Calculators · Consulting
Fabric heat-loss calculator
Per element · ΣUA · peak transmission load
List the envelope element by element. We sum the U×A, add thermal bridging and (optionally) ventilation, and give the peak heat loss at your design temperatures, the fast sanity check before the full PHPP model.
The envelope
One row per element. U-value × area.
PHPP default for a non-optimised detail set is around 0.05; a Passivhaus-detailed envelope approaches 0.
Heat-recovery efficiency is not applied here, this is the unrecovered case. Set a lower effective ACH for an MVHR system.
A first-pass sense-check, not a PHPP result. PHPP applies reduction factors (ground, adjacency), solar gains and a full ventilation balance the simple ΣUA here does not.
How it’s calculated
- Fabric:
HT_fabric = Σ (U × A)across every element. - Thermal bridging:
y × A_envelope, a flat uplift across the total area, the PHPP simplified approach. - Ventilation (optional):
0.33 × n × V, the unrecovered air-change loss. - Peak load:
Q = HT × (T_int − T_ext).
Why trust it: it is the standard steady-state heat-loss method, the same arithmetic underneath PHPP and SAP. Every model we issue is checked by one of our 7 accredited Passivhaus certifiers.
Take it further
Want the real number, with gains and reductions?
This sizes the fabric. The energy balance, the heating demand and the certification figure need the full PHPP. Send us the build-up and we will model it properly.