Pipe Insulation 18 mm vs 22 mm vs 28 mm vs 35 mm – How to Correctly Measure the Diameter
Pipe Insulation 18 mm vs 22 mm vs 28 mm vs 35 mm – How to Correctly Measure the Diameter and Choose the Right Size
It seems like a trivial question – you take the insulation, slide it onto the pipe, and you're done. In practice, however, an incorrectly chosen insulation size is one of the most common problems we deal with among customers. The insulation is too loose, the longitudinal slit can't be closed, or on the contrary – the insulation can't even be slid onto the pipe. And the result is always the same: either condensation, heat loss, or both. This article will therefore explain not only how to correctly measure the pipe diameter, but also why the specific sizes 18 mm, 22 mm, 28 mm, and 35 mm are established this way, what lies behind these numbers, and how to avoid the most common mistakes when choosing.
What does "insulation diameter" actually mean – outer vs inner diameter
This is the first and most important point to understand. When you see insulation labeled "18 mm" or "22 mm", this number does not indicate the outer diameter of the finished insulation, but the inner diameter of the insulation's opening – i.e., the opening into which the pipe itself is inserted. Insulation labeled 18 mm therefore has an inner opening of exactly 18 mm, so it fits perfectly onto a pipe with an outer diameter of 18 mm.
The outer diameter of the finished insulation is then the inner diameter plus twice the wall thickness of the insulation. For example, insulation with an inner diameter of 18 mm and a wall thickness of 9 mm will have an outer diameter of 18 + 2 × 9 = 36 mm. This is important to know when planning space in walls, installation ducts, or floor routing.
So when you search for insulation in a catalogue or online store, you always search by the outer diameter of your pipe – and this must match the inner diameter of the insulation. Not the other way around, not by estimation, not "roughly".
Why the diameters 18, 22, 28 and 35 mm – where these numbers come from
These dimensions are not random. They come directly from the standardized outer diameters of copper pipe according to the EN 1057 standard, which is the European standard for plumbing systems. Copper pipe is supplied in the following series: 10, 12, 15, 18, 22, 28, 35, 42, 54 mm – these are outer diameters. Insulations are therefore designed exactly for these dimensions.
In Slovak and Czech installations, imperial (inch) sizes are also commonly found (adopted from British/American tradition), which are encoded in brackets next to the insulation label:
- 18 mm = 3/8" – corresponds to the outer diameter of a 3/8-inch pipe
- 22 mm = 1/2" – corresponds to the outer diameter of a 1/2-inch pipe
- 28 mm = 3/4" – corresponds to the outer diameter of a 3/4-inch pipe
- 35 mm = 1" – corresponds to the outer diameter of a 1-inch pipe
This inch notation is historical and is still used in practice – installers will say "half-inch pipe" and mean a 22 mm outer diameter. So if you ever hear this kind of naming, you now know what it means and which insulation you should look for.
Important note: for plastic pipes (PPR, PEX, PE-RT, multilayer pipes) the dimensions differ from copper. For example, a PPR pipe with a nominal diameter of 20 mm has an outer diameter of 20 mm, not 22 mm. Before ordering insulation, always physically measure the outer diameter of your specific pipe.
How to correctly measure the outer diameter of a pipe – 4 proven methods
Here is the core of the whole article. We have dealt with dozens of orders where the customer ordered insulation "by eye" or based on what they remembered from an installation years ago, and the result was not good. We therefore recommend four reliable measuring methods, from which you can choose the one that suits you best.
Method 1: Calipers (vernier caliper)
The most reliable and accurate method. Place the jaws of the calipers on the outer circumference of the pipe and read the outer diameter directly in millimetres. An accuracy of 0.1 mm is more than sufficient. If you don't have calipers, borrow one – it's worth it. For everyday household use, even a cheap plastic caliper for a few euros from a hardware store will suffice.
Method 2: Measuring with a tape (circumference)
If you don't have calipers, wrap a piece of string or measuring tape around the pipe, measure the length of the circumference, and calculate the diameter using the formula: diameter = circumference ÷ π (3.1416). For example, if the pipe circumference is 69 mm, the diameter is 69 ÷ 3.1416 ≈ 22 mm. This method is less accurate, but usable for larger diameters, where practicality of access to the pipe also plays a role.
Method 3: Visual identification by pipe type
If you know the material and nominal size of the pipe (e.g., from project documentation or a purchase receipt), you can read the outer diameter from the standard's table. For copper pipe, the outer diameter equals the nominal size (e.g., a Cu 22 × 1 mm pipe has an outer diameter of 22 mm). For plastic pipes this is not so clear-cut – always check the specific type.
Method 4: Comparison with an insulation sample
Do you have old insulation or a leftover piece from a previous installation? Check its inner diameter – that is the diameter of the pipe it was fitted onto. If it still fits well on the pipe, you have the exact size.
Overview of insulation by size – where each is used and why
Insulation 18 mm (3/8") – the thinnest common pipe
Insulation for an 18 mm pipe is most commonly used for hot and cold water lines in bathroom cores, capillary lines to outlet fittings, and refrigerant lines in air conditioning (where Cu 3/8" is a common size for the suction/discharge line). In practice you'll encounter it in bathroom piping, behind toilets, and on connection lines to washing machines and dishwashers. For hot water piping indoors, 18 mm / 6 mm insulation (6 mm wall thickness) will suffice, while for cold water in areas with a risk of condensation or for lines in utility rooms we recommend 18 mm / 9 mm insulation with a 9 mm wall thickness, which better prevents condensation forming on the cold pipe.
Insulation 22 mm (1/2") – the most common size in residential installations
This is in practice the best-selling size. A 22 mm pipe (1/2" Cu) is effectively the standard line for hot and cold water distribution in apartment buildings, family houses, and residential complexes. On every larger plumbing installation job you will come across metres of this dimension. 22 mm / 6 mm insulation is suitable for hot domestic water lines in a heated interior. If you are insulating a cold line in a basement, utility room, or an area with higher relative humidity, reach for 22 mm / 9 mm insulation – this will provide sufficient thermal resistance and condensation will not form on the pipe even in summer, when the temperature difference is highest.
Insulation 28 mm (3/4") – for distribution lines and heating
A 28 mm (3/4") pipe appears in installations where higher flow is required – main risers in apartment buildings, the primary line of a heat pump, distribution lines for underfloor heating, lines to storage water heaters, and in boiler rooms. It is also a common dimension for lines between a solar collector and a storage tank. 28 mm / 6 mm insulation covers most common indoor applications for hot water and heating. For lines where minimizing heat loss over long runs is a priority (for example, main lines in a boiler room or lines in unheated areas), consider a thicker wall – in this case it's worth looking at 9 mm or 13 mm thickness.
Insulation 35 mm (1") – primary distribution lines, storage tanks, boiler rooms
Insulation for a 35 mm pipe is used on the primary lines of larger systems – higher-output boiler rooms, district heating (DH) distribution at the building level, connections to storage tanks over 300 litres, lines to large heat pumps, and commercial applications. In a family house you will encounter this dimension only rarely, but in apartment buildings, schools, or commercial buildings it is fairly common.
Table: Overview of insulation by diameter and common use
| Inner ∅ of insulation | Inch equivalent | Outer ∅ of pipe | Typical use |
|---|---|---|---|
| 18 mm | 3/8" | 18 mm | Outlet connections, capillary lines, air conditioning |
| 22 mm | 1/2" | 22 mm | Apartment hot/cold water lines, radiator heating |
| 28 mm | 3/4" | 28 mm | Risers, distribution lines, storage tanks, solar |
| 35 mm | 1" | 35 mm | Primary boiler room lines, apartment buildings, commercial buildings |
Plastic pipes – where dimensional pitfalls lurk
One of the most common sources of confusion when choosing insulation is when the installation is not made of copper but of a plastic material – PPR, PEX, PEX-AL-PEX (multilayer pipes), PE-RT, or PE. These pipes have different labeling systems than copper pipe.
Multilayer pipes (aluminum-plastic) are, for example, sold in dimensions of 14 × 2 mm, 16 × 2 mm, 20 × 2 mm, 26 × 3 mm. The outer diameter is therefore not the same as with copper. A 16 × 2 mm pipe has an outer diameter of 16 mm – and for such a pipe there is no "16 mm insulation" in the standard range, so 18 mm insulation (inner opening 18 mm) is generally used, which is slightly larger but still holds well. For a 20 × 2 mm pipe, 22 mm insulation is used for the same reason.
PPR pipes have outer diameters of 20, 25, 32, 40, 50 mm – here the match with insulation exists only partially. PPR DN20 (outer diameter 20 mm) – 22 mm insulation is commonly used for this (opening 2 mm larger), which is acceptable. PPR DN25 (outer diameter 25 mm) – here 28 mm insulation will suit you. A slightly larger opening isn't ideal, but works in practice – with a decent longitudinal adhesive strip, the insulation holds and insulates sufficiently. If you have doubts, also see our article How to Choose Pipe Insulation – Diameter, Wall Thickness, and Material Type, which is dedicated specifically to this topic.
What happens when you choose insulation with the wrong diameter
From experience we know both scenarios – insulation too small and too large. Both are problematic, just in different ways.
Insulation too small (inner diameter smaller than the pipe's outer diameter): It simply won't fit on. You can try to force it on, but the foam insulation tears in the process. Even if you manage to fit it, the inner surface of the insulation will be stretched, the material will be under constant mechanical stress, which leads to premature cracking and loss of sealing properties. The longitudinal slit will not close without a gap.
Insulation too large (inner diameter larger than the pipe's outer diameter): You can easily fit the insulation on, but an air gap forms between the pipe and the insulation. On cold pipes (air conditioning, cold water) this gap allows moisture to condense directly on the pipe, inside the insulation – i.e., in a place you can't see and which can't dry out. The result is corrosion (greenish patina on copper, rust on steel), biological growth, and degradation of the insulation from the inside. On warm pipes, on the other hand, you lose the thermal insulating effect because the air in the gap circulates and carries away heat.
The ideal condition is when the inner diameter of the insulation exactly matches the outer diameter of the pipe – the insulation tightly hugs the pipe, the longitudinal slit closes without a gap, and the entire cross-section of the insulation works as it should. We discuss this topic in more detail in the article Condensation on Pipes – Why It Occurs and How Insulation Eliminates It.
Insulation wall thickness – 6 mm, 9 mm, 13 mm – quick overview
You need the correct insulation diameter (inner opening), but besides that there is a second dimensional parameter – the wall thickness of the insulation. This determines how well the insulation prevents the transfer of heat (or cold). This is a separate topic that we cover in the article Differences Between 6 mm, 9 mm, and 13 mm Wall Thickness – When to Use Which, but for completeness here is a brief overview:
- 6 mm – basic insulation for warm indoor lines, where condensation is not an issue and the main goal is to reduce heat loss to a reasonable level
- 9 mm – a higher level of insulation, suitable for cold lines (where there is a risk of condensation), lines in unheated areas, or where more emphasis is placed on energy savings
- 13 mm – significantly increased thermal protection, for more demanding applications, some outdoor installations (combined with a UV-resistant layer), and energy-efficient systems
So when ordering insulation you always specify two numbers: the inner diameter (= the outer diameter of the pipe) and the wall thickness. For example, "insulation 22/9" means an inner diameter of 22 mm and a wall thickness of 9 mm.
Practical examples from job experience
When we look at what people actually deal with, there are several situations that repeat over and over:
Scenario 1: Bathroom renovation, the customer doesn't know what pipe is there. An old apartment, copper piping from the 1980s behind the tiles. The customer only knows they want to insulate it. We always recommend measuring with calipers at any accessible section (e.g., before the main shut-off valve or at the inlet to the toilet). It comes out to 22 mm = order 22 mm insulation. Done, no problem.
Scenario 2: New building, the designer wrote "DN20 multilayer pipe". The customer automatically ordered 20 mm insulation – but this doesn't exist in the standard range. The correct insulation is 22 mm (inner opening 22 mm), which fits snugly onto a pipe with an outer diameter of 20 mm. This is a very common mistake and we always explain it: DN20 doesn't mean an outer diameter of 20 mm for plastic pipes with a 2 mm wall thickness – the outer diameter is indeed 20 mm, but the closest insulation is 22 mm.
Scenario 3: The customer has a solar system with a 28 mm copper line, and the route runs through an unheated attic space. Here 28/6 insulation is not enough. The minimum recommended thickness for an unheated space is 9 mm, ideally 13 mm. At the same time, UV resistance needs to be addressed if the route also runs partially outdoors. We recommend a combination of sufficiently thick insulation and an outer cladding of foil or a UV-stable surface.
Scenario 4: The customer ordered the correct insulation, but couldn't fit it onto fittings and elbows during installation. Here the problem is not the diameter, but the fact that fittings have a larger outer diameter than the pipe itself. The solution is an insulated section on the fittings with an oversized cut, or the use of special fitting-shaped insulation pieces. You can find more about installation in the article Installing Pipe Insulation – Procedure, Tools, and Common Mistakes During Installation.
Most common mistakes when choosing insulation diameter – checklist
- Measuring the inner diameter of the pipe instead of the outer diameter
- Confusing inch sizes – mixing up 1/2" (22 mm outer diameter of copper) with a 1/2" threaded size (15–16 mm outer diameter of the thread)
- Ordering insulation based on the nominal size of a plastic pipe without accounting for the pipe's wall thickness
- Visual estimation without measuring – the pipe "looks like 22 mm", but it's actually 28 mm
- Ignoring the fact that fittings and elbows are larger than the pipe
- Confusing the outer diameter of the insulation with the inner one (measuring insulation from the external dimension)
Inch vs metric sizes – quick converter
In practice you will come across both systems. For reference, here is an overview of the most common inch sizes and their metric equivalents for the outer diameter of copper pipe:
| Inch designation | Outer ∅ Cu (mm) | Inner ∅ of insulation | Typical application |
|---|---|---|---|
| 3/8" | 18 mm | 18 mm | outlets, air conditioning |
| 1/2" | 22 mm | 22 mm | apartment water/heating lines |
| 3/4" | 28 mm | 28 mm | risers, storage tanks, solar |
| 1" | 35 mm | 35 mm | primary lines, boiler rooms |
| 1 1/4" | 42 mm | 42 mm | larger distribution systems |
Insulation for plastic pipes – conversion table for PEX and multilayer pipes
| Pipe (outer ∅ × wall thickness) | Outer ∅ of pipe (mm) | Recommended insulation (inner ∅) |
|---|---|---|
| PEX/PE-RT 14 × 2 | 14 mm | 15 mm (if available) or 18 mm |
| PEX/multilayer 16 × 2 | 16 mm | 18 mm |
| PEX/multilayer 20 × 2 | 20 mm | 22 mm |
| PPR 25 / multilayer 26 × 3 | 25–26 mm | 28 mm |
| PPR 32 / multilayer 32 × 3 | 32 mm | 35 mm |
The table shows that plastic pipes mostly don't have an exact metric equivalent in insulation – you always use the next larger insulation. This slight tolerance (2–4 mm) is acceptable in practice, because foam insulation is elastic enough and the longitudinal slit closure takes up the slack. For a larger gap (e.g., 5 mm or more), it's advisable to use insulation tape to seal the joint.
Energy implications – why the insulation diameter matters economically too
You might wonder – why focus so much on the exact diameter? Insulation is insulation, after all. But this is about physics. Heat loss through insulation depends on the logarithm of the ratio between the outer and inner diameter of the insulation. If the inner diameter of the insulation doesn't match the pipe – whether it's too large (air gap) or too small (mechanical stress and cracking) – the actual thermal insulating capacity is significantly lower than what the manufacturer states for that wall thickness.
In other words: insulation 22/9 on a 22 mm pipe provides the guaranteed thermal resistance corresponding to a 9 mm wall. The same insulation fitted onto an 18 mm pipe (because the customer couldn't find 18 mm insulation and bought 22 mm) will have a 2 mm air gap around the circumference – and the actual thermal insulating efficiency will be lower. Not catastrophic, but measurable. Over dozens of metres of piping, this can mean several euros of wasted energy per year – which, over an installation lifespan of 20 years or more, is not negligible. More on the economics of heat loss can be found in the article Frequently Asked Questions About Pipe Insulation – Diameter, Inch Sizing, Heat Loss, and Energy Savings.
Special case: outdoor insulation and UV stability
If insulation runs outdoors – for example, lines from solar collectors on a roof, piping near an outdoor heat pump unit, or lines on a facade – the diameter selection is the same, but there is an additional requirement for UV stability of the insulation surface. Standard black foam rubber insulation (EPDM/NBR) is more resistant to UV radiation than white foam PE insulation, but for long-term outdoor use we always recommend either a UV-stable version or an outer protective covering (foil or a plastic protective jacket). More on this aspect can be found in the article Pipe Insulation Outdoors vs Indoors – Different Requirements and Suitable Types.
Frequently Asked Questions (FAQ)
Can I use insulation one size larger if I don't have the exact size on hand?
In an emergency, yes, but only one size larger and only for short sections or temporarily. For example, 22 mm insulation on an 18 mm pipe will have an air space of about 2 mm around the entire circumference. For cold water, this is a risk of condensation inside the insulation. For hot water, there will be a slight reduction in thermal resistance. It is always better to use the correct size – it costs the same and works substantially better.
How do I find out the pipe size when it's already built into the wall and I have no access to it?
Find out the type of installation from the project documentation or construction log. If there is no documentation, look for accessible parts of the line – valves, joints, points of entry into bathroom cores, outlets. If it's a copper installation from the 1990s or newer, it will almost certainly be 22 mm for apartment lines and 28 mm for risers. If it's PEX or multilayer pipe from after 2000, the most likely size is 16 × 2 (→ 18 mm insulation) or 20 × 2 (→ 22 mm insulation).
Why is the value on insulation given in both inches and millimetres? Are these the same pipes?
Yes, the label "18 mm (3/8")" means it's one and the same insulation – both equivalent designations are simply given, because copper pipe is labeled metrically in European practice (18 mm outer diameter), but plumbing terminology still uses the inch name (3/8"). Both designations describe the same physical pipe dimension.
What if I have a pipe with an outer diameter that doesn't match any standard insulation – for example, 24 mm or 30 mm?
Such sizes occur with some plastic systems. The solution is to use the next larger insulation (e.g., for 24 mm → 28 mm insulation) and seal the longitudinal slit well with insulation tape. If it's a critical application (strong condensation, long route), you can consider cutting the insulation and joining it with adhesive sealant – but this is more of a last resort. In any case, the rule applies: always larger, never smaller.
Can I use 22 mm insulation on a 22 mm copper pipe if the pipe has a plastic sleeve (PE-RT protective sleeve) on it?
No. If there is a protective plastic sleeve on the pipe, you must measure the outer diameter of the sleeve, not the pipe itself. The sleeve increases the outer dimension, and the insulation must fit over the sleeve. In this case, the outer diameter of the pipe + sleeve combination will be...
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