Calculating Heat Loss: A Step-by-Step Guide

Heat loss refers to the amount of heat energy that escapes from a building or a home, usually through doors, windows, floors, walls, and the roof.

In simple terms, the lower the heat loss, the less energy you need to keep your home warm. Calculating heat loss correctly allows for proper heating system design and can make a building more energy efficient and reduce heating bills.

How to Reduce Heat Loss

• Insulate your home:
• Proper insulation is the most effective way to prevent heat loss. Therefore, consider insulating your walls, roof, and floors.
• Upgrade your windows:
• Double-glazed or triple-glazed windows can significantly reduce heat loss.
• Seal any drafts:
• Make sure doors and windows are properly sealed to prevent drafts.
• Install a heat recovery system:
• These heating systems can capture and reuse heat that would otherwise be lost, particularly from ventilation.

Why is Calculating Heat Loss Important?

• Accurate calculations allow you to properly design and size a heating system.
• Precise calculations allow for more efficient energy use, reducing costs and environmental impact.
• Can help you identify problem areas in your home where you could improve heat retention.

How to Calculate Heat Loss

To calculate heat loss, it’s first important to understand the two key types:

• loss of transmission (heat escaping through surfaces like walls, windows, roofs), and
• loss of ventilation (heat loss due to air changes per hour).

Transmission losses

Each component of the building (walls, roof, windows, etc.) has its own U-value, which measures how much heat it allows to pass through, and must be calculated separately.

Each component is calculated using the basic formula:

Ventilation losses

These types of losses occur when hot air inside the building is replaced by colder outside air through ventilation or infiltration.
They can be calculated using the formula:

Where the Air Rate Change represents how often the air in the building is completely replaced.

Thermal bridging

Another key factor to consider is thermal bridging, which occurs when a part of the building envelope conducts more heat than surrounding areas. This phenomenon can work against your efforts to promote heat retention, as the conductive components create a pathway for heat to escape.

Thermal bridging is added to the heat loss calculation through a ‘Y-value’, which represents the total extra heat loss from thermal bridges.

The final heat loss formula, considering all these factors, is:

Click here to try out our free Heat Loss Calculator to find your heat losses and temperature change for a building.

Common U-Values for Materials

Walls:

• Solid Brick: 2.1 W/m²K (0.370 BTU/hr·ft²·°F).
• Solid Brick Insulated: 0.28 W/m²K (0.049 BTU/hr·ft²·°F).
• Cavity Wall Uninsulated: 1.3 W/m²K (0.229 BTU/hr·ft²·°F).
• Cavity Wall Insulated: 0.55 W/m²K (0.097 BTU/hr·ft²·°F).
• Solid Stone: 2.25 W/m²K (0.396 BTU/hr·ft²·°F).
• Solid Stone Insulated: 0.32 W/m²K (0.056 BTU/hr·ft²·°F).
• Plasterboard: 0.16 W/m²K (0.028 BTU/hr·ft²·°F).
• Solid Concrete: 3 W/m²K (0.528 BTU/hr·ft²·°F).
• Solid Concrete Insulated: 0.31 W/m²K (0.055 BTU/hr·ft²·°F).
• Hardwood: 0.18 W/m²K (0.032 BTU/hr·ft²·°F).
• Softwood: 0.13 W/m²K (0.023 BTU/hr·ft²·°F).

Doors:

• Solid Wood Door: 3 W/m²K (0.528 BTU/hr·ft²·°F).
• Glazed Wood Single: 5.7 W/m²K (1.004 BTU/hr·ft²·°F).
• Glazed Wood Double: 3.4 W/m²K (0.599 BTU/hr·ft²·°F).
• Glazed Wood Triple: 2.6 W/m²K (0.458 BTU/hr·ft²·°F).
• Metal Single: 5.7 W/m²K (1.004 BTU/hr·ft²·°F).
• Metal Double: 3.4 W/m²K (0.599 BTU/hr·ft²·°F).
• Metal Triple: 2.6 W/m²K (0.458 BTU/hr·ft²·°F).

Windows:

• Metal Single Glazed: 5.7 W/m²K (1.004 BTU/hr·ft²·°F).
• Metal Double Glazed: 3.4 W/m²K (0.599 BTU/hr·ft²·°F).
• Metal Triple Glazed: 2.6 W/m²K (0.458 BTU/hr·ft²·°F).
• Wood Single Glazed: 4.8 W/m²K (0.846 BTU/hr·ft²·°F).
• Wood Double Glazed: 2.8 W/m²K (0.493 BTU/hr·ft²·°F).
• Wood Triple Glazed: 2.1 W/m²K (0.370 BTU/hr·ft²·°F).
• PVC Single Glazed: 4.8 W/m²K (0.846 BTU/hr·ft²·°F).
• PVC Double Glazed: 2.8 W/m²K (0.493 BTU/hr·ft²·°F).
• PVC Triple Glazed: 2.1 W/m²K (0.370 BTU/hr·ft²·°F).

Floors:

• Concrete: 1.35 W/m²K (0.238 BTU/hr·ft²·°F).
• Steel: 50 W/m²K (8.806 BTU/hr·ft²·°F).
• Screed: 1.2 W/m²K (0.211 BTU/hr·ft²·°F).
• Softwood: 0.13 W/m²K (0.023 BTU/hr·ft²·°F).
• Hardwood: 0.18 W/m²K (0.032 BTU/hr·ft²·°F).
• Wood Blocks: 0.14 W/m²K (0.025 BTU/hr·ft²·°F).

Roof:

• Aerated Concrete: 0.16 W/m²K (0.028 BTU/hr·ft²·°F).
• Asphalt: 0.5 W/m²K (0.088 BTU/hr·ft²·°F).
• Felt/Bitumen: 0.3 W/m²K (0.053 BTU/hr·ft²·°F).
• Screed: 0.41 W/m²K (0.072 BTU/hr·ft²·°F).
• Stone Chippings: 0.96 W/m²K (0.169 BTU/hr·ft²·°F).
• Clay Tiles: 1 W/m²K (0.176 BTU/hr·ft²·°F).
• Concrete Tiles: 1.5 W/m²K (0.264 BTU/hr·ft²·°F).
• Wood / Wool: 0.1 W/m²K (0.018 BTU/hr·ft²·°F).

Calculating Heat Loss: Example

Building information

Room dimensions (length x width x height): 5m x 4m x 2.5m (16.4ft x 13.1ft x 8.2ft)

External wall total area (minus window and door): 40 m² (430.6 ft²)

Window area: 4 m² (43.1 ft²)

Door area: 2 m² (21.5 ft²)

Roof area: 20 m² (215.3 ft²)

Floor area: 20 m² (215.3 ft²)

Chimney area: 0.5 m² (5.4 ft²)

U-values for building components

External walls: 0.3 W/m²K (0.053 BTU/hr·ft²·°F)

Windows: 1.6 W/m²K (0.282 BTU/hr·ft²·°F)

Door: 1.5 W/m²K (0.264 BTU/hr·ft²·°F)

Roof: 0.25 W/m²K (0.044 BTU/hr·ft²·°F)

Floor: 0.25 W/m²K (0.044 BTU/hr·ft²·°F)

Chimney: 2.0 W/m²K (0.352 BTU/hr·ft²·°F)

Indoor and outdoor design average temperature:

Indoor design temperature (inside temperature): 21°C (69.8°F)

Outdoor design temperature (outside temperature): -3°C (26.6°F)

Difference in temperature: 24°C (43.2°F difference)

Heat loss calculations:

• Heat loss through walls, windows, door, roof, and floor:
• Walls: 0.3 W/m²K x 40 m² x 24°C = 288 W (0.053 BTU/hr·ft²·°F x 430.6 ft² x 43.2°F = 984.4 BTU/hr)
• Windows: 1.6 W/m²K x 4 m² x 24°C = 153.6 W (0.282 BTU/hr·ft²·°F x 43.1 ft² x 43.2°F = 524.5 BTU/hr)
• Door: 1.5 W/m²K x 2 m² x 24°C = 72 W (0.264 BTU/hr·ft²·°F x 21.5 ft² x 43.2°F = 245.4 BTU/hr)
• Roof: 0.25 W/m²K x 20 m² x 24°C = 120 W (0.044 BTU/hr·ft²·°F x 215.3 ft² x 43.2°F = 409.2 BTU/hr)
• Floor: 0.25 W/m²K x 20 m² x 24°C = 120 W (0.044 BTU/hr·ft²·°F x 215.3 ft² x 43.2°F = 409.2 BTU/hr)
• Total: 288 W + 153.6 W + 72 W + 120 W + 120 W = 753.6 W (984.4 + 524.5 + 245.4 + 409.2 + 409.2 = 2,572.7 BTU/hr)
• Heat loss through ventilation:
• Air changes per hour (ACH): 0.5
• Room volume: 5m x 4m x 2.5m = 50 m³ (16.4ft x 13.1ft x 8.2ft = 1,766 ft³)
• Thermal energy:
• Thermal bridge correction factor: 0.05
• Heat loss from thermal bridging: Original heat loss x Thermal bridge correction factor = 753.6 W x 0.05 = 37.68 W (2,572.7 BTU/hr x 0.05 = 128.6 BTU/hr)

Total heat loss:

• Total heat loss = Original heat loss + Ventilation heat loss + Thermal bridging =
• 753.6 W + 198 W + 24 W + 37.68 W = 1013.28 W (2,572.7 + 675.7 + 81.9 + 128.6 = 3,459.0 BTU/hr)

R-Value

The measurement of “R-Value” is an essential component in calculating heat loss.

R-values denote the thermal resistance of building materials. As such, they play a crucial role in determining a structure’s ability to retain heat and resist temperature fluctuations, due to heat transfer.

R-Value and Heat Loss Calculation

Beyond just insulation, R-values significantly impact mechanical ventilation and overall energy models.

For example, when evaluating a house, particularly in the context of a shared party wall and accounting for the surface area, the effective R-value becomes paramount in your heat loss calculations. Accurately measuring and accounting for R-values helps you design a system that is better able to maintain a comfortable indoor temperature.

By accurately measuring the difference in temperature between the inside and outside environments, and considering factors such as windows and doors, engineers can ensure their design retains heat in the most efficient way possible.

Finally, capturing R-values accurately along the front wall, surface areas, and points of potential heat transfer like windows and doors can make all the difference in achieving a sustainable and cozy living space.

h2x R-Value Calculator

h2x’s free online R-value calculator considers variables such as the material type and thickness, thermal conductivity, and the number of layers.

This calculation helps engineers determine the energy of watts divided by time needed to maintain a comfortable indoor temperature while factoring in the overall energy efficiency of the building, and the appropriate R-value of insulation materials, to achieve optimal thermal resistance.

Conclusion

Calculating heat loss correctly is essential in designing a high-performing heating system.

This is because doing so allows you to design a suitable, efficient heating system or domestic hot water system and identify where additional insulation could help reduce your energy bills.

Always remember: the lower the U-value, the less heat lost from your building. In turn, this raises the heat transfer coefficient and makes your building more energy efficient.

Just as important, different types of materials used in the building, thermal bridging, and air infiltration also play a significant role in heat loss.

Therefore, it’s crucial to take all these factors into account while doing your calculations to get accurate results.

By following the simple step-by-step guide above, you can easily calculate the heat loss from your home or building.

Frequently Asked Questions (FAQs)

1. What is a U-value?

The U-value measures heat loss in a building element, such as a wall, ground floor, or roof.

Essentially, it measures how well different components of a building transfer heat.

The lower the U-value, the better the material is at insulating.

2. How do you calculate U-value?

To calculate the U-value, divide the rate of heat transfer by the internal temperature gradient and external temperature gradient difference between the building, and the surface area through which the heat required is being transferred.

This calculation often requires specific knowledge of the materials being used.

3. What other factors affect heat loss in a building?

Many factors affect heat loss, including the materials used in construction, the building’s shape and orientation, the level of insulation, the number of windows and doors, thermal bridging, and ventilation rates.

4. What is thermal bridging?

Thermal bridging occurs when a part of the building envelope is more conductive than the surrounding materials, which results in a path of least resistance for heat transfer.

Common locations for thermal bridging include gaps in insulation, window and door openings, and where walls meet the roof or ground floor.

5. How does insulation give you a lower temperature?

Insulation works by providing a highly resistant barrier to the flow of heat.

It keeps warm air in during the winter and hot air out during the summer, which helps maintain a comfortable internal temperature for the building year-round.

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