Why a universal thickness table is a trap
A thickness table is a lookup of one variable against another, with everything else frozen. The frozen parts are usually ambient temperature, relative humidity and the acceptable surface temperature — and those three are exactly what changes between a plant room in Santiago and a plant room in Jeddah.
The consequence is well documented in the field: a wall thickness that performs perfectly in a 24 °C / 50 % RH building is undersized on the same pipe in a 32 °C / 80 % RH room. Nothing about the pipe changed. The dew point moved from 12.9 °C to 28.1 °C, and the insulation surface is now below it for most of the year.
So the honest answer to "what thickness do I need" is: give me four numbers about your line and I will calculate it. Below is the method, so you can check our result rather than trust it.
The four inputs
| Input | What we need | Why it dominates the result |
|---|---|---|
| Pipe outside diameter | OD in mm, plus the material (copper, steel, stainless) | Sets the surface area per metre and the curvature. A 15 mm and a 108 mm pipe need different walls for the same duty. |
| Medium temperature | The coldest temperature the line actually reaches — not the design set point | A chilled water line running at +7 °C but shut down overnight at +2 °C is sized on +2 °C. Off-coil and stand-by conditions are where systems fail. |
| Ambient temperature and relative humidity | Both, as a pair, for the space the pipe runs through | These two set the dew point. Neither number means anything alone: 30 °C at 40 % RH is a mild case, 30 °C at 85 % RH is a hard one. |
| Location and code | Indoor or outdoor, exposed or in a void, and which classification your authority requires | Decides the surface finish (UV protection), whether a jacket is needed, and the fire classification your market calls for. |
What actually happens at the pipe surface
Moisture condenses on a surface that is colder than the dew point of the air touching it. That is the whole mechanism. Insulation does not "prevent condensation" by being thick — it keeps the outside surface of the insulation warmer than the dew point, so the air next to it never reaches saturation.
For a cold line, adding wall thickness moves the surface temperature up towards ambient. Too little wall and the surface sits at or below the dew point: water forms on the outside, the closed-cell wall is wet on the surface, and if the vapour barrier is incomplete the moisture keeps travelling inwards until it reaches the pipe. That is where corrosion under insulation starts.
The dew point values below are arithmetic, not opinion — computed with the Magnus formula from the two ambient numbers. You can check any of them against a psychrometric chart:
| Ambient condition | Dew point | What it means for the surface |
|---|---|---|
| 24 °C / 50 % RH | 12.9 °C | Surface must stay above ~12.9 °C. A comfortable office or European plant room. |
| 27 °C / 65 % RH | 19.9 °C | Surface above ~19.9 °C. Typical summer plant room in a temperate climate. |
| 30 °C / 75 % RH | 25.1 °C | Surface above ~25.1 °C. Tropical coastal plant room — the same pipe now needs noticeably more wall. |
| 32 °C / 80 % RH | 28.1 °C | Surface above ~28.1 °C. The hardest common case; a wall sized for the first row will run wet here. |
Why 'exactly at the dew point' still fails
Designers normally target a surface temperature one to two degrees above the calculated dew point, not equal to it. Three practical reasons:
- The dew point moves. Any plant room has days above the design condition, and a system designed with zero margin is wet on those days.
- Real installations are not ideal. A compressed wall at a hanger, a 1 mm gap at a joint, a bare flange — each of these is a local cold spot that the average calculation does not see.
- Humidity control is often absent. In a room with no dehumidification, relative humidity follows the weather; the design figure is frequently optimistic.
This is also why the thickness answer cannot be a single number for a whole building. The same pipe passing from a conditioned space into an unconditioned plant room needs different treatment at each location.
Where the calculation stops and the conversation starts
The heat-transfer step is steady-state: heat flow through the wall, surface resistance, then solve for the thickness that keeps the surface above the target. It is a solved problem with a closed-form answer — but the answer is only as good as the four inputs, and it has to be run per pipe size and per duty. We run it and send you a table: pipe size, medium temperature, ambient condition, required wall, and the resulting surface temperature so you can see the margin.
That is also why we do not publish a thickness table here. A published table with unstated assumptions is right for someone else's project and wrong for yours, and it is the kind of error that only shows up on site a season later.
Four installation failures that undo a correct calculation
- Compression at supports. A metal cleat compresses the wall to a fraction of its thickness and creates a local cold bridge. Use a vapour-tight saddle or a sleeve of the same material.
- Joints that do not close. A gap at a butt joint is an open door for vapour. Glue both faces, then tape the joint with the same closed-cell tape.
- A vapour barrier with holes. The closed-cell wall and μ ≥ 10,000 resist vapour; a bare end at a flange, valve or support defeats all of it.
- Outdoor surfaces without a jacket. UV degrades the surface within a season or two. Foil facing, coating or metal jacket — not optional outdoors.
Send us four numbers
Pipe outside diameter, the coldest medium temperature, the ambient temperature and relative humidity of the space, and whether it is indoor or outdoor with the classification your project requires. We return the wall thickness, the class, and the surface temperature margin for each pipe size on your list — usually within one working day.
Questions engineers ask
Is there a rule of thumb I can use for a quick estimate?
"One inch everywhere" is the rule of thumb in circulation, and it is wrong often enough to be expensive. It happens to be conservative for a small chilled pipe in a cool room and clearly undersized for a large pipe in a hot humid plant room — the two cases where the rule is applied most. If you want one number for budgeting, use the thickest wall in the range for cold lines and confirm it with the calculation before ordering.
Does a thicker wall always mean less condensation?
For a cold line, yes: more wall holds the surface closer to ambient, further above the dew point. The limit is economic and physical — cost, weight, clearance, and on small-bore pipe the bending radius. For hot lines the return diminishes quickly: after a point you are paying for foam that saves a fraction of a watt.
What standard do you size to?
The calculation is the standard steady-state surface-condensation method used in ASHRAE Fundamentals and in most national codes; the physics does not differ between them. Where a project specifies a particular code's tabulated thickness, tell us which one and we work to that table instead, so your inspector sees the number they expect.
Can you calculate it for a whole project, not one pipe?
Yes — send the pipe schedule (size, medium, location) and the ambient conditions per space, and we return one table covering every size. That is how the calculation should be delivered: one table the installer can work from, not a single number for a single pipe.