What formula does this use?

It models steady-state radial conduction through the cylindrical insulation shell, R = ln(r2/r1) / (2πk), in series with the outside air film resistance, R = 1 / (h × 2πr2). Heat loss per foot equals the temperature difference divided by the sum of those two resistances.

Why does the first inch of insulation matter most?

The insulation resistance grows with the natural log of the radius ratio, which increases slowly. Going from bare to one inch is a huge jump, but going from one inch to two adds far less, so most of the savings come from that first layer.

What conductivity values are used?

Approximate design values in BTU·in per hr·ft²·F: fiberglass 0.25, foam rubber or elastomeric 0.27, mineral wool 0.26, and polyiso 0.20. Conductivity rises with temperature, so verify k at your mean insulation temperature for very hot or chilled service.

Does this work for chilled water pipes?

Yes. If the fluid is colder than the ambient air, the temperature difference is negative and the pipe gains heat instead of losing it. The magnitude is the same calculation; the tool reports the absolute heat flow either way. For chilled lines also check the surface stays above the dew point to avoid condensation.

What air film coefficient should I use?

Still indoor air gives an outside film coefficient around 1.5 to 1.8 BTU per hr·ft²·F, which is the default range. A draft or outdoor wind raises it, which slightly increases bare-pipe loss but barely affects a well-insulated pipe because the insulation dominates.

Pipe Insulation Heat Loss Calculator

Uninsulated hot-water and heating pipes bleed energy continuously, and chilled lines both gain heat and risk condensation. This calculator computes heat loss per linear foot for bare versus insulated pipe so you can choose a thickness that pays for itself.

How it works

Heat flows radially out through the insulation shell and then through the air film at the surface, two resistances in series per foot of pipe:

R_insulation = ln(r2 / r1) / (2π·k)
R_film       = 1 / (h × 2π·r2)
q (BTU/h·ft) = (T_fluid − T_ambient) / (R_insulation + R_film)

r1 is the bare pipe outer radius, r2 is r1 plus the insulation thickness, k is the insulation conductivity, and h is the outside air film coefficient. For a bare pipe the loss is just the film term acting on the metal surface.

Example and notes

A 3/4 in pipe carrying 140 F water in 70 F air loses on the order of 30 to 40 BTU/h per foot bare, but only a few BTU/h per foot with one inch of fiberglass — a reduction well above 80 percent. Over a 50 ft run that is a large standing loss eliminated. Because the log-radius term grows slowly, the second inch of insulation saves far less than the first, so size to the economic optimum rather than piling on thickness. For chilled service, verify the surface temperature stays above the dew point to prevent dripping.