Heat Loss Calculator

1Room dimensions and exterior walls

2Construction and insulation

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About Heat Loss Calculation

The results are approximate. Before use, verify the calculations against the applicable standards and consult a specialist. The developer is not responsible for the consequences of use without project verification.

This heat loss calculator provides a simple home energy estimate by calculating heat loss through exterior walls, windows, an exterior door, ceiling, floor, and air exchange. It can be used to estimate the heating power required to maintain a selected indoor temperature at a specified winter outdoor temperature and to assess the influence of wall, window, roof, and insulation parameters.

Each component of the heat loss is calculated separately and then added together. The result is shown in watts and kilowatts, as well as specific heat loss per 1 m2 of floor area and per 1 m3 of room volume.

Guidelines and recommendations

Room geometry

Floor area and volume. The floor area is calculated from the room length L and width B. The volume also includes the room height H.

A = L × B

V = L × B × H

Dimensions are entered in metres. Therefore, area A is obtained in m2 and volume V in m3. The floor area is used to calculate heat loss through the ceiling and floor, while the volume is used to calculate heat loss caused by air exchange.

Exterior perimeter. Only room sides that border outdoor air are included in the wall calculation. The actual length of each selected side is used. When all four walls are exterior walls, the exterior perimeter of a rectangular room is:

Pext = 2 × (L + B)

If there are no exterior walls, heat loss through walls, windows, and the exterior door is taken as zero.

Temperature difference

Temperature difference. All heat loss components use the difference between the selected indoor temperature Ti and outdoor temperature Te.

ΔT = Ti - Te

For example, with an indoor temperature of +20 °C and an outdoor temperature of -15 °C, the design temperature difference is 35 K. A temperature difference in kelvins is numerically equal to the same temperature difference in degrees Celsius. If the outdoor temperature is equal to or higher than the indoor temperature, ΔT = 0 is used for the heat loss calculation.

Heat loss through building elements

Basic relationship. Heat loss through walls, windows, the door, ceiling, and floor is calculated as the product of the element area A, thermal transmittance U, and temperature difference ΔT.

Q = U × A × ΔT

Here Q is heat loss in W, U is thermal transmittance in W/(m2·K), A is the area of the element in m2, and ΔT is the temperature difference in K. A larger area, higher U-value, or greater temperature difference results in greater heat loss.

Exterior walls

Wall area. The exterior perimeter is first multiplied by the room height. The window area and the assumed exterior door area are then subtracted because these elements are calculated separately.

Awall = Pext × H - Awindow - Adoor

If the combined area of the windows and door exceeds the total area of the exterior walls, the result is not calculated until the input data is corrected. This prevents a negative wall area from being used.

Wall insulation. Instead of calculating every construction layer separately, the calculator uses predefined approximate thermal transmittance values:

  • poor insulation or an older house - U = 1.30 W/(m2·K)
  • medium insulation - U = 0.60 W/(m2·K)
  • good insulation or a modern house - U = 0.25 W/(m2·K)

These values represent the overall thermal performance of the wall. If the exact U-value of a specific wall construction is known, the simplified insulation options should be treated as approximate reference values.

Windows and exterior door

Glazing. One thermal transmittance value is applied to the entire entered window area according to the selected energy-efficiency level:

  • older windows with high heat loss - U = 2.80 W/(m2·K)
  • modern insulated glazing - U = 1.40 W/(m2·K)
  • energy-efficient modern windows - U = 0.90 W/(m2·K)

Exterior door. Door dimensions are not entered separately. Typical areas and thermal transmittance values are used for the available door types:

  • insulated single door - area 2.0 m2, U = 1.40 W/(m2·K)
  • insulated double door - area 4.0 m2, U = 1.40 W/(m2·K)
  • uninsulated exterior door - area 2.0 m2, U = 3.00 W/(m2·K)
  • glazed single door - area 2.0 m2, U = 1.60 W/(m2·K)
  • glazed double or patio door - area 4.0 m2, U = 1.60 W/(m2·K)

The window and door areas are excluded from the wall area, so heat transfer through the same part of the building envelope is not counted twice.

Ceiling and floor

Ceiling. Heat loss is calculated over the full room area A. The following effective coefficients are used depending on the space or construction above:

  • heated room above - U = 0
  • well-insulated roof or attic - U = 0.18 W/(m2·K)
  • medium roof or attic insulation - U = 0.35 W/(m2·K)
  • poor insulation or cold attic - U = 0.90 W/(m2·K)

Floor. Effective coefficients are also used for the lower boundary of the room to represent typical differences between a heated space, basement, and insulated floor:

  • heated room below - U = 0
  • unheated basement or garage - U = 0.35 W/(m2·K)
  • good floor insulation - U = 0.20 W/(m2·K)
  • medium floor insulation - U = 0.40 W/(m2·K)
  • poor insulation or no insulation - U = 0.80 W/(m2·K)

For both the ceiling and floor, the full room area and the overall indoor-to-outdoor temperature difference are used. These coefficients are simplified effective values, so the temperature of an attic, basement, or ground is not calculated separately.

Air exchange and infiltration

Air heat loss. Part of the heat is used to warm outdoor air entering through ventilation and air leakage. The calculation uses the room volume V, air change rate n, and temperature difference ΔT.

Qair = 0.34 × n × V × ΔT

The coefficient 0.34 represents the approximate volumetric heat capacity of air for this calculation method. The air change rate n indicates how many room volumes of air are replaced by outdoor air per hour.

  • n = 0.3 h-1 - low air exchange
  • n = 0.5 h-1 - normal air exchange
  • n = 0.8 h-1 - moderately increased air exchange
  • n = 1.0 h-1 - increased air exchange
  • n = 1.5 h-1 - high air exchange
  • n = 2.0 h-1 - very high air exchange

For a simple home energy estimate, around 0.5 h-1 is often used for an ordinary residential room. Higher values increase the calculated ventilation heat loss in direct proportion to the air change rate.

Total heat loss

Summation. After each component is calculated separately, the calculator adds the heat loss through exterior walls, windows, the door, ceiling, floor, and air exchange.

Qtotal = Qwall + Qwindow + Qdoor + Qceiling + Qfloor + Qair

Qtotal is the total heat loss in watts under the selected conditions. The value in kilowatts is obtained by dividing the result in watts by 1000.

Specific values. To compare rooms with different floor areas and ceiling heights, heat loss per unit of area and volume is also calculated:

qA = Qtotal / A

qV = Qtotal / V

The first value is expressed in W/m2, and the second in W/m3. The value in BTU/h is calculated using 1 W = 3.41214 BTU/h. The percentage share of each heat loss component is calculated as its heat loss divided by Qtotal.

Related European standards

EN 12831-1 "Energy performance of buildings - Method for calculation of the design heat load - Part 1: Space heating load". This standard establishes the general methodology for determining the design heat load of rooms and buildings, including heat transfer through the building envelope and ventilation heat losses. The calculator uses a simplified sequence for these main heat loss components with predefined coefficients.

EN ISO 6946 "Building components and building elements - Thermal resistance and thermal transmittance - Calculation methods". This standard describes how thermal resistance and thermal transmittance U are determined for building elements. In this calculation, multi-layer walls are not assembled from individual materials. Instead, predefined approximate U-values are used for several insulation levels.

FAQs

What outdoor temperature should be used for a heat loss calculation?

For winter heating calculations, the design outdoor temperature for the cold season in the local area is commonly used rather than the current outdoor temperature. The lower the selected outdoor temperature at the same indoor temperature, the greater ΔT becomes and the higher the calculated heat loss.

How should the exterior wall insulation level be selected?

The selected level determines the thermal transmittance used for the wall. The calculator uses U = 1.30 W/(m2·K) for poor insulation, U = 0.60 for medium insulation, and U = 0.25 for good insulation. If a calculated U-value for the actual wall construction is available, these values can be used as reference points for selecting the closest option.

Why are the window and door areas subtracted from the wall area?

Windows and the exterior door have their own thermal transmittance values and are calculated separately. If their areas were also left within the wall area, the same part of the building envelope would be counted twice and the total heat loss would be overestimated.

How strongly does air exchange affect home heat loss?

Heat loss through air exchange is directly proportional to the air change rate n. For example, with the same room volume and temperatures, increasing n from 0.5 to 1.0 doubles the ventilation component of the heat loss. This means ventilation and infiltration can represent a significant part of the total heating load.

What do the W/m² and W/m³ values indicate?

W/m2 shows how much of the total heating load corresponds to one square metre of floor area, while W/m3 shows the load per cubic metre of room volume. These values are useful for comparing rooms of different sizes, while the total required heat output is represented by Qtotal.