Heat Loss and U-Value Calculator
Add wall, roof or floor layers to get total R-value and U-value in SI and US units, then heat loss in watts and BTU/h plus a degree-day seasonal estimate.
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About this toolWhat is the Heat Loss and U-Value Calculator?
Heat leaves a building through its walls, roof and floor in proportion to three things: how well the construction resists heat flow, how much of it there is, and how much colder it is outside. This calculator handles the first part properly. You list the layers from the inside face to the outside, give each a thickness and a thermal conductivity (or a known R-value for an air gap or a branded product), and it adds the resistances in series with the inside and outside surface films to give the total R-value and the U-value.
From there the heat loss is a multiplication: U times area times the indoor to outdoor temperature difference, shown in watts and BTU per hour. Add heating degree-days for your location and you also get a seasonal energy figure in kWh and therms. A breakdown table shows what share of the resistance each layer contributes, and a temperature profile shows the steady-state temperature at every interface, which is where you look first when you are worried about condensation inside a wall.
- Up to ten layers by thickness and conductivity or by known R-value
- ISO 6946 and ASHRAE surface resistances for walls, roofs and floors
- R-value and U-value in both SI and US units at once
- Heat loss in watts and BTU/h, plus seasonal kWh and therms from degree-days
- Layer share table and steady-state interface temperatures
How to use the Heat Loss and U-Value Calculator
- Choose metric or US units. Switching converts every number already entered so the construction stays the same.
- Edit the layers from inside to outside: pick a material to fill its typical conductivity, then set the thickness. Use "Known R-value" for air cavities, reflective foils or a product rated as a whole.
- Pick the surface resistance set that matches the element: ISO 6946 or ASHRAE, for a wall, a roof or a floor.
- Enter the net area (without windows and doors), the indoor temperature and the outdoor design temperature.
- Optionally add heating degree-days to see the seasonal energy loss, then read the U-value, R-values, heat loss and layer breakdown.
Worked examples
Timber frame wall with brick cladding (page default)
Input
12.5 mm plasterboard, 140 mm mineral wool, 11 mm plywood, 0.18 cavity, 102.5 mm brick; ISO 6946 wall films; 20 m², 21 °C inside, −3 °C outside, 2,000 K·day
Output
R 4.302 m²·K/W (R-24.43), U 0.2325 W/m²·K, 111.6 W (380.7 BTU/h), 223 kWh per season
The insulation provides 3.68 of the 4.30 total, about 86%. The brick, often assumed to be a good insulator, adds only 0.13.
US 2x4 wall with an R-13 batt
Input
R-0.45 gypsum, R-13 batt, R-0.62 OSB, R-0.62 siding; ASHRAE wall films; 1,000 ft², 70 °F inside, 10 °F outside
Output
R-15.54, U 0.0644 BTU/h·ft²·°F, 3,861 BTU/h
This is the clear-field value through the cavity. With studs at 16 in centres the whole-wall figure is closer to R-11 or R-12.
Background and insight
The method is the one in ISO 6946 and the ASHRAE Handbook of Fundamentals: thermal resistances of layers that the heat crosses one after another simply add, R = d / k for each homogeneous layer, and U is the reciprocal of the total. Surface resistances account for the thin film of still air on each face. ISO 6946 uses 0.13 m²·K/W inside for walls, 0.10 for heat flowing up through a roof or ceiling, 0.17 for heat flowing down through a floor, and 0.04 outside. ASHRAE tabulates the same idea in US units: R-0.68 for still air on a vertical surface and R-0.17 for a 15 mph winter wind.
US and SI R-values describe the same property in different units, which is why a product sold as R-13 in the United States is RSI 2.29 elsewhere. One m²·K/W equals 5.678 ft²·°F·h/BTU. Conductivities in the material list are typical design values from ISO 10456, CIBSE Guide A and manufacturer literature; insulation products vary with density and age, so the declared lambda value on the datasheet should replace the default whenever you have it.
A clear-field calculation like this ignores thermal bridges. In a timber stud wall the studs occupy roughly 15 to 25% of the area and conduct three times better than mineral wool, so the whole-wall U-value is noticeably worse than the clear-field one. Steel studs are far worse again. Full heat-load methods (ACCA Manual J, EN 12831) add bridging, ventilation and infiltration on top of the fabric losses calculated here.
Common use cases
Comparing insulation upgrades
See how much an extra 50 mm of PIR board or a thicker batt changes the U-value and the seasonal kWh before pricing the work.
Building regulations checks
Check that a proposed wall or roof meets a target U-value such as 0.18 or 0.26 W/m²·K, then confirm with a full ISO 6946 calculation including bridging.
Sizing a room heater
Add up the fabric losses of each wall, ceiling and floor to get a first estimate of the heat a room needs at the design outdoor temperature.
Condensation screening
Use the interface temperatures to see where a wall drops below the indoor dew point and where a vapour control layer belongs.
Tips and limitations
- Calculate windows and doors separately with their own manufacturer U-values; a double-glazed window loses several times more heat per square metre than an insulated wall.
- Ventilated cavities and the layers outside them do not count. If the air gap behind cladding is well ventilated, stop the build-up at the gap and use an outside film of 0.13 instead.
- Compressed or wet insulation performs worse than its rated R-value. A batt squashed into a thinner cavity loses resistance even though its density rises.
- Degree-day energy is heat loss through this element only, before boiler or heat pump efficiency. Divide by the seasonal efficiency to get fuel or electricity bought.
- Ground floors and basement walls lose heat to the ground, not the air. ISO 13370 has the method for those.
Frequently asked questions
How do you calculate the U-value of a wall?
Add the thermal resistance of every layer (thickness divided by conductivity) to the inside and outside surface resistances, then take the reciprocal. A wall with total R of 4.3 m²·K/W has U = 1 / 4.3 = 0.23 W/m²·K.
What is the difference between R-value and U-value?
R-value is resistance to heat flow, so higher is better. U-value is the heat flow per unit area per degree of temperature difference, the reciprocal of total R, so lower is better. U includes the surface films; an R-value printed on insulation usually does not.
How do I convert US R-value to RSI?
Multiply by 0.1761. R-13 is RSI 2.29 and R-30 is RSI 5.28. To go the other way, multiply RSI by 5.678.
What surface resistances should I use?
ISO 6946 gives 0.13 m²·K/W inside for walls, 0.10 for roofs with heat flowing up, 0.17 for floors with heat flowing down, and 0.04 outside. ASHRAE uses R-0.68 inside on a wall and R-0.17 outside with a 15 mph wind.
How is heat loss calculated from the U-value?
Heat loss Q = U x area x temperature difference. At U 0.25 W/m²·K, 20 m² and 24 K between inside and outside, the loss is 0.25 x 20 x 24 = 120 W.
How do heating degree-days give annual heat loss?
Multiply U x area by the degree-days and by 24 hours. A wall with U·A = 5 W/K in a 2,000 K·day climate loses 5 x 2,000 x 24 = 240,000 Wh, which is 240 kWh, over the heating season.
Does the calculator include thermal bridging from studs?
No. It computes the clear-field value through the insulated part. Studs, joists, wall ties and steel framing raise the real U-value, often by 10 to 30% in timber frame and much more with steel studs.
Is there an in-depth guide to the Heat Loss and U-Value Calculator?
Yes. “U-Value, R-Value and Heat Loss: Calculating a Wall Layer by Layer” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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