Bare and insulated pipe heat loss or gain, insulation thickness comparison, and condensation risk.
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Pipe & Insulation Parameters
Heat Loss / Gain Results
Bare Pipe Heat Loss
—
BTU/hr
Insulated Heat Loss
—
BTU/hr
Heat Loss Reduction
—
%
Insulated Loss per ft
—
BTU/hr per ft
Jacket Surface Temp
—
°F (insulated)
Energy Cost & Condensation Analysis
Insulation Thickness
Heat Loss (BTU/hr)
% Reduction
Surface °F
Est. Annual Cost
Per foot of pipe: q = (Tf − Ta) ÷ (Rins + Rsurf), Rins = ln(r2/r1) ÷ (2πk), Rsurf = 1 ÷ (h·2πr2). k is evaluated at the insulation’s mean temperature; h is still-air natural convection plus radiation at the actual surface temperature, solved iteratively. Bare pipe: surface film only (steel, ε ≈ 0.8). Indoor still air — wind raises outdoor losses considerably. Pipe wall and inside film resistances are neglected.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This calculator estimates heat loss (or heat gain on cold lines) for bare and insulated steel pipe in still indoor air, using the cylindrical conduction resistance of the insulation in series with the outer surface film. Enter the pipe size, insulation type and thickness, jacket, fluid and ambient temperatures, and length; the tool returns bare and insulated heat rates in BTU/hr, the percent reduction, the loss per foot, and the insulated jacket surface temperature.
A thickness comparison table shows diminishing returns as insulation is added, with the surface temperature at each thickness, and an annual energy-cost estimate prices the heat for a gas boiler, electric heat, or a chiller. For chilled or cold-water lines, the tool compares the jacket surface temperature with the ambient dew point to flag condensation.
Formula & Method
Heat rate
q = |Tf − Ta| ÷ (Rins + Rout)
Insulation R
Rins = ln(r₂ ÷ r₁) ÷ (2π · k · L), k at the mean insulation temperature
Film R
Rout = 1 ÷ (h · 2π · r₂ · L), h = hconv + hrad
Convection
hconv = 0.27 (ΔT ÷ D)0.25 (still air, horizontal pipe, D in ft)
Radiation
hrad = εσ(Ts4 − Ta4) ÷ (Ts − Ta)
This is the steady-state cylindrical conduction model of standard heat-transfer practice (the same approach as ASTM C680). r₁ is the pipe outer radius and r₂ = r₁ + insulation thickness, in feet, so k is converted from the published Btu·in/hr·ft²·°F by dividing by 12. Conductivity varies with temperature, so k is read at the mean of the fluid and jacket temperatures from typical manufacturer data (Johns Manville Micro-Lok HP fiberglass, ROCKWOOL ProRox PS 960NA mineral wool, Armacell AP ArmaFlex elastomeric, cellular glass); the surface coefficient combines still-air natural convection from a horizontal cylinder with radiation (bare steel ε ≈ 0.8; ASJ, PVC, or painted jacket 0.9; aluminum 0.1), and the surface temperature is solved iteratively. Indoor still air only — wind raises outdoor losses considerably. The pipe wall and inside film are neglected. Dew point uses the Magnus formula; a cold line is flagged when its jacket surface is within 1°F of the dew point. Energy cost uses 100,000 BTU/therm for gas, 3,412 BTU/kWh for electricity, and the chiller COP for cooling. This tool does not check energy-code minimum thicknesses (ASHRAE 90.1 Tables 6.8.3-1 and 6.8.3-2, or your local energy code), which depend on fluid temperature and pipe size.
Frequently Asked Questions
How is pipe insulation heat loss calculated?
Heat loss is found from the cylindrical (radial) conduction resistance of the insulation in series with the outer air-film resistance. The heat rate q equals the temperature difference between fluid and ambient divided by the sum of those resistances. The insulation resistance grows with the natural log of the outer-to-inner radius ratio, so each added inch of thickness returns less benefit than the last.
What thermal conductivity (k) should I use?
Thermal conductivity k is a property of the insulation, published in BTU-inch per hour per square foot per degree Fahrenheit at a stated mean temperature, and it rises with temperature. This tool reads k at the mean insulation temperature from typical manufacturer data: fiberglass about 0.23 at 75 degrees F mean and 0.28 at 200 degrees F, mineral wool 0.25 at 100 degrees F and 0.29 at 200 degrees F, flexible elastomeric about 0.245 at 75 degrees F, and cellular glass about 0.29 at 75 degrees F. You can also enter your product's own k. Lower k means less heat loss for the same thickness.
What is the critical radius of insulation?
The critical radius equals k divided by the outer film coefficient h, with k per foot of thickness. Below it, adding insulation can slightly increase heat loss because the growing surface area outpaces the added resistance. For real pipe insulation in still air (k about 0.02 BTU per hour-foot-degree F, h about 1.5) the critical radius is only about 0.15 to 0.2 inch, smaller than any standard pipe, so on HVAC piping adding insulation always reduces heat loss. The tool still checks and will note it if it ever applies.
Why does the tool check for condensation on cold pipes?
When the fluid is colder than the surrounding air, the outer surface of the insulation can fall below the dew point and sweat. The calculator finds the dew point from ambient temperature and relative humidity using the Magnus formula, solves for the jacket surface temperature, and warns when the surface is below or within 1 degree F of the dew point; the thickness table shows the surface temperature at each thickness so you can pick one that stays dry. Low-emissivity aluminum jackets run colder than painted or PVC jackets. Cold-pipe insulation also needs a continuous, sealed vapor retarder to keep moisture from reaching the cold pipe.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one
Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.