Insulation & Heat Loss Calculator

Add up the layers of a wall or a roof and see how much heat escapes through it. The resistances printed on the packaging, the R-values, add together, and only at the very end are they turned into a U-value — averaging U-values instead is the classic mistake, and it flatters the answer.

How to use it

You build the wall or roof up layer by layer, entering the resistance of each one, and the total is converted into a U-value for the whole thing at the end.

  1. List the layers from outside in Every material in the build-up, with its thickness in millimetres. Leave a layer set to “none” if you have fewer than three.
  2. Set the temperatures 20°C inside and your winter average outside. The difference between them is what drives the loss — the absolute values do not matter.
  3. Read the U-value Lower is better. Compare it against the Part L figures shown: 0.18 for a new-build wall, 0.30 for an extension.
Full instructions
In depth

Working out the U-value of a wall, roof or floor

What R-value and U-value actually mean, why you must add the resistances and not the rates, how to set up the layers in the calculator, and a worked example that shows the difference between mineral wool and rigid board.

Screenshot: Why you add R and not U
Why you add R and not U as it appears when the page opens.

Two numbers that say the same thing backwards

R-value is resistance to heat flow. A higher R-value means the material resists more heat. U-value is the rate at which heat passes through. A lower U-value means less heat gets through. They are reciprocals: U is one divided by R, and R is one divided by U.

The critical rule is that R-values add across the layers of a wall, but U-values do not. If you have a layer of brick with an R of 0.13 and a layer of mineral wool with an R of 2.63, the total R is 2.76. The U-value is one divided by 2.76, which is 0.36. You add R, then invert once at the end.

Averaging U-values is the classic mistake. It gives an answer that looks better than the truth every time, because the arithmetic of inverses does not work that way.

What this page calculates

You list the layers of a wall, roof or floor from outside to inside. The tool adds the thermal resistance of each layer, includes the surface film resistance on each face, converts the total to a U-value, and works out how much heat that area loses in a year at the temperatures you set. If you enter an energy price, it shows the annual cost of that heat loss as well.

It does not model draughts, ventilation, or the cold bridges at junctions and around window openings. On a badly detailed job those can add a quarter to the real loss. For Building Control you need a full calculation from someone qualified to sign it. This is for checking your own build-up and seeing what each change does to the number.

Who needs this

A self-builder choosing between mineral wool and rigid board wants to see the U-value of each option before ordering. A homeowner adding insulation to a loft wants to know whether 100 mm or 150 mm brings the wall below the target. A builder quoting an extension needs to show the building inspector that the walls and roof meet Part L. An energy adviser comparing a solid wall before and after internal insulation wants the heat loss in watts.

The tool is not a replacement for a professional assessment. It covers one element at a time, not the whole house. But it shows the effect of each layer clearly enough to make informed choices.

Setting up the layers

The panel is headed The build-up. The first field is Area, the total area of wall, roof or floor in square metres.

Three layer slots follow, each with a material dropdown and a Thickness field in millimetres. The dropdown lists fifteen choices: PIR board, Phenolic board, Mineral wool, EPS polystyrene, XPS polystyrene, Sheep wool, Wood fibre board, Blown cellulose, Brickwork, Dense concrete block, Aircrete block, Plasterboard, Cement render, Softwood, and Unfilled air cavity. Each one shows its thermal conductivity in watts per metre kelvin beside the name.

The outermost layer goes in Layer 1. If you have fewer than three layers, leave the others set to none. The tool automatically adds the surface film resistance on the inside and outside faces, which together are worth about 0.17 square metre kelvin per watt. Calculators that leave those out make thin build-ups look worse than they really are.

Below the layers are Inside temperature and Outside temperature, both in degrees Celsius. The default is 20 inside and 5 outside, which is a typical winter average for a cool climate. The gap between them is what drives the heat loss. The absolute values do not matter on their own.

The cost side

Hours of heating a year is the number of hours your heating runs over a full winter. The default is 4380, which is roughly half the year. In a mild climate you might use less. In a cold one you might use more.

Price per kWh is optional. If you enter your electricity or gas price, the result includes the annual cost of the heat lost through this element. That lets you compare two insulation choices not just by their U-value but by what each one saves in money.

Currency sets the symbol. These two fields are there to make the comparison real. Knowing that an extra 50 mm of PIR saves you a certain amount a year makes the spending decision concrete.

Reading the output

The result shows the total R-value of the build-up and the U-value derived from it. Below that it shows the heat loss through the area in watts and in kilowatt hours over the heating season. If you gave a price per kWh, the annual cost appears as well.

Reference figures from the UK building regulations are shown beside the result so you can see how your build-up compares. The Part L target is about 0.18 for a new-build wall and about 0.30 for an extension or a refurbishment. A roof target is lower still. These figures are a useful benchmark even outside the UK, because the physics does not change with the border.

A worked example: a cavity wall with insulation

The wall is 10 square metres. Layer 1 is brickwork at 102.5 mm. Layer 2 is mineral wool at 100 mm. Layer 3 is aircrete block at 100 mm. Inside temperature is 20. Outside temperature is 5.

The brick has a conductivity of 0.77, so 102.5 mm gives an R of about 0.13. The mineral wool has a conductivity of 0.038, so 100 mm gives an R of about 2.63. The aircrete block has a conductivity of 0.15, so 100 mm gives an R of about 0.67. Add the surface films, roughly 0.17 between them, and the total R is about 3.60. The U-value is one divided by 3.60, which is about 0.28.

Now swap the mineral wool for PIR board at the same 100 mm. PIR has a conductivity of 0.022, giving an R of about 4.55 from that layer alone. The total R jumps to roughly 5.52 and the U-value drops to about 0.18. That single change takes the wall from just above the extension target to the new-build target.

This is why conductivity matters so much. PIR reaches the same U-value in roughly half the thickness of mineral wool. The wall ends up thinner, and in a retrofit where space is tight, that difference can decide whether the insulation fits at all.

Diminishing returns and where to stop

The first layer of insulation does far more than the last. Going from 50 mm of PIR to 100 mm saves nearly three times what going from 100 mm to 150 mm saves. Each extra millimetre is worth less than the one before it, because the U-value curve flattens steeply.

At some point the cost of the extra insulation outweighs the energy saving. That point depends on the price of the insulation, the price of energy, and how long you plan to keep the building. The cost field on this page helps you find it, because you can try different thicknesses and see the saving in money rather than in abstract units.

Mistakes worth avoiding

  • Averaging U-values of two layers. The answer is always wrong in the flattering direction. Add the R-values, then invert once.
  • Forgetting the surface films. The still air on each face is worth about 0.17 between them. Leaving it out makes a thin build-up look much worse than it is.
  • Treating the order of layers as unimportant. The U-value does not change with the order, but the position of the dew point does. Putting insulation on the wrong side of a wall can cause condensation inside it.
  • Using this for a whole-house estimate. This covers one element. Your heating bill includes every wall, roof, floor, window and door, plus ventilation and hot water.
  • Ignoring cold bridges. The junctions between wall and floor, wall and roof, and around window openings all lose heat that this steady-state calculation does not count.

When another tool fits

  • To size a heater or air conditioner from the finished room, the Air Conditioner and Heater Sizing page takes the whole room into account.
  • To count the timber in a stud wall before filling it with insulation, the Stud Wall and Framing Calculator lists every piece.
  • To work out the aggregate for a floor build-up, the Gravel, Sand and Soil Calculator handles the sub-base.

The numbers stay on your own device

All of the maths runs in your browser. The layers, thicknesses, temperatures and prices you enter are not sent to any server. There is no login, no cloud storage, and no copy of your build-up kept after you close the tab. The page works without an internet connection for the same reason: there is nothing at the other end to connect to.

Help

Why you add R and not U

You build the wall or roof up layer by layer, entering the resistance of each one, and the total is converted into a U-value for the whole thing at the end. Give it the area as well and that becomes the heat escaping through it, and then what replacing that heat costs over a heating season, the figure that decides whether insulation pays.

List the layers from outside in

Every material in the build-up, with its thickness in millimetres. Leave a layer set to “none” if you have fewer than three.

Set the temperatures

20°C inside and your winter average outside. The difference between them is what drives the loss — the absolute values do not matter.

Read the U-value

Lower is better. Compare it against the Part L figures shown: 0.18 for a new-build wall, 0.30 for an extension.

Fabric loss only

This is steady-state loss through the area you gave. It does not model draughts, ventilation, or the cold bridges at junctions and around openings — which on a poorly detailed job can add a quarter again. For Building Control you need a full calculation from someone qualified to sign it.

Good to know

Never average U-values

U-values do not add and they do not average. Add the R-values of the layers, then take one over the total. Averaging the U-values of two layers gives an answer that is wrong in the flattering direction every time.

Surface films are worth having

The still air clinging to each face is worth about 0.17 m²K/W between them — roughly as much as a sheet of glass. It is included here; calculators that leave it out make thin build-ups look far worse than they are.

Diminishing returns are steep

Going from 50 to 100 mm of PIR saves nearly three times what going from 100 to 150 saves. The first inch is always worth far more than the fourth.

Common questions

What is the difference between R-value and U-value?
They are reciprocals. R is resistance to heat flow — bigger is better, and R-values add across the layers of a wall. U is the rate heat gets through — smaller is better, and it is one divided by the total R. You add R, then invert once.
How thick does my insulation need to be?
Set the other layers, then raise the insulation thickness until the U-value drops below your target. For a wall that is usually 0.18 for new build or 0.30 for an extension. PIR gets there in roughly half the thickness of mineral wool.
Why does 100 mm of mineral wool beat 100 mm of brick so heavily?
Conductivity. Mineral wool is 0.038 W/mK and brick is 0.77 — twenty times higher. The same 100 mm gives R 2.63 as wool and R 0.13 as brick, so the wool does about twenty times the work.
Does the order of the layers change the U-value?
Not the U-value — resistances add in any order. It changes where the dew point falls inside the wall, which is a condensation question rather than a heat-loss one, and it is why insulation position is specified as carefully as its thickness.
Is the cost figure my actual heating bill?
No — it is the cost of the heat lost through this one element, at the price per kWh you entered. Your bill covers every wall, roof, floor, window and door, plus ventilation and hot water.

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