Heat Loss / Gain Calculator

Unit system:

SI IP

1) General

Floor area (m²)

Ceiling height (m)

Elevation pressure (kPa)

Indoor setpoint (°C)

Outdoor design (°C)

Indoor RH (%)

Outdoor RH (%)

2) Envelope areas, U-values

Walls area (m²)

U walls (W/m²·K)

Roof area (same unit)

U roof (W/m²·K)

Floor area

U floor

Window area

U windows

3) Infiltration & ventilation

ACH (air changes/hour)

Fresh air (L/s)

4) Solar & internal gains

Solar irradiance (W/m²)

Window SHGC (0–1)

People count

People sensible (W/person)

People latent (W/person)

Lighting (W)

Appliances (W)

Results

Component	Sensible (W)	Latent (W)	Sensible (BTU/h)	Latent (BTU/h)
Total Cooling Sensible	0		0
Total Cooling Latent		0		0
Total Cooling Load (W + BTU/h)	0	0	0	0
Required Heating (net)	0		0

Cooling sensible includes envelope, infiltration, ventilation, internal and solar gains. Cooling latent includes people and moisture from infiltration/ventilation. Heating requirement net subtracts internal and solar gains.

Example data table

Parameter	Value	Notes
Area	120	m², single‑storey home
Ceiling height	2.7	m
Indoor / Outdoor	24 / 40	°C summer design
RH in / out	50 / 45	%
Walls A · U	160 · 0.45	m² · W/m²·K
Roof A · U	120 · 0.25	—
Floor A · U	120 · 0.35	—
Windows A · U	18 · 1.6	—
ACH / Fresh air	0.6 / 30	h⁻¹ / L·s⁻¹
Irradiance · SHGC	500 · 0.5	W/m² · —
People	4	75 W sensible, 55 W latent each
Lighting / Appliances	600 / 800	W

Formulas used

Transmission: Q = U × A × ΔT per element (W).
Infiltration sensible (SI): Q = 0.33 × ACH × V × ΔT where V = area × height (W).
Infiltration sensible (IP): Q(BTU/h) = 1.08 × CFM × ΔT.
Ventilation sensible (SI): Q = 1.207 × L/s × ΔT (W), with 1.207 ≈ ρ × c_p.
Solar gain: Q = SHGC × Irradiance × A_window (W).
Humidity ratio: W = 0.62198 × p_w / (p_a − p_w), with p_w = RH × p_ws(T).
Latent (vent/infil): P = ṁ × h_fg × (W_out − W_in) (W), using ṁ = ρ × flow.
Conversions: 1 W = 3.412142 BTU/h, 1 ft² = 0.092903 m², 1 °FΔ = 5/9 K.

How to use this calculator

Select SI or IP units. Enter geometry, temperatures, and humidity.
Fill envelope areas and U‑values for walls, roof, floor, and windows.
Provide infiltration rate (ACH) and any outdoor air flow for ventilation.
Enter window SHGC and solar irradiance for peak sun conditions.
Add internal gains: people, lighting, and appliances. Click Calculate.
Review results. Export the summary as CSV or a formatted PDF.
Use the example button to preload a typical small home scenario.

This tool offers simplified peak design estimates. For code compliance or critical facilities, corroborate with detailed load procedures and local standards.

Notes & assumptions

Air density ρ ≈ 1.2 kg/m³; c_p ≈ 1.006 kJ/kg·K; h_fg ≈ 2.45 MJ/kg.
Magnus formula for saturation pressure; barometric pressure set by elevation input.
Internal and solar gains offset heating requirement but add to cooling.

Typical envelope U-values

Assembly	U (W/m²·K)	U (BTU/h·ft²·°F)	Notes
Insulated cavity wall	0.35–0.50	0.062–0.088	Varies by insulation and studs
Well-insulated roof	0.16–0.30	0.028–0.053	Thicker insulation lowers U
Slab on grade	0.30–0.45	0.053–0.079	Edge insulation improves
Double glazing	1.6–2.8	0.28–0.49	Low‑e coatings reduce U
Triple glazing	0.7–1.2	0.12–0.21	Best for cold climates

Air infiltration classes and ACH ranges

Building tightness	ACH (h⁻¹)	Comments
Tight construction	0.20–0.60	New builds, air-sealed
Typical home	0.70–1.00	Average leakage
Leaky / old	1.50–3.00	Poor seals and cracks

Internal gains quick references

Source	Sensible	Latent	Unit	Notes
Person, seated	70–85	45–60	W per person	Activity dependent
Office lighting (LED)	5–10	—	W/m²	Heat equals power
Appliances, typical	3–8	—	W/m²	Intermittent use
Kitchen equipment	15–30	5–15	W/m²	High latent from cooking

Quick tips

Typical SHGC: 0.25–0.55; adjust by glazing and shading.
ACH: 0.2–0.6 tight homes, 0.7–1.0 typical, 1.5+ leaky.
Ventilation: 7–10 L/s per person for offices; homes vary widely.
Use lower outdoor temperature for heating, higher for cooling.
Solar irradiance varies by orientation; noon sun can exceed 700 W/m².