Pipe Insulation Heat Loss (bare vs insulated)
Cylindrical conduction + convection + radiation; effectiveness % vs bare.
Example
You enter
- Pipe OD (in) 2.375
- Pipe surface T (°F) 200
- Ambient T (°F) 70
- Insulation type fiberglass
- Thickness (in) 1.5
- Jacket emissivity (0-1) 0.9
You get
- Q bare 202.6 BTU/hr·ft
- Q insulated 22.9 BTU/hr·ft
- Effectiveness 88.7 %
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeSix insulation k-values with manufacturer attribution.
Formula or tableR_cond = ln(r2/r1) / (2π × k); h_outside = h_conv + h_rad; R_outside = 1 / (h_outside × 2π × r2); Q = (T_s − T_a) / (R_cond + R_outside). Since 2026-10-09 h_outside is the bare-pipe-heat-loss model: Churchill-Chu natural convection for a horizontal cylinder with air properties at the film temperature, Churchill-Bernstein forced convection for the air velocity, combined as the cube-root sum of cubes, and h_rad = eps × σ × (T_s⁴ − T_a⁴)/(T_s − T_a). Iterate for the outer-surface temperature.
Source and editionASHRAE Handbook Fundamentals chapter 25 (insulation) by name; ASTM C680 (cylindrical surface conditions) by name; manufacturer k-values from data/hvac/insulation-k-values.json.
Free accessASTM C680 licensed; manufacturer technical bulletins free at each manufacturer site.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namespipe_OD_in, surface_T_F, ambient_T_F, insulation, thickness_in, jacket_emissivity, Q_bare_BTU_hr_ft, Q_insulated_BTU_hr_ft, effectiveness_pct
- Stefan-Boltzmann constant σ 0.1714×10⁻⁸ BTU/(hr·ft²·°R⁴)physical fact
- Default jacket emissivity 0.9 (painted steel / fabric jacket)engineering practice
- Film air properties Sutherland viscosity and conductivity at the film temperature, 1 atm (the bare-pipe-heat-loss model); until 2026-10-09 a simplified ASHRAE form with standard-air properties, which read 5 to 17% higher in convection on small pipeChurchill and Chu (1975); bare-pipe-heat-loss
- Iterative outer-surface T solve 12 fixed-point iterations for the R_outside ↔ T_s2 couplingengineering practice