Whole-Building Heat-Loss Coefficient UA
The whole envelope rolled into one number a degree-day estimate and a balance-point calc both start from.
Example
You enter
- Assemblies area 1200, r 17; area 1500, r 38; area 200, r 3; area 1500, r 19
- Natural infiltration (cfm) 50
- Design temperature difference (F, optional) 70
You get
- Conduction + infiltration conductance 255.7
- Heat-loss coefficient UA 309.7 Btu/h-F
- Design heating load 21677
Details, formula, and sources
UA = sum(A_i/R_i) + 1.08 x CFM (Btu/h-F). A small house of R-17 walls, R-38 ceiling, R-3 windows, and R-19 floor with 50 cfm infiltration sums to 309.7 Btu/h-F - times a 70 F design difference, a 21,700 Btu/h load. Air-seal to 25 cfm and upgrade the glass to R-5 and the load drops 17%, the two biggest levers a roll-up exposes. Sensible only; enter the natural infiltration cfm. An energy-audit aid, not a stamped Manual J.
cond = sum(A_i / R_i); UA_inf = 1.08 x CFM; UA = cond + UA_inf; design_load = UA x dT.
The whole-building heat-loss-coefficient roll-up UA = sum(A/R) + 1.08 x CFM with the design load Q = UA x dT, as compiled in the ASHRAE Fundamentals and RESNET energy-audit references, by name.
The UA roll-up and the design-load relation are public building-science results in the ASHRAE Fundamentals and RESNET references.
Estimate. AHJ and licensed professional govern.
Field names used by the API: assemblies, cfm_inf, dt_f, cond, ua, design_load
- Conduction sum of each assembly's area over its clear-field R-valuebuilding science
- Infiltration 1.08 x CFM sensible conductance; enter the natural infiltration cfmASHRAE Fundamentals
- Design load Q = UA x dT at the design temperature difference; sensible onlyASHRAE Fundamentals