What Insulation Thickness Is Needed for My Heating or Water System

What Insulation Thickness Is Needed for My Heating or Water System?

This is one of the questions customers ask most often - and also one that has no single universal answer. The correct insulation thickness depends on several factors at once: what medium flows through the pipe, its temperature, where the pipe is located, the diameter of the pipe, and what you actually expect from the insulation. Do you want to prevent heat loss? Stop condensation? Protect pipes from freezing? Each of these goals requires a slightly different approach and a different wall thickness.

In this article, we will break down the whole topic systematically - from the basics of heat loss physics through specific applications to tables with recommended thicknesses for common installation scenarios. If this is your first time here and you're not yet sure about the basic concepts, we recommend first reading the article How to Choose Pipe Insulation - Diameter, Wall Thickness and Material Type, which you can find in the same Knowledge Center. Here we will assume you already know the basics and go deeper.

Why Wall Thickness Matters at All - Brief Physics for the Installer

Heat flow through a cylindrical layer of insulation doesn't follow the same logic as heat flow through a flat wall. For a flat wall, a simple rule applies: a layer twice as thick means half the heat flow. This doesn't work the same way for a pipe, because the outer circumference of the insulation grows with thickness - meaning the heat exchange area facing outward increases as well. The result? The first 6 mm of insulation saves you dramatically more heat than the next 6 mm, and the following 6 mm saves even less than the previous ones. The law of diminishing returns applies very specifically in heat engineering.

From this comes a practical conclusion: for heating systems (where the main goal is minimizing heat loss), it's worth going for a thickness of 9 mm or more, but jumping from 13 mm to 19 mm brings substantially less savings than jumping from 0 mm to 6 mm. For cold water systems and air conditioning, where the main concern is preventing condensation, it makes sense to go for a thickness of at least 9-13 mm, because condensation depends on the surface temperature of the insulation, which decreases more evenly with thickness.

Three Basic Scenarios: Heating, Domestic Hot Water and Cold Water

Before we start talking about specific numbers, we need to distinguish between three basic situations that occur again and again in practice, and for each of them the logic of thickness selection is slightly different.

Scenario 1: Central Heating Distribution (Radiator, Underfloor)

The temperature of heating water typically ranges from 35-80 °C depending on the system type and current outdoor weather. Modern condensing boilers and heat pumps increasingly work with low temperatures (35-55 °C), while older cast iron radiators require 70-80 °C. In both cases, the difference between the medium temperature and the ambient air temperature in the room or shaft is 20-60 °C, which represents a significant driving factor for heat loss.

The STN EN 12828 standard and the EU Energy Performance of Buildings Directive (EPBD) recommend a minimum insulation thickness equal to the pipe diameter for distribution in tempered spaces (basements, utility rooms) - meaning at least 22 mm of insulation for a 22 mm pipe. In practice, this is hard to achieve with common foam insulation, and many old installations use 9 mm, which is a compromise between cost and performance.

An important practical detail: if you route distribution through an unheated space (unheated garage, under the roof, exterior shaft), heat losses are much higher and a thickness of 9 mm is the necessary minimum here, with 13 mm recommended. For exterior distribution, you should use insulation with UV protection and a thickness of at least 13-19 mm.

Scenario 2: Domestic Hot Water (DHW) Distribution

Hot water circulates at 55-65 °C. The reason why exactly this temperature is recommended is legionella - the bacteria dies above 55 °C. At a lower temperature, there is a risk of it multiplying. DHW distribution must therefore be properly insulated so that the water doesn't lose temperature before reaching the outlet tap - otherwise either cold water flows out, or the circulation pump runs constantly and consumes extra energy.

For DHW distribution in tempered interior spaces, a thickness of 9 mm is recommended as a reasonable base, and for distribution in unheated spaces, at least 13 mm. If you have a circulation pipe (return circuit), this rule applies equally to it as well.

Scenario 3: Cold Water Distribution and Air Conditioning

Here the situation is reversed: the problem isn't heat loss but moisture condensation on the pipe surface. Cold water has a temperature of 8-15 °C, and air conditioning refrigerant is even lower. If the pipe surface is colder than the dew point of the surrounding air, moisture from the air condenses on the pipe - and within a few months you have damp ceilings, moldy walls, and corroded fixings.

The insulation thickness for cold water applications must be sufficient to keep the surface temperature of the insulation above the dew point. In normal interior conditions (temperature 22-24 °C, relative humidity 50-60%), 9 mm is sufficient. In humid spaces (bathroom, laundry room, basement), 13 mm is the safer choice. For summer air conditioning, when outdoor humidity is high and refrigerant temperatures are low, the standard is 13-19 mm.

Recommended Thicknesses by System Type - Overview Table

The following table summarizes recommendations based on a combination of Slovak and European standards, insulation manufacturer recommendations, and practical installation experience. Treat it as a starting point, not an absolute rule - every building is a bit different.

System Type	Pipe Location	Min. Thickness	Recommended Thickness
Heating (70-80 °C)	Heated room	6 mm	9 mm
Heating (70-80 °C)	Unheated space / basement	9 mm	13 mm
Heating (35-55 °C, heat pump)	Heated room	6 mm	9 mm
DHW (55-65 °C)	Heated interior	6 mm	9 mm
DHW (55-65 °C)	Unheated space	9 mm	13 mm
Cold water (8-15 °C)	Dry interior	6 mm	9 mm
Cold water (8-15 °C)	Humid interior / basement	9 mm	13 mm
Air conditioning / cooling	Interior	9 mm	13 mm
Exterior distribution	Outdoor / roof / balcony	13 mm	19 mm

Specific Products and When to Use Them

Let's look at the specific sizes you encounter most often in practice, and which products from our range are suitable for particular situations.

18 mm Pipes (1/2", typically copper or PE-X for underfloor heating)

Pipes with an outer diameter of 18 mm are one of the most common sizes in residential installations. You'll find them as DHW distribution in apartment buildings, as radiator connection lines, or as underfloor heating manifold circuits.

For these pipes we offer two wall thickness variants:

• Insulation 18 mm / 6 mm - suitable for DHW and heating distribution in tempered heated spaces where you need to prevent heat loss, but space is limited (for example, an installation shaft with tightly arranged pipes).
• Insulation 18 mm / 9 mm - the recommended choice for most common applications at this diameter. Significantly better thermal insulation properties than 6 mm while still maintaining a manageable overall diameter. Use it whenever you route distribution through unheated spaces, or when you want to truly minimize heat loss.

From practice: in family houses, it very often happens that DHW distribution runs through an unheated basement or garage. We have seen installations with 6 mm thickness where the water cooled by more than 5 °C during a longer draw (for example, filling a bathtub) before reaching the tap. Upgrading to 9 mm significantly mitigates this problem.

22 mm Pipes (3/4", the most common diameter for heating and DHW)

Twenty-two millimeter pipes are something of a gold standard for residential installations. Main DHW distribution, heating supply and return circuits, boiler connections - all of this typically runs through a 22 mm copper or steel pipe.

• Insulation 22 mm / 6 mm - a minimalist variant for spaces where there isn't enough room, or where heat losses are less critical (for example, a short section in a boiler room, which is a warm space anyway, so heat losses aren't "thrown outside").
• Insulation 22 mm / 9 mm - the standard and recommended choice for 22 mm distribution. In an average family house with 15-20 meters of uninsulated or poorly insulated DHW and heating distribution, properly installed 9 mm insulation can save €200-400 per year in energy, depending on the energy source price and the length of the heating season.

28 mm Pipes (1" and larger, main distribution, boiler room)

Pipes with an outer diameter of 28 mm appear in larger houses with two or more heating circuits, or as main distribution in apartment buildings. They are also used in solar systems and in the primary circuits of heat pumps.

• Insulation 28 mm / 6 mm - suitable for short sections in a utility room where accessibility is limited, or as a quick solution for distribution in a heated boiler room. For longer sections and unheated spaces, we clearly recommend looking for a variant with a thickness of 9 mm or more.

How the Calculation Changes Under Different Boundary Conditions

The table above is a good starting point, but the actual choice of thickness depends on several variables that vary from house to house. Let's look at the three most important ones.

Ambient Temperature

The colder the space through which the pipe passes, the greater the temperature difference between the medium and the surroundings - and therefore the greater the heat loss. A utility room at 18 °C and a basement at 8 °C represent completely different conditions for the same 22 mm pipe with DHW at 60 °C. In the basement, the temperature difference (ΔT) is 10 °C higher, which increases heat flow by roughly 20%. That's a difference that adds up to tens of kilowatt-hours per year over 20 meters of distribution.

For exterior conditions, a much stricter rule applies: at an outdoor temperature of -15 °C and DHW at 60 °C, you have ΔT = 75 °C. Without sufficient insulation (at least 13 mm, preferably 19 mm), you risk not only enormous energy loss but also pipe freezing during a longer pump shutdown.

Length of Distribution Lines

For short sections (up to 1-2 meters), the energy loss through the pipe is relatively small, and the difference between 6 mm and 9 mm insulation may not be economically decisive. For long distribution lines (10, 20, 50 meters), the differences multiply and the investment in thicker insulation pays off much faster.

A practical example: an older family house with 35 meters of uninsulated DHW and heating distribution in an unheated basement. Heat losses from the pipe distribution alone were estimated at 1,200-1,800 kWh per year. After installing 9 mm insulation along the entire route, heat losses dropped to an estimated 350-500 kWh - a saving of roughly 70%, which at a price of €0.12/kWh (natural gas) means €100-160 per year. The payback period for the insulation investment was less than 2 years.

Relative Humidity of the Environment

For cold water distribution and air conditioning, the relative humidity of the air at the installation site is decisive. The dew point at 22 °C and 50% relative humidity is about 11 °C. This means that if the surface of your insulation is colder than 11 °C, water will start condensing on it. The surface temperature of the insulation depends on its thickness: the thicker it is, the higher the surface temperature (closer to the ambient air temperature). In a bathroom with 70% relative humidity, the dew point at 22 °C is around 16 °C - this is a significantly more critical situation, and 6 mm insulation is not enough here.

6 mm vs. 9 mm Thickness - When Does the Difference Really Matter?

This question comes up very often, because in terms of price and space, the difference between 6 mm and 9 mm insulation is relatively small, but in terms of performance it can be significant. This topic is covered in more detail in a separate article, Differences Between Wall Thicknesses of 6 mm, 9 mm and 13 mm - When to Use Which, in this Knowledge Center; here we'll summarize it practically.

For heating and DHW, the following applies: 6 mm insulation represents roughly 38% of the heat loss of an uninsulated pipe, while 9 mm reduces it to roughly 27%. This means that jumping from 6 to 9 mm saves you another 11 percentage points of heat loss - which, for long runs and high energy prices, is not negligible. In practice, this means that if you have enough space and the installation isn't complicated, it's almost always worth choosing the 9 mm variant.

Exceptions where 6 mm insulation is sufficient and also a rational choice:

• Short radiator connections (up to 50 cm) within the heated room itself
• Distribution in enclosed installation shafts where space doesn't allow a larger diameter
• Cold water distribution in dry interiors with a stable temperature above 18 °C
• Temporary or provisional solutions during renovation

Special Situations: Underfloor Heating, Solar System, Heat Pump

Underfloor Heating

Underfloor heating distribution operates at low temperatures (35-50 °C) and is mostly embedded in the floor structure. The pipe itself in the anhydrite or concrete screed is not insulated - quite the opposite, heat should transfer into the floor. What is insulated are the supply lines from the manifold to the circulation pump and from the pump to the boiler. These sections are typically in an unheated space or shaft, and the same rules apply here as for regular heating: at least 9 mm.

Solar Collectors

Solar systems operate at very high temperatures (80-200 °C) with a special antifreeze medium. Distribution between the collectors and the storage tank must be insulated with insulation resistant to high temperatures - regular PE foam is not sufficient here. Mineral wool or special rubber insulation with a temperature resistance above 150 °C is used. Thickness should be at least 13 mm, and for exterior routing, 19-25 mm.

Heat Pumps - Primary Circuit (Brine)

The primary circuit of a geothermal heat pump operates at temperatures well below zero (typically -5 to +5 °C). This distribution must be insulated thoroughly enough to prevent condensation even in summer. The insulation thickness should be at least 13 mm, and for lines in unheated spaces, 19 mm. The material must have a closed-cell structure (rubber) - open-cell PE foam is not sufficient here, because it absorbs moisture.

Economics: When Is Thicker Insulation Worth It and When Is It Unnecessary?

Pipe insulation is an investment that pays back through energy savings. Thicker insulation costs more, but saves more. The question is at what ratio. Simplified figures for orientation (at a gas price of €0.80/m³ and a calorific value of 10 kWh/m³, i.e. an effective price of around €0.12/kWh):

• Installing 9 mm insulation instead of 6 mm on 20 meters of 22 mm pipe with 60 °C DHW in a basement at 10 °C: savings of roughly 40-60 kWh/year, which is €5-7 per year. Upgrade cost: €10-15. Payback: 2-3 years.
• Installing 13 mm instead of 0 mm on the same section: savings of 300-400 kWh/year, which is €36-48 per year. Insulation cost: €40-60. Payback: less than 2 years.

This leads to a fundamental practical conclusion: the greatest economic effect comes from the transition from no insulation to some insulation. Every additional millimeter brings a smaller and smaller increase in savings. Therefore, it's reasonable to choose a good middle value (9 mm for most interior applications, 13 mm for unheated spaces) and it's not worth chasing every extra millimeter unless you're in extreme conditions.

Correctly Measuring Pipe Diameter Before Ordering

Insulation wall thickness is one thing, but you also need to choose the correct inner diameter of the insulation, which must match the outer diameter of your pipe. You can find more about measuring and inch designations in the article Pipe Insulation 18 mm vs 22 mm vs 28 mm vs 35 mm - How to Measure the Diameter Correctly in this Knowledge Center. Here's just a brief reminder: always measure the outer diameter of the pipe, not the inner diameter, and not the diameter in inches according to the thread designation.

Basic equivalents that help with orientation:

• Copper pipe 15 × 1 mm → outer diameter 15 mm → insulation for 15 mm
• Copper pipe 18 × 1 mm → outer diameter 18 mm → insulation for 18 mm
• Copper pipe 22 × 1 mm → outer diameter 22 mm → insulation for 22 mm
• Copper pipe 28 × 1.5 mm → outer diameter 28 mm → insulation for 28 mm
• PE-X/AL pipe 16 × 2 mm → outer diameter 16 mm → insulation for 16 mm
• PE-X/AL pipe 20 × 2 mm → outer diameter 20 mm → insulation for 20 mm

Watch out for a common misunderstanding: a pipe labeled as "1/2-inch" in the threaded system has an outer diameter of 21.3 mm and needs insulation for 22 mm (the nearest larger size). A "3/4-inch" pipe has an outer diameter of 26.9 mm, for which insulation for 28 mm is commonly used.

Installation and Completeness of Insulation - Why Details Matter

Even the best insulation won't help you if it's installed incorrectly. Classic mistakes we encounter in the field when inspecting old installations:

• Gaps and holes in the insulation - every uninsulated centimeter is a thermal bridge, which not only directly leaks heat but also causes condensation and dampness in the surrounding insulation.
• Uninsulated fittings and valves - elbows, T-pieces, valves and shut-offs are commonly left out of the insulation because insulating them seems complicated. Yet fittings have a large heat exchange area and can account for 20-30% of the total losses along a route.
• Insulation cut along the longitudinal seam but not glued - the longitudinal seam must be glued or taped shut, otherwise the insulation opens up with temperature changes and loses its function.
• Insulation inner diameter too small - insulation forced onto a pipe with too small an opening loses its effectiveness because it's compressed and the thickness of the active layer is reduced.

You can find a detailed installation guide in the article Installing Pipe Insulation - Procedure, Tools and Common Installation Mistakes.

Frequently Asked Questions (FAQ)

If I have a 22 mm heating pipe in the basement, is 6 mm thickness enough?

Technically, 6 mm isn't "insufficient" - it's better than nothing. But for distribution in an unheated basement, we recommend at least 9 mm, ideally 13 mm. A basement typically has 5-10 °C in winter, and at a heating water temperature of 70-80 °C, ΔT is 60-75 °C - that's a high load for a 6 mm insulation layer. With 9 mm you achieve about 30% lower heat loss compared to 6 mm, which, over a longer route during the heating season, significantly shows up on your gas bill.

Do I need to use special insulation for cold water in the bathroom, or is regular PE foam enough?

For a bathroom with regular cold water (8-15 °C), closed-cell PE foam with a thickness of 9-13 mm is sufficient. The closed-cell structure prevents moisture absorption, which is crucial in a bathroom. Open-cell foam would absorb moisture and stop insulating. Common tubular insulation made of PE foam sold in building and installation stores has a closed-cell structure, so it's suitable for bathrooms.

For air conditioning my technician recommends 13 mm insulation, but the store only sells 9 mm for my diameter. What should I do?

There are two options here: either look for rubber insulation (Armaflex and similar), which is also available in 13 mm and 19 mm thicknesses for all common diameters, or use 9 mm insulation but with an additional vapor barrier (aluminum tape along the entire length), which partially compensates for the lower thickness. The ideal solution is still 13 mm rubber - it's specifically designed for cooling applications and has significantly higher diffusion resistance to water vapor than PE foam.

I have a family house with an air-to-water heat pump. The distribution runs from the outdoor unit through the wall into the house. What thickness for that external section?

The external section of a heat pump's distribution is exposed to weather conditions and must meet several requirements at once: thermal insulation, UV resistance and frost protection. The recommended minimum thickness here is 13 mm, but for areas with severe frost and wind, 19 mm is better. The material must be either rubber with a UV-resistant surface, or PE foam covered with a UV-resistant jacket (rubber sheet, HDPE pipe). For very long exterior runs (5 meters or more), consult a specialized designer.

Can I use two layers of 6 mm insulation instead of one 13 mm layer?

Yes, from a thermal engineering point of view, it's almost equivalent - the total thickness is 12 mm, which is close to 13 mm. The thermal insulation performance...