House Heat Loss Calculator

Calculator

Unit system

Thermal input mode

Indoor temperature (°C)

Outdoor design temperature (°C)

Wall area (m²)

Wall U-value

Roof or ceiling area (m²)

Roof U-value

Floor area (m²)

Floor U-value

Window area (m²)

Window U-value

Door area (m²)

Door U-value

House volume (m³)

Air changes per hour

Planned ventilation (m³/h)

Heat recovery efficiency (%)

Thermal bridge allowance (%)

Safety factor (%)

Internal gains to subtract (W)

Formula Used

Transmission heat loss is calculated with:

Q = U × A × ΔT

Here, Q is heat loss in watts, U is thermal transmittance, A is area, and ΔT is indoor minus outdoor temperature.

Air heat loss is calculated with:

Qair = 0.33 × airflow × ΔT

The airflow is measured in cubic metres per hour. The constant includes air density and specific heat. The calculator adds thermal bridges, subtracts chosen internal gains, and applies the safety factor.

How to Use This Calculator

Select metric or imperial units.
Choose U-value mode or R-value mode.
Enter indoor and outdoor design temperatures.
Add areas and thermal values for each building part.
Enter house volume, leakage, and planned ventilation.
Add bridge allowance, heat recovery, and safety factor.
Press Calculate to view the result above the form.
Use CSV or PDF buttons to save the result.

Example Data Table

Input	Example value	Purpose
Indoor temperature	21 °C	Target room temperature
Outdoor temperature	-5 °C	Cold design condition
Wall area and U-value	160 m², 0.35	Wall transmission loss
Roof area and U-value	90 m², 0.18	Roof transmission loss
Windows	24 m², 1.80	Glazing heat loss
Air leakage	450 m³, 0.60 ACH	Infiltration heat loss
Estimated result	8.16 kW	Heating capacity estimate

Heat Loss Planning Basics

House heat loss shows how fast warmth leaves a building. It uses design weather, surface areas, insulation levels, and air leakage. The result helps size heaters, compare upgrades, and find weak envelope parts. A good estimate keeps rooms comfortable without selecting equipment that is far too large.

Why Each Surface Matters

Walls, roofs, floors, windows, and doors lose heat by conduction. Each part has an area and a U-value. A high U-value means heat moves faster. A low U-value means stronger insulation. Windows often lose more heat per square metre than insulated walls, so small window changes can matter.

Air Leakage and Ventilation

Cold outdoor air also enters through gaps, vents, doors, and mechanical systems. This calculator uses air changes per hour and house volume. It can also include planned ventilation flow. Heat recovery reduces that planned ventilation loss. Infiltration is not treated as recovered heat, because leaks usually bypass heat exchangers.

Thermal Bridges and Margin

Beams, studs, slab edges, and poor junctions can bypass insulation. The bridge allowance adds a percentage to the transmission loss. A safety margin is then applied to the full design load. This margin can cover uncertainty, aging materials, and slightly colder weather.

Using Better Inputs

Use measured areas when possible. Subtract window and door areas from wall area if your wall value is net. Keep all U-values in the selected unit system. Use R-values only when you choose the R-value mode. Pick outdoor design temperature from local winter data. Use the indoor temperature you actually want during cold hours.

Reading the Output

The final result is shown in watts, kilowatts, and BTU per hour. Line results show which path drives the load. This is useful before buying a boiler, furnace, heat pump, or room heater. The numbers are planning estimates, not a certified energy model. For permits or large investments, ask a qualified energy professional.

Practical Upgrade Choices

Start with the largest line item. Air sealing may help when leakage is high. Attic insulation often gives strong savings. Window upgrades may improve comfort near glass. Floor loss may matter over crawl spaces. Recalculate after each change to see the effect.

This makes the calculator useful for planning staged improvements carefully.

FAQs

What is house heat loss?

House heat loss is the rate at which indoor heat leaves through surfaces and air movement. It helps estimate required heating capacity during cold weather.

Should I use U-values or R-values?

Use the value type you already know. U-value measures heat transfer. R-value measures resistance. The calculator converts R-values to U-values internally.

What outdoor temperature should I enter?

Use a local winter design temperature. It should represent a cold condition your heating system must handle, not the yearly average temperature.

Does air leakage matter?

Yes. Leaky houses can lose large heat amounts through infiltration. Enter a realistic ACH value to improve the heating load estimate.

What is a thermal bridge allowance?

It is an added percentage for studs, beams, slab edges, and junctions that bypass insulation. Use higher values for complex or poorly insulated envelopes.

Can this size a heat pump?

It gives a planning heat load. A heat pump should also be checked against its cold weather output, local climate, and installer recommendations.

Why include a safety factor?

A safety factor covers uncertainty in measurements, insulation data, leakage, and weather. Avoid excessive margins because oversizing can reduce comfort and efficiency.

Are the results certified?

No. The result is a practical estimate for planning. For code compliance, major renovations, or equipment contracts, use a qualified professional assessment.