Home Envelope Heat Loss Simulator

Building Envelope

Wall U-value 0.30 W/m²K

Roof U-value 0.20 W/m²K

Window U-value 1.40 W/m²K

Air infiltration 0.60 ACH

Outdoor temperature -5 °C

Live Readouts

Total heat loss (instantaneous)

— W

Annual heating energy

— kWh

Est. annual cost

—

walls — · roof — · windows — · infiltration —

How it works

Steady-state conductive loss through each envelope surface follows Fourier's law in its building-physics form:

Q = U · A · ΔT   (watts)
U = 1 / R   (W/m²K, lower U = better insulation)
ΔT = T_indoor − T_outdoor

Air leaking through gaps and vents carries an additional load set by the volumetric heat capacity of air (≈0.34 Wh/m³K):

Q_infiltration = 0.34 · ACH · Volume · ΔT

This fixed 10×8×2.5 m house (200 m³, 21°C indoors, 13.5 m² of glazing) sums four such terms live as you move the sliders — walls, roof, windows, infiltration — for the total instantaneous loss shown above, exactly as an energy auditor's heat-loss calculation works.

The annual figure scales that same total heat-loss-per-degree (UA, W/K) by 2,200 heating-degree-days — a temperate-climate seasonal constant — converted to kWh, then priced at $0.15/kWh. It answers "how much does this envelope cost to heat over a whole winter," not just right now.

Single-glazed, uninsulated — a poorly-built older home: high U-values, leaky (2.5 ACH).
Passive-house grade — heavily insulated, triple glazing, airtight (0.15 ACH) — the practical efficiency ceiling.
Orange particles stream outward through each surface at a rate proportional to that surface's own share of the loss — thicker streams mean more energy escaping there.

Heat flux (particle rate ∝ loss) House envelope

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