Get the ASHRAE 90.1 minimum insulation thickness for hot and chilled water pipes, correct it for your insulation's conductivity, and check the thickness needed to stop condensation on cold lines in humid climates.
Code minimums apply to the edition of ASHRAE 90.1 (or the local energy code) adopted for the project. The condensation check assumes still air and neglects the pipe wall and fluid film, which is conservative. A continuous vapour barrier is essential on cold pipes regardless of thickness.
| Fluid temperature | k range* | Mean rating temp. | < 1″ | 1 to < 1½″ | 1½ to < 4″ | 4 to < 8″ | ≥ 8″ |
|---|---|---|---|---|---|---|---|
| >350 °F (>177 °C) | 0.32–0.34 | 250 °F | 4.5″ | 5″ | 5″ | 5″ | 5″ |
| 251–350 °F (122–177 °C) | 0.29–0.32 | 200 °F | 3″ | 4″ | 4.5″ | 4.5″ | 4.5″ |
| 201–250 °F (94–121 °C) | 0.27–0.30 | 150 °F | 2.5″ | 2.5″ | 2.5″ | 3″ | 3″ |
| 141–200 °F (61–93 °C) | 0.25–0.29 | 125 °F | 1.5″ | 1.5″ | 2″ | 2″ | 2″ |
| 105–140 °F (41–60 °C) | 0.21–0.28 | 100 °F | 1″ | 1″ | 1.5″ | 1.5″ | 1.5″ |
| Fluid temperature | k range* | Mean rating temp. | < 1″ | 1 to < 1½″ | 1½ to < 4″ | 4 to < 8″ | ≥ 8″ |
|---|---|---|---|---|---|---|---|
| 40–60 °F (4–16 °C) | 0.21–0.27 | 75 °F | 0.5″ | 0.5″ | 1″ | 1″ | 1″ |
| below 40 °F (below 4 °C) | 0.20–0.26 | 50 °F | 0.5″ | 1″ | 1″ | 1″ | 1.5″ |
* Conductivity in Btu·in/(h·ft²·°F). Thickness in inches by nominal pipe size.
The table thickness assumes insulation whose conductivity sits within the listed range. For a different material, ASHRAE 90.1 gives the equivalent thickness:
T = r × [(1 + t ÷ r)K/k − 1] — r = pipe outside radius, t = table thickness, K = conductivity of your insulation, k = upper value of the table range.
A better insulator (lower K) is allowed to be thinner; a poorer one must be thicker. The tool rounds the result up to the next commercial thickness.
The 90.1 table is an energy rule. On a chilled water pipe in a humid plant room or ceiling void the real limit is often condensation: if the outer surface falls below the dew point, water forms on the jacket, soaks the insulation and drips onto the ceiling. In hot, humid climates the anti-condensation thickness is frequently double the code minimum. The tool calculates the surface temperature through the insulation and the outer air film and picks the smallest commercial thickness that keeps it above the dew point.
4″ chilled water pipe (OD 114.3 mm) at 6 °C, insulation at the top of the table range (k 0.27), plant room at 35 °C and 80% RH, foil jacket.
ASHRAE 90.1 requires 0.5 in for pipes under 1 in and 1.0 in for 1½ in and larger at 40–60 °F. Below 40 °F the 8 in and larger pipes need 1.5 in. In humid locations the thickness needed to prevent condensation is usually greater.
For 141–200 °F systems ASHRAE 90.1 requires 1.5 in on pipes under 1½ in and 2.0 in on larger pipes; for 105–140 °F domestic hot water, 1.0 in under 1½ in and 1.5 in above.
Use enough insulation to keep the outer surface above the dew point of the surrounding air and seal it with a continuous vapour barrier. The thickness depends on fluid temperature, ambient temperature, humidity and the jacket's surface coefficient.
Yes. ASHRAE 90.1 allows the thickness to be reduced with T = r[(1 + t/r)^(K/k) − 1] when the insulation's conductivity K is below the upper value of the table range.
Shiny jackets have low emissivity, so they exchange less radiant heat with the room and the outer surface runs colder. A colder surface is closer to the dew point, so more insulation is needed than with a painted or plain finish.
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