Passive House Heat Loss Calculator

Advanced Heat Loss Inputs

Indoor design temperature (°C)

Outdoor design temperature (°C)

Heated floor area (m²)

Building air volume (m³)

Heating degree days (K·days)

Heating season length (days)

Wall area (m²)

Wall U-value (W/m²K)

Roof area (m²)

Roof U-value (W/m²K)

Floor area to ground (m²)

Floor U-value (W/m²K)

Window area (m²)

Window U-value (W/m²K)

External door area (m²)

Door U-value (W/m²K)

Thermal bridge allowance (W/K)

Airtightness n50 (ACH at 50 Pa)

n50 to natural ACH divisor

Ventilation airflow (m³/h)

Heat recovery efficiency (%)

Internal gains (W/m²)

Useful solar gains before factor (kWh/year)

Gain utilization factor (%)

Design safety factor (%)

Example Data Table

Case	Floor area	n50	Heat recovery	HLC	Peak load	Annual intensity
Compact passive design	140 m²	0.60 ACH	85%	85.42 W/K	2349.05 W	17.50 kWh/m²·year
Improved window package	140 m²	0.40 ACH	90%	73.01 W/K	2007.78 W	11.19 kWh/m²·year
Leaky early concept	140 m²	1.50 ACH	70%	112.92 W/K	3105.30 W	31.72 kWh/m²·year

Formula Used

Temperature difference: ΔT = indoor design temperature − outdoor design temperature.

Transmission coefficient: H_T = Σ(U × A) + thermal bridge allowance.

Infiltration coefficient: H_I = 0.33 × ACH_natural × volume.

Ventilation coefficient: H_V = 0.33 × airflow × (1 − heat recovery efficiency).

Total heat loss coefficient: H = H_T + H_I + H_V.

Peak heat loss: Q_peak = H × ΔT × safety factor.

Annual gross demand: E = H × heating degree days × 24 ÷ 1000.

Annual net demand: Net demand = annual gross demand − useful gains.

How to Use This Calculator

Enter the indoor and outdoor design temperatures first. Add the heated floor area and indoor air volume.

Enter each envelope area and U-value. Use project drawings when possible. Add a thermal bridge allowance if details are not modeled.

Enter n50 airtightness, the conversion divisor, ventilation flow, and heat recovery efficiency. Adjust gains only for annual demand.

Press Calculate to view the result above the form. Use the CSV or PDF button to save the same calculation.

Passive House Heat Loss Overview

A passive house heat loss estimate studies how heat leaves a building. The aim is simple. Keep comfort high while heating demand stays low. Heat moves through walls, roof, floor, windows, doors, leakage paths, and ventilation air. This calculator brings those paths into one design view.

Why This Physics Matters

Heat flow follows a temperature difference. When indoor air is warmer than outdoor air, energy moves outward. A better envelope lowers that flow. A tight shell reduces uncontrolled air change. A heat recovery unit cuts ventilation loss by passing warmth from exhaust air to fresh supply air.

Good passive design starts with the heat loss coefficient. This value shows watts lost per degree of temperature difference. Low values mean the building resists heat flow well. The peak load then uses the design temperature gap. It tells you the heater size needed during a cold condition. The annual demand uses heating degree days. It gives a seasonal energy estimate.

Inputs That Change Results

U-values and areas control transmission loss. Window quality matters because glass usually loses more heat than insulated opaque parts. Thermal bridges add extra loss at junctions, balconies, slabs, and framing lines. Air leakage is entered as n50. The calculator converts it to an estimated natural air change rate using a shielding divisor for early estimates.

Ventilation also matters. Passive houses need clean air. Heat recovery efficiency reduces the heat penalty of that fresh air. Internal gains and solar gains can offset annual heating demand. Use them carefully, because peak sizing should not rely on uncertain sunshine.

Design Use

Use this tool early in planning. Test wall insulation, better windows, tighter leakage, and stronger heat recovery. Compare the watts per square meter result with low energy targets. A passive house often aims for very low peak load and annual demand. Final certification still needs detailed software, local climate data, and professional review.

Practical Advice

Enter realistic dimensions. Avoid guessing window areas. Check each U-value source. Use conservative outdoor design temperatures. Add thermal bridge loss when details are unknown. Recalculate after every design change. The best result is not only a low number. It is a balanced design that feels steady and comfortable all winter.

FAQs

What is passive house heat loss?

It is the rate at which heat leaves a low energy building through surfaces, air leakage, ventilation, and thermal bridges. It helps estimate peak heating load and annual heating demand.

Is this the same as certified passive house software?

No. This is a planning calculator. Certification needs detailed climate files, verified assemblies, shading, monthly balances, and approved methods. Use this tool for early design checks.

Why do U-values matter so much?

U-values measure heat flow through each building part. Lower U-values mean less heat escapes through walls, roofs, floors, windows, and doors during cold weather.

What does n50 mean?

n50 is air changes per hour during a blower door test at 50 pascals. Passive projects often aim for very low leakage to reduce uncontrolled heat loss.

Why is heat recovery included?

Fresh air is needed for health. Heat recovery reduces the energy lost with exhaust air by warming incoming air through a heat exchanger.

Should solar gains reduce peak heating load?

Usually not for conservative sizing. Winter sunshine can be uncertain during the coldest hour. This calculator uses gains for annual demand, not peak load.

What is a thermal bridge allowance?

It is extra heat loss from junctions and structural paths. Examples include balcony connections, slab edges, framing interruptions, and poorly insulated corners.

How can I lower the final result?

Improve insulation, reduce window U-values, cut air leakage, increase heat recovery, and remove thermal bridges. Compact building shapes also reduce envelope area.