Heat loss through a wall:
what the U-value does to your heating bill

What the U-value says

A U-value is the rate of heat flow, in watts, through one square metre of a building element for every degree of temperature difference across it. A lower U-value means better insulation. A bare solid wall might have a U-value of 2 or more, a well-insulated one a few tenths, and modern standards push lower still.

It already includes the materials in the wall, the air films on both faces and any cavity, so you do not add up layers yourself unless you are computing the U-value from scratch.

Because it is already per square metre and per degree, scaling it up to a whole wall is a straightforward multiplication.

The formula and a worked wall

Over a full day at that constant difference, 180 W × 24 h = 4,320 Wh, which is 4.32 kWh. That equals the output of a 1 kW heater running for a little over four hours.

How the U-value changes the answer

Everything else in the sum is fixed by the room and the weather, so the U-value is where an upgrade makes its difference. The table uses the same 30 m² wall and 15 °C difference.

The values are examples to show the scale, not guarantees for any construction. Use the declared or calculated U-value for your wall, which depends on the materials and thickness.

The temperature difference does the rest

Heat loss is proportional to the difference between inside and outside, so it is not constant through the year. The same wall that loses 180 W at 15 °C loses 360 W when the difference doubles to 30 °C, as it might on a freezing night.

This is why design calculations use a design outdoor temperature for the local climate. That figure comes from local weather data or standards, so look it up for your location instead of using the example number.

A wall is only part of the picture

Most rooms have glazing, and glass loses heat much faster per square metre than an insulated wall. Applying the same formula to each element and adding the results shows how much of the loss the windows account for.

The 4 m² of window loses nearly half as much as the 30 m² of wall, which is why glazing upgrades and curtains often pay back well. The U-values here are illustrative, so use the declared values for your own products.

From watts to a season of energy

The instantaneous loss is useful for sizing equipment, but the energy bill depends on how long and how cold the heating season is. The industry shortcut is the degree-day: the sum, over the season, of the daily difference between a base temperature and the outdoor average.

Divide by the efficiency of the heating system to find the fuel or electricity bought. A heat pump with a coefficient of performance of 3 would use about a third of that as electricity.

What the formula leaves out

The sum describes steady conduction through the wall only. A house loses heat in several other ways, and a complete heat-loss estimate adds them up.

• Windows and doors, which usually have higher U-values than the walls around them.
• Roof and ground floor, treated with the same U × A × ΔT approach.
• Air leakage and ventilation, which are measured with air changes per hour.
• Thermal bridges at junctions, where heat bypasses the insulation.
• Solar and internal gains, which offset some of the loss.

A general building energy load tool collects these terms into a whole-building estimate, though it still depends on correct U-values and areas as inputs. For a quick check of one wall, the three-factor formula is enough, and it makes clear what an insulation upgrade might save.

Where the loss can be reduced

The formula shows three levers. The area is fixed by the building, the temperature difference by the climate and the comfort you want, and the U-value by the construction. That leaves the U-value as the practical lever, together with the setpoint.

• Add insulation to the wall, internally or externally, which lowers U directly.
• Lower the indoor setpoint by one degree; the loss falls in proportion to the difference.
• Replace the weakest elements first, since the highest-U parts lose the most per square metre.
• Seal gaps, because air leakage adds loss that the conduction formula does not count.

Cost and disruption vary a lot between these options, so compare the saving in kWh with the cost of the work. The earlier table shows a sizeable cut for each step down in U-value.

Finding a U-value for your own wall

The U-value of an existing wall can be taken from the manufacturer's data, from a survey report, from building regulations tables for the construction age, or calculated by adding the thermal resistance of each layer. The U-value is the reciprocal of the total resistance, including the air films on each face.

A measured value from a heat flux meter is the most reliable for an old wall whose build-up is unknown, because the real moisture content and gaps affect performance.