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Room Transmission Heat Loss with Fixed Opening Sizes

Estimate a room’s simplified transmission heat loss using wall U-value, captured window and door assumptions, and indoor/outdoor temperatures.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

For a 5 m by 5 m room, 2.7 m high, with four external walls, no openings and wall U-value 1 W/(m²·K), a 40 °C temperature difference gives 2160 W of modeled wall heat loss.

Transmission power = modeled heat-loss coefficient × (indoor temperature − outdoor temperature)

Worked example

Enter these known values and leave the other values blank.

Length
5 m
Width
5 m
Height
2.7 m
Number of external walls
4
Insulation
Preset wall U-value: 1 W/(m²·K)
Level
Intermediate level
Window count (1.08 m² each)
0
Door count (1.8 m² each)
0
Internal
20 °C
Ambient
-20 °C
Wall U-value
1 W/(K⋅m²)
Modeled heat-loss coefficient
54 W/K
Transmission heating power only
2160 W

Assumptions and limitations

  • External wall area is the full room perimeter times height times external-wall count divided by four. This equal-share approximation cannot identify which long or short walls are exposed in a non-square room.
  • Every window is fixed at 1.08 m² with U-value 2.5 W/(m²·K); every door is 1.8 m² with U-value 2.4 W/(m²·K). Total opening area cannot exceed modeled external-wall area.
  • The captured floor-level coefficient adds floor area times 1 W/(m²·K) for ground floor, 0.7 for top floor, and zero for intermediate floors. These are fixed assumptions, not measured construction properties.
  • The model omits ventilation, air leakage, thermal bridges, warm-up demand, solar gains and equipment efficiency. It estimates transmission only and does not provide a complete heating-system size.
  • Indoor temperature must be at least outdoor temperature for this heating estimate. Equal temperatures give zero transmission power.

Common questions

Can I use the preset insulation labels as measured U-values?

No. The choices are fixed illustrative coefficients. Enter a verified assembly U-value in custom mode when one is available; the opening and floor assumptions still remain fixed.

References

Bookify permits zero heat flow at equal temperatures, rejects negative heating demand and prevents fixed-size openings from exceeding the modeled wall area. Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor. The power unit uses the International Table BTU per hour consistently with the reviewed energy convention.

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