loss in watts, A is the area in square metres, and ΔT is the temperature difference in kelvin (or °C, they are the same size step). A 10 m² wall at U 0.18 with 20 °C inside and 0 °C outside loses about 36 watts. Add up every element with the whole-house heat loss calculator, then size the kit with the AC tonnage calculator or the mini split sizing calculator. To turn watts into money, try the electricity cost calculator.

Typical U-Value Targets

These are common reference values for new buildings. Always check your local rules, since they change.

External wall: around 0.18 W/(m²·K)
Roof: around 0.15 W/(m²·K)
Ground floor: around 0.13 W/(m²·K)
Windows: around 1.4 W/(m²·K)

Thermal Bridging

A standard U-value is a "clear-field" number. It covers the plain, repeating part of the build-up only. Timber studs, mortar joints, wall ties, and steel fixings all carry heat faster than the insulation around them. These weak spots are called thermal bridges, and they push the real U-value higher. ISO 6946 adds a correction (ΔU) for them, and junctions at corners, eaves, and window reveals need their own psi-value check. If you are working out stud spacing for a framed wall, the stud calculator and framing calculator show how much timber sits in the insulation zone.

Imperial Units

In the US, U-values use BTU/(hr·ft²·°F) and insulation is sold by R-value in hr·ft²·°F/BTU. To swap between systems:

1 BTU/(hr·ft²·°F) = 5.678 W/(m²·K)
1 hr·ft²·°F/BTU = 0.1761 m²·K/W

So a US R-20 wall is about R-3.52 in metric, giving a U-value near 0.28 W/(m²·K). The feet to meters calculator and inches to mm calculator are handy when reading mixed data sheets.

Why It Matters

Heating and cooling use most of a building's energy. Getting U-values right cuts fuel bills, lowers carbon, stops cold spots, and helps prevent condensation and mould on inside surfaces. Insulation is also the cheapest fix at design stage — far cheaper than tearing a finished wall apart later.

Related Construction Calculators

Insulation Calculator — how much insulation material to order
Heat Loss Calculator — total heat loss for a whole room or house
BTU Calculator — heating and cooling load in BTU
Drywall Calculator — plasterboard sheets for the inner lining
Roof Pitch Calculator — pitch and slope for roof build-ups
CFM Calculator — ventilation airflow for the same space
Solar Panel Calculator — pair a low-energy fabric with on-site generation
Carbon Footprint Calculator — see the emissions saved by better insulation

Formulas used

Layer thermal resistance

R_{layer} = \frac{d}{\lambda}

Total thermal resistance

R_T = R_{si} + \sum_{i=1}^{n} \frac{d_i}{\lambda_i} + R_{se}

Thermal transmittance (U-value)

U = \frac{1}{R_T}

Steady-state heat loss

Q = U \times A \times \Delta T = U \times A \times (T_{in} - T_{out})

Goal mode: required insulation thickness

d_{req} = \left( \frac{1}{U_{target}} - R_{others} \right) \times \lambda

Unventilated air cavity resistance (ISO 6946 linear interpolation)

R_{cav} = R_a + (R_b - R_a) \cdot \frac{d - d_a}{d_b - d_a}

Ventilation correction for air cavity

R_{cav} = \begin{cases} R_{cav} & \text{unventilated} \\ 0.5 \cdot R_{cav} & \text{slightly ventilated} \\ 0 & \text{well ventilated} \end{cases}

Layer share of total resistance

\%R_i = \frac{R_i}{R_T} \times 100

Frequently asked questions

Does the order of my layers change the U-value?

No. The U-value is the same no matter what order you list the layers in, because the R-values are simply added up.

Order still matters for two reasons:

Condensation risk depends on where the insulation and vapour layers sit
The cross-section drawing only looks right if you keep the outside layer at the top

Can I use this U-value for building control or a SAP submission?

Use it as a design check, not as the final paperwork. Official submissions normally need accredited software and a thermal bridging correction (ΔU) added on top.

Your result here should be close, so it is great for testing ideas before you pay for a full assessment.

My wall has timber studs. How do I deal with them?

This tool gives the clear-field value, which is the bay between the studs. Timber carries more heat than insulation, so the real wall is worse.

Two ways to handle it:

Run the calculator twice — once through the insulation, once with timber in place of the insulation — then combine the two by area share
Or add a small ΔU allowance, often about 0.02–0.05 W/(m²·K) for a framed wall

Should I use the declared or design lambda value?

Use the design value (λ) from the product data sheet. It is a little higher than the declared value because it allows for moisture and ageing.

The defaults in this calculator are typical design values. Always swap in the maker's figure when you have it.

Is this accurate for a ground floor?

It gives the resistance of the floor build-up only. A real ground floor also depends on the soil and the shape of the floor, worked out with the perimeter-to-area (P/A) ratio in ISO 13370.

The ground helps you, so the true U-value is usually better than the number shown here. Treat this as a starting point.

What if my product is not in the material list?

Pick Custom Material and type the lambda (λ) from the data sheet or the product label. Then set the thickness as normal.

Do not guess a lambda. A small error in λ moves the U-value a lot.

Why is my air cavity showing an R-value of zero?

You have it set to well ventilated. Outside air flows straight through, so it adds nothing.

Use unventilated for a sealed cavity and slightly ventilated for small weep holes or gaps. A sealed cavity 25 mm or wider gives about 0.18 m²·K/W.

If I double the insulation, does the U-value halve?

No. The R-value of that layer doubles, but the rest of the wall and the surface resistances stay the same.

Example: going from 100 mm to 200 mm of mineral wool might take a wall from 0.30 to 0.19, not to 0.15. Each extra millimetre saves a bit less than the last.

How thick does insulation need to be to hit 0.18 W/(m²·K)?

Switch to Goal Mode, type 0.18, pick your insulation layer, and the tool tells you exactly.

Rough guide for a typical cavity wall:

Mineral wool (λ 0.035): about 160–170 mm
EPS (λ 0.038): about 175–185 mm
PIR board (λ 0.022): about 100–110 mm

Why can't I build a window up from layers?

A window is glass, gas fills, spacer bars, and a frame all working together. Simple layer maths does not model it.

Instead, type the whole-unit U-value (Uw) from the maker's data sheet. That number already covers the glass, spacers, and frame.

Does this include air leakage, draughts, or moisture?

No. It is a steady-state, dry, clear-field calculation.

Real buildings also lose heat through gaps, open vents, and damp materials. Air leakage alone can add a lot to your heating bill, so treat the heat loss figure as a floor, not a ceiling.

Can I model foil-faced or reflective insulation in a cavity?

The built-in cavity table assumes plain surfaces, so it will not credit the foil. To include it, add a Custom Material layer instead of an air gap.

Set the thickness and lambda so that thickness ÷ lambda equals the R-value the maker quotes for that cavity.

What is a realistic U-value target for an old house?

An uninsulated solid brick wall is around 2.0 W/(m²·K). Getting it down to 0.30 is a big, worthwhile win.

New-build targets like 0.18 are often hard in a retrofit because of room space, damp risk, and window reveals. Improve as far as the building allows.

Why does plasterboard barely change my result?

It is thin and only a fair insulator. At 12.5 mm with a lambda of 0.21, its R-value is about 0.06 m²·K/W.

Look at the percentage column in the breakdown table. It shows that insulation usually does 70–90% of the work.

How many layers can I add?

Up to 20. That is plenty for almost any build-up.

If you run out, merge thin layers of similar material into one layer with the combined thickness.

I deleted the wrong layer. Can I get it back?

Yes. A yellow bar appears with an Undo button. Click it and the layer comes back in the same spot.

The undo clears once you move, copy, or add another layer, so use it right away.

Why do the two percentage figures differ for the same layer?

They measure different things.

The percentage in the breakdown table is the share of the total R, including the two surface resistances
The badge next to each layer is the share of the material layers only

My heat loss result looks very small. Is it wrong?

Probably not. It is the loss through one element only, at one moment, in watts.

A whole house adds up every wall, roof, floor, and window, plus heat lost through ventilation. Run each element here, then total them in a whole-house heat loss calculator.

What outside temperature should I use?

Use your local winter design temperature, not the average. In much of the UK that is about -3 °C to 0 °C. Colder regions use lower figures.

Designing to the coldest normal day makes sure the heating still copes when it matters.

Why did my numbers change when I switched to Imperial?

They were converted, then rounded for display. The physical build-up is the same.

For example 100 mm becomes 3.937 in, and a lambda of 0.035 W/(m·K) becomes 0.2427 BTU·in/(hr·ft²·°F). The U-value stays the same, just in different units.

Can I use this for an internal wall or a party wall?

You can work out the R-value, but the surface resistances are wrong by default. Both faces of an internal wall are indoors, so set Rse to 0.13 as well.

Party walls also have their own rules and cavity requirements, so check local guidance.