Calculate the heat a heated building leaks to the cold winter outdoors. Adjust the envelope area, insulation performance (U-value), indoor-to-outdoor temperature difference and air-change rate to see the heat lost through walls and windows and the heat carried away by ventilation, for energy-efficient design and heating-load estimates.
Parameters
Total envelope area A
m²
Combined area of external walls, roof, floor and windows
How many times per hour the indoor air is replaced
Room volume V
m³
Total volume of the heated space
Results
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Temperature difference (K)
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Fabric (transmission) loss (W)
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Ventilation loss (W)
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Total heat loss (kW)
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Heat-loss coefficient (W/K)
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Heat loss per m² (W/m²)
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Building cross-section — heat-escape animation
Heat escaping the warm interior through the walls, roof, windows and floor (orange arrows), and the exchange of warm air leaving and cold air entering (blue arrows). Arrow sizes are scaled to the two loss components.
Fabric (transmission) loss and ventilation loss. A: envelope area, U: average U-value, ΔT: indoor-to-outdoor temperature difference, n: air-change rate, V: room volume. 0.33 is the volumetric heat capacity of air [W·h/(m³·K)].
The total heat loss is the sum of the two components and is proportional to the indoor-outdoor temperature difference ΔT. The heat-loss coefficient (Q-value) is the loss per kelvin and summarises the building's thermal performance independently of the weather.
What is Building Heat Loss?
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In winter, even with the heating on, the room goes cold again after a while. Does that mean the heat I put in is escaping somewhere?
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Exactly. Because there is a temperature difference between the warm room and the cold outdoors, heat is constantly flowing out — that is "heat loss". So to hold the room at a steady temperature, the heating has to replace exactly the heat that leaks away, watt for watt. Knowing where and how much heat is escaping is the starting point for energy-efficient design, for sizing a heater, and for estimating fuel bills.
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Where exactly are the "escape routes" for the heat?
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There are two main ones. The first is transmission (fabric) loss — heat conducted straight through the walls, roof, floor and especially the windows. Each part of the envelope has a "U-value" (thermal transmittance) that says how readily heat passes through a square metre of it for each degree of temperature difference. A low U-value means good insulation. The fabric loss is simply "envelope area × U-value × indoor-to-outdoor temperature difference". Lower the U-value slider on the left and you will see the fabric loss drop right away.
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I see. And the second route?
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The second is ventilation (infiltration) loss. Every time warm indoor air leaves the building, cold outdoor air comes in to replace it and has to be heated from scratch. It is the same whether the air leaves deliberately through a vent or leaks through cracks and gaps. This loss grows with the air-change rate and the volume of the room. The formula is "0.33 × air-change rate × room volume × temperature difference", and the number 0.33 is the volumetric heat capacity of air.
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Both formulas have the temperature difference in them. Is that why heating gets harder on a cold day?
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That is exactly it. Both the fabric and the ventilation loss are directly proportional to the temperature difference, so the building's heating demand tracks the weather. Combine the two losses per kelvin of difference and you get the heat-loss coefficient — the single most compact summary of how many watts the building loses per degree. Driving that number down is the whole purpose of insulation, high-performance glazing, airtight construction and heat-recovery ventilation. The two great levers are clear from the equations: lower the U-value, and control the air-change rate.
Frequently Asked Questions
Heat loss is the sum of two routes. The fabric (transmission) loss is Q_fabric = A·U·ΔT, where A is the total envelope area, U is the average thermal transmittance and ΔT is the indoor-to-outdoor temperature difference. The ventilation loss is Q_vent = 0.33·n·V·ΔT, where 0.33 is the volumetric heat capacity of air (W·h/(m³·K)), n is the air-change rate and V is the room volume. The total heat loss is their sum, and in steady state it equals the heat the heating system must supply.
The heat-loss coefficient is the heat a building loses per kelvin of temperature difference: Q-value = A·U + 0.33·n·V [W/K]. It does not depend on the outdoor temperature, so it is a property of the building itself and the most compact measure of its thermal performance. A smaller value means better insulation and airtightness and a lower heating load. Since the actual heat loss is just this coefficient times the temperature difference, the formula also explains why heating demand rises on colder days.
The formulas show two effective levers. The first is lowering the U-value: thicker insulation, high-performance glazing (double or Low-E) and fewer thermal bridges all cut the fabric loss directly. The second is controlling the air-change rate: stop unintended infiltration with airtight construction, and provide the required fresh air through heat-recovery ventilation that reclaims the heat in the leaving warm air. Note that windows, despite a small area, often dominate the fabric loss because their U-value is so high.
Both the fabric loss and the ventilation loss are proportional to the indoor-to-outdoor temperature difference ΔT, so when the outdoor temperature equals the indoor temperature (ΔT = 0) the total heat loss is zero. When the outdoor temperature rises above the indoor temperature, ΔT becomes negative and so does the heat loss — that is, heat flows from outside into the building. This tool keeps the sign and shows it correctly as a region where no heating, and possibly cooling, is needed.
Real-World Applications
Energy-efficient home design and insulation ratings: When designing a new house or a renovation, lowering the average envelope U-value (the U_A value) is central to meeting energy codes. Separating the fabric loss (from envelope area and U-value) from the ventilation loss (from air-change rate and volume), as this tool does, builds the foundation for the design decision of whether to strengthen insulation or revisit the airtightness and ventilation plan. You can also feel how much the fabric loss moves when window glazing is upgraded from double to triple.
Sizing heating equipment: The capacity in kilowatts of an air conditioner, underfloor heating system or boiler must be able to replace the heat the building loses on its coldest day. Enter the design outdoor temperature (the regional severe-cold value) in the outdoor-temperature slider and read the total heat loss to get a first estimate of the required heating capacity. Too little capacity leaves the room cold in mid-winter; too much causes frequent on-off cycling that hurts efficiency and comfort.
Estimating fuel bills and carbon emissions: Multiplying the heat-loss coefficient by the local heating degree-days (the indoor-to-outdoor temperature difference summed over a year) gives an estimate of the annual heating energy. Multiply that by the fuel price and the equipment efficiency (COP) for the cost, or by an emission factor for the CO₂ output. It is the starting point for estimating the payback period of an insulation retrofit.
Pre-study for building thermal-load simulation: Before running a detailed dynamic thermal-load calculation in EnergyPlus or TRNSYS, a steady-state estimate like this tool gives a first read on whether transmission or ventilation dominates and on the order of magnitude of the total loss. If the detailed result differs from this estimate by an order of magnitude, it is a useful sanity check pointing to an error in the entered area, U-value, ventilation rate or weather data.
Common Misconceptions and Pitfalls
The first big misconception is that "windows are small in area, so they do not matter for heat loss". It is true that windows are only 20-30% of the whole envelope area, but a window's U-value can be close to that of an uninsulated wall — several times that of an insulated wall. Since the fabric loss is "area × U-value", a window with an outsized U-value becomes the largest single route of fabric loss even with a small area. In a poorly insulated house, nearly half the heat escapes through the windows. This tool uses a single average U-value and is meant for estimates; in detailed design, split the calculation by part, with its own U-value and area.
Next, the hasty conclusion that "raising airtightness and cutting ventilation saves energy". If you throttle ventilation below the required fresh-air rate just to cut the ventilation loss, indoor CO₂, humidity and chemicals build up, causing condensation, mould and health problems. That is why building codes require a minimum of about 0.5 air changes per hour for occupied rooms. The correct answer is to stop unintended infiltration with airtight construction, and to provide the required fresh air with heat-recovery ventilation that reclaims the heat from the leaving warm air. Airtightness and ventilation must be planned together.
Finally, assuming that "this calculation alone fixes the heating load". This tool deals only with steady-state heat loss; the actual heating load also includes solar heat gain through windows (an ally in winter), internal gains from occupants, appliances and cooking, the time lag from the building's thermal mass, and the special heat transfer of a floor in contact with the ground. The heat loss gives the upper-bound skeleton of the heating load, but once solar and internal gains are accounted for, the real demand is often smaller. This tool is for conceptual understanding and quick estimates.