Differences between 6 mm, 9 mm and 13 mm wall thickness – when to use which
Differences between 6 mm, 9 mm and 13 mm wall thickness – when to use which
One of the most frequently asked questions we encounter when choosing pipe insulation is precisely the question of wall thickness. Customers usually take care to get the correct inner diameter right (so that the insulation fits the pipe), but they treat wall thickness as secondary – sometimes even choosing it based solely on what's currently in stock or what's cheaper. This is a mistake that can have real consequences: condensation on the pipe, higher heat losses, or, conversely, an unnecessarily overpriced installation. In this article, we will go through in detail what the insulation wall thickness actually affects, what the physical differences are between the 6 mm, 9 mm and 13 mm variants, and in which specific situations each of these thicknesses is suitable. We will also look at practical examples from common installations – family houses, apartment building cores, technical rooms, and outdoor piping.
What does insulation wall thickness actually affect
Before we move on to a specific comparison, it's important to understand what insulation wall thickness physically means and what it actually changes in the behaviour of the whole system.
Pipe insulation made of foamed polyethylene (PE) or elastomeric foam (e.g. Armaflex and similar types) works on the principle of thermal resistance. The thicker the layer of material surrounding the pipe, the greater the thermal resistance created between the warm medium inside and the outside environment (or vice versa – for cooling pipes, between the cold medium and the warm surrounding air). Thermal resistance increases linearly with thickness, but in practice this relationship is a bit more complicated – it depends on the pipe diameter, the material's thermal conductivity coefficient (λ), and the temperature difference.
The second key function of insulation is protection against condensation. On pipes carrying cold water (cold water supply, cooling systems), the pipe surface is cold, and if this surface is colder than the dew point of the surrounding air, moisture condenses on it. Thicker insulation shifts the outer surface into a warmer zone, thereby eliminating this phenomenon. You can read more about this phenomenon in the article Condensation on pipes – why it occurs and how insulation eliminates it.
The third function is protecting pipes from freezing – in spaces where the temperature may drop below 0 °C, insulation slows down the heat loss from the water in the pipe, thereby extending the time it takes for the water to freeze. On its own it will not prevent freezing, but combined with occasional flow or a heating cable, it can be sufficient.
6 mm wall thickness – when it's sufficient
Insulation with a 6 mm wall thickness is the thinnest commonly available variant for home and installation use. It is light, flexible, cheap, and easy to install even in tight conditions. But it has clear limits.
Typical use of 6 mm insulation
Domestic hot water (DHW) piping indoors. If hot water runs a short route within an apartment core or an installation shaft, where the ambient temperature is relatively high (15–22 °C) and the pipe run is not too long, 6 mm insulation can do its job – it protects the pipe surface from unnecessary heat loss into the walls and minimizes thermal losses. Of course, thermal losses will be higher than with thicker insulation, but on short sections this is an acceptable compromise.
Indoor cold water piping in apartments – if the humidity in the space is normal (not elevated), the air temperature is not extremely high, and the cold-water pipe does not run through an unventilated, damp space. In this case, a 6 mm layer is enough to keep the insulation surface above the dew point, i.e. to prevent moisture from condensing on it.
Heating (heating distribution pipes) in a tempered interior – for heating medium temperatures up to 70 °C and short indoor runs (e.g. pipes from the manifold to individual rooms in underfloor heating). Here the primary function of the insulation is not thermal protection but preventing condensation on cooler parts of the pipe (e.g. the return line).
Air conditioning cooling pipes in mild environments – for a short run of refrigerant or chilled water through a tempered space, 6 mm may be sufficient, but at higher ambient temperatures or longer runs a thicker variant should be chosen.
Practical example: When renovating an apartment core in panel buildings, 6 mm insulation is typically installed on DHW and cold water pipes running in the riser or in grooves. Space is limited, the runs are short, and the temperature in the shaft does not drop below 12 °C. In this scenario, the 6 mm solution is functional and economically justified.
For pipes with an outer diameter of 18 mm (typically 1/2" copper or 15 mm Cu), a suitable option is, for example, Insulation 18 mm / 6 mm, for 22 mm pipes (3/4" Cu or 22 mm copper) it's Insulation 22 mm / 6 mm, and for 28 mm pipes Insulation 28 mm / 6 mm.
Where 6 mm is not enough – the most common mistakes
In practice, 6 mm insulation is often installed where 9 mm or 13 mm should have been used – either to save money or out of lack of knowledge. The most common problematic situations:
- Running cold water through a technical room with a boiler, where in summer the air temperature is 28–35 °C with high relative humidity. Water condenses on the thin 6 mm insulation and the walls get wet.
- Long horizontal DHW runs in an attic or unheated cellar – thermal losses are unnecessarily high.
- Outdoor piping of any kind – 6 mm is never enough outdoors, even in Slovakia's mild climate zone.
9 mm wall thickness – the golden mean for most common installations
Insulation with a 9 mm wall thickness is the most commonly used variant in practice – and for good reason. It offers significantly better thermal protection than 6 mm, while remaining flexible enough for installation under normal conditions. For most home installations, 9 mm is the right choice.
Where 9 mm insulation is ideal
DHW piping over long indoor runs. If hot water runs through a hallway, technical room, attic, or unheated cellar, 9 mm insulation can significantly reduce heat losses compared to the 6 mm variant – we're talking about a loss reduction of roughly 30–40%, depending on the medium temperature and environment. For a system with DHW circulation, this means lower heating costs and shorter waiting times.
Central heating distribution pipes in a tempered or partially unheated space (garage, technical room, cellar). The heating medium typically has a temperature of 55–80 °C, and the temperature difference with the surroundings is large, so insulation makes a real difference. 9 mm is the minimum recommended standard here; many technical standards for pipes in unheated interiors require exactly this thickness.
Cold water in spaces with elevated humidity – a bathroom with poor ventilation, a cellar, a technical room with a boiler. Here the main threat is condensation. A 9 mm layer of insulation shifts the outer surface into a warmer zone and, under normal air humidity, eliminates condensation even on cold-water pipes (8–15 °C).
Air conditioning pipes and cooling systems indoors. For chilled water or refrigerant pipes (e.g. split systems, fan coils) indoors, 9 mm is the standard thickness. It also acts as a vapour barrier – preventing moisture from penetrating from inside the insulation to the cold pipe surface.
Practical example: Renovation of heating pipes in a family house, where the owner installed a new condensing boiler and extended the piping to a newly built extension. The route ran through a garage (winter temperatures drop to 5–8 °C). We chose 9 mm insulation – on 18 mm Cu pipe clamps, for example Insulation 18 mm / 9 mm, and on 22 mm Cu runs Insulation 22 mm / 9 mm. Result: no condensation, and measurably lower gas consumption compared to the previous season, when the pipes ran without insulation.
Comparison 6 mm vs 9 mm – when a thickness change really helps
The difference between 6 mm and 9 mm may seem small at first glance – only 3 mm. But in terms of thermal resistance, it is a significant difference, because the thermal resistance of cylindrical insulation increases logarithmically with the ratio of outer to inner radius, not linearly. In practical terms: going from 6 mm to 9 mm on a 22 mm pipe reduces heat losses by roughly 30–35%. Going from 9 mm to 13 mm brings a further reduction of 20–25%. In other words, each millimetre on a thinner profile does more than on a thicker one.
13 mm wall thickness – when it's essential
Insulation with a 13 mm wall thickness is a relatively exceptional choice for home use, but in certain situations it is truly essential. This thickness provides significantly higher thermal resistance, better protection against condensation, and better frost protection.
Situations where 13 mm is the right choice
Outdoor piping of any kind. Pipes running outdoors – garden water connections, outdoor shower supply pipes, solar collectors, heat pumps – must have at least 13 mm insulation, combined with a UV-resistant jacket (standard PE insulation is not UV-resistant and degrades in sunlight). Outdoors, the temperature can drop well below zero, and the temperature difference between the medium and the ambient air can reach 50–60 °C, so thicker insulation is an investment that pays off.
Unheated spaces at risk of frost – attics, unheated garages, cellars, where winter temperatures can drop below 0 °C. If water pipes run through these spaces, 13 mm insulation is the minimum measure. Combined with a heating cable or regular flow through the pipe, this is usually a sufficient solution.
Cooling and air conditioning pipes with large temperature differences – for example industrial cooling, central air conditioning with chilled water at 6–8 °C in an environment with high humidity and air temperatures of 25–35 °C. The risk of condensation here is extremely high, and neither 6 mm nor 9 mm provides sufficient protection.
Long runs of heating pipes in an unheated interior – for example a boiler room in the basement, from which the pipe runs 15–20 metres through an unheated cellar to another part of the building. On such a run with a medium at 75 °C, heat losses with 6 mm insulation are economically painful. 13 mm significantly improves the energy balance here.
Regulatory requirements. The technical standard STN EN 12828 and EU directives on energy performance of buildings (EPBD) set minimum insulation thicknesses for various types of piping. For piping in unheated environments, this is generally a thickness equal to the outer diameter of the pipe (in a simplified interpretation), which for common pipes works out to 13–20 mm. For any certified installation or project where a revision report or energy certificate is issued, these values must be observed.
Practical example: A customer installed an air-to-water heat pump, with the pipe running from the outdoor unit through the exterior wall into the technical room. The outdoor section was only about 60 cm long, but given direct sunlight and frost, we recommended 13 mm insulation with a UV jacket. When we returned for inspection a year later, the insulation was in perfect condition – unlike the neighbour's installation, where 9 mm without a UV jacket was used and the surface had cracked and become brittle.
How to choose the right thickness – a practical decision process
When facing an insulation thickness choice, we recommend answering these five questions:
- What is the temperature of the medium in the pipe? – Cold water (up to 20 °C), hot water (40–65 °C), or heating medium (65–90 °C)? Higher temperature = greater temperature difference = greater need for insulation.
- What is the temperature around the pipe? – Interior 20 °C, garage 5 °C, exterior -15 °C? The colder the surroundings (for a warm medium) or the warmer the surroundings (for a cold medium), the thicker the insulation should be.
- How long is the run? – A short indoor run (up to 3 m) versus a long run (10+ m). Long runs = accumulation of heat losses = a more significant economic benefit from thicker insulation.
- What is the relative humidity around the pipe? – Normal (40–60%), elevated (above 70%)? High humidity significantly increases the risk of condensation on cold-water pipes.
- Are there regulatory requirements? – If so, follow the relevant standard. You can find more on this topic in the article What insulation thickness is needed for pipes in my heating or water system.
A more detailed look at selection based on all parameters – including material type and diameter – can be found in the article How to choose pipe insulation – diameter, wall thickness and material type.
Differences in terms of installation and space constraints
In addition to thermal-technical parameters, wall thickness also has a practical dimension during installation. Thicker insulation takes up more space, is stiffer, and bending it in tight spaces can be problematic.
6 mm insulation is the most flexible, easily laid in grooves, behind wall cladding, in installation shafts, and in places with minimal space. It can be bent to small radii without risk of breaking.
9 mm insulation is still flexible enough for normal installations, but at small bending radii (below 3–4 times the pipe diameter) it may start to deform and lose its round shape. When routing pipes behind plaster or in a groove, allow for more space.
13 mm insulation is significantly stiffer, harder to bend, and can be a real problem in tight spaces. Installation around corners requires either special fittings or an experienced installer who knows how to properly cut and glue individual pieces. You can find more on proper installation and common mistakes in the article Installing pipe insulation – procedure, tools and common installation mistakes.
Practical note: with 13 mm insulation on smaller-diameter pipes (e.g. 18 mm), the outer diameter of the insulation reaches up to 44 mm. This is important to know when planning space for pipe routing, wall penetrations, and installation shaft dimensions.
The economic side of things – is it worth paying more for thicker insulation?
Thicker insulation costs more – not dramatically, but there is a difference. For a family house with total pipe runs of 30–50 metres, the material price difference between the 6 mm and 9 mm variant may be €15–30, and similarly between 9 mm and 13 mm. The question is what the return on this investment is.
For heating pipes with a medium temperature of 75 °C running through an unheated garage (temperature 5 °C) over a 15-metre run with a 22 mm pipe: heat losses with 6 mm insulation are roughly 20–25 W/m, which for 15 m gives 300–375 W of continuous loss. Over a heating season (5,000 hours), that's 1,500–1,875 kWh. At a gas price of €0.08/kWh, that's €120–150 per year just on this run. With 9 mm insulation, losses drop to about 13–17 W/m, giving annual losses of €65–85. The saving is therefore €50–80 per year – the investment in thicker insulation (a difference of about €8–12) pays for itself in a week.
This simple calculation shows that skimping on insulation thickness for heating pipes in unheated environments is a short-sighted decision. Of course, for short runs in a tempered interior the difference is smaller, and choosing 6 mm insulation can be economically rational.
Special cases – solar systems, heat pumps, underfloor heating
Solar thermal systems operate with medium temperatures that can reach 80–120 °C in summer. Here, 6 mm insulation is not enough even indoors. Also, most standard PE insulation is not designed for such temperatures – insulation materials with higher heat resistance (elastomeric foam, mineral wool) should be chosen. A thickness of 13 mm is a common standard for solar piping.
Heat pumps have piping on the primary circuit side (brine or refrigerant) with temperatures as low as -10 to +15 °C. Here, water condensation on the pipes is an extreme risk, especially in humid spaces. The minimum recommended thickness is 9 mm, and 13 mm for spaces with higher humidity or long runs.
Underfloor heating – the pipes in the floor are of course uninsulated (we want the heat to transfer into the floor), but the supply and return lines from the boiler/manifold to the underfloor circuit should be insulated. For these runs indoors, 6 mm is sufficient, provided they are in a tempered space.
Mutual comparison – summary in numbers
For clear orientation, here is a summary of key parameters for a common 22 mm pipe (e.g. 22 mm copper, outer diameter):
| Parameter | 6 mm wall | 9 mm wall | 13 mm wall |
|---|---|---|---|
| Outer diameter of insulation | 34 mm | 40 mm | 48 mm |
| Heat losses (75 °C / 20 °C) | ~22 W/m | ~15 W/m | ~11 W/m |
| Protection against condensation | basic | good | excellent |
| Installation flexibility | excellent | good | limited |
| Suitability for outdoor use | no | only with jacket | yes (with UV jacket) |
| Relative price (per metre) | 1× | ~1.3× | ~1.7× |
The heat loss values are indicative and calculated for λ = 0.036 W/(m·K), which is a typical thermal conductivity coefficient for foamed polyethylene (PE insulation). For elastomeric foams, λ may be slightly higher, which would slightly reduce their insulating performance at the same thickness – more on material types in the article How to choose pipe insulation – diameter, wall thickness and material type.
Frequently Asked Questions (FAQ)
Can I use different thicknesses on the same pipe run – for example 6 mm in a tempered space and 9 mm in the garage?
Yes, and this is actually the right approach. There is no reason to use the same thickness along the entire run if conditions change. Insulate different sections according to the environment they pass through – 6 mm in a tempered interior, 9 mm in a garage, 13 mm outdoors with a UV jacket. Joints between different thicknesses can be made with contact adhesive and insulation tape to avoid thermal bridges. Details on joining and common mistakes can be found in the article Installing pipe insulation – procedure, tools and common installation mistakes.
Is 6 mm insulation enough for cold water pipes in a bathroom if we use a fan in the bathroom?
It depends on the specific conditions. If the fan keeps relative humidity below 65% and air temperature below 25 °C, then 6 mm is usually enough to protect against condensation on cold water (temperature 10–15 °C). But if the fan doesn't work sufficiently, or if the bathroom temperature rises above 28 °C in summer, the risk of condensation remains even with 6 mm insulation. In such cases, we recommend 9 mm as a safer solution.
Are the 6 mm, 9 mm and 13 mm thicknesses standardized, or do they vary from manufacturer to manufacturer?
These values are essentially standard – most European pipe insulation manufacturers produce exactly these three basic wall thicknesses. However, their declared thermal-technical properties may vary slightly depending on the foam quality and thermal conductivity coefficient (λ). When choosing, therefore, look not only at the thickness but also at the λ value stated by the manufacturer – the lower it is, the better the insulator at the same thickness.
I have pipes in an unheated cellar where the summer temperature reaches 18 °C and the winter temperature is 5 °C. What thickness is suitable for heating and what for cold water?
For heating pipes (medium 70–80 °C) in this cellar, we recommend at least 9 mm, ideally 13 mm, since the temperature difference between the medium and surroundings is large (55–75 °C) and continuous heat losses would be unnecessarily high with thinner insulation. For cold water, 6 mm is sufficient in this cellar – a summer temperature of 18 °C is close enough to the cold water temperature that condensation is not a risk, and in winter the cellar is colder, but so is the water – the difference remains small. Of course, if in doubt, 9 mm is always the safer choice.
The market also offers insulation with a thickness of 19 mm or 25 mm. When are these extra-thick variants used?
These thicknesses are intended for industrial applications, high-temperature solar thermal systems, long outdoor piping runs, or places where standards require higher thermal resistance. You will rarely encounter them in a typical home installation. If you are working in an area where a standard (e.g. building regulations or an energy performance decree) prescribes a specific thickness, it must be followed regardless of whether it seems unnecessarily thick to you. More on regulatory requirements in the article What insulation thickness is needed for pipes in my heating or water system.
How can I tell if existing pipe insulation is insufficient?
The most common signs are: moisture or water droplets on the insulation surface (condensation on cold pipes), a noticeably warm insulation surface on hot pipes (high heat losses), or visible damage – cracking, brittleness, peeling. A detailed overview of faults and their causes can be found in the article Pipe insulation faults and damage – cracking, peeling, dampness and repairs.
Conclusion – how to decide without unnecessary complication
If you take away just one thing from this article, let it be this: always choose the insulation wall thickness based on where the pipe runs and what the function of the piping is – not on what's cheaper or what happens to be in stock.
Simple rule: 6 mm for a heated interior with short runs and normal humidity; 9 mm for an unheated interior, tempered spaces, long DHW and heating runs, or elevated humidity; 13 mm for outdoors, spaces at risk of frost, cooling pipes with a large temperature difference, and anywhere prescribed by a standard or technical design.
You can find the entire range of available pipe insulation, including variants for different diameters and thicknesses, directly in our online store. If you're not sure which specific product to choose, the article Pipe insulation 18 mm vs 22 mm vs 28 mm vs 35 mm – how to correctly measure the diameter can also help, showing you how to correctly measure the outer diameter of your pipe before choosing insulation.
Have a question about this topic?
Not sure what to decide, or dealing with a specific situation in your household? Write to us - we're happy to help.