Why tight sheathing improves energy efficiency in refrigeration and hydronic lines

A technical analysis of the role of the radial air gap, “apparent” thermal conductivity, and design implications for ACR systems.

Foreword

In construction practice, the issue of insulation of refrigeration and hydronic lines is often reduced to two parameters: sheathing thickness and declared λ. This is a simplification that does not stand up to comparison with the experimental literature of the past 15 years. The figure that appears on the data sheet of a PE foam or elastomer insulation is almost always measured under conditions that do not correspond to those of the actual installation, and the difference is not a detail: in some configurations the actual performance deviates from that stated by more than 15 percent.

The objective of this article is to clarify why, for the same insulation material and nominal thickness, a system in which the sheathing is co-extruded or otherwise made integral with the copper pipe at the manufacturing stage offers measurably higher performance than an insulator strung on site-and why this difference weighs in thermal load calculations and operating costs.

1. The physical model: the thermal resistance of an insulated pipe

For a cylindrical tube in steady state, the thermal resistance per unit length is the sum of three contributions in series:

R_tot = R_conv_int + R_cond_parete + R_cond_isol + R_conv_est

In the case of an insulated copper refrigeration line, the copper conduction and internal convection terms are negligible compared with the insulation resistance. The reference formula for insulation resistance is the classical formula for the cylindrical wall:

R_cond_isol = ln(D2 / D1) / (2 · π · λ)

Where:

D2 = outer diameter of the conduit [m].
D1 = outer diameter of the pipe [m]
λ = thermal conductivity of the material [W/(m-K)]

The whole calculation holds on an often unstated assumption: that there is nothing between D1 (copper) and the inner diameter of the sheath. In practice there is almost always a radial air gap, and that gap changes the physics of the system.

2. The radial air gap: the structural problem of retrofitted insulators

Commercial conduits made of PE foam, FEF (flexible elastomeric foam), PUR, and the like are produced with an inner diameter slightly larger than the nominal diameter of the pipe to allow it to be threaded or opened longitudinally and sealed with adhesive. This is an installability requirement, not a product defect.

The problem is that that clearance generates an air gap of varying thickness, in which two phenomena are triggered that worsen thermal performance:

1. Natural convection in the cavity. Air is not a good insulator when it can move: the conductivity of still air is about λ_air = 0.026 W/(m-K), but with natural convection in a vertical or horizontal annulus the equivalent conductivity increases nonlinearly with gap thickness.

2. Chimney effect along the axis of the pipe. On long straight sections, the gap functions as a preferential pathway for convective heat transport along the axial direction, especially in vertical sections and in the presence of significant thermal gradients (hot gas lines of heat pump air conditioners, for example).

Porzuczek, in a 2024 experimental study conducted at Cracow University of Technology and published in Materials in accordance with ISO 8497, points out that the “apparent” (or actual) thermal conductivity of a product can only be measured properly with pipe insulation testing apparatus: the guarded hot plate (GHP) method, by which many of the λ values claimed on the board are produced, does not reproduce the cylindrical geometry nor the natural convection that is generated around real insulation.

In design terms, it means that the value of λ printed on the sheath box can be correct as laboratory data but not as system data. This is a distinction that, until the 2000s, was often overlooked even in product standards.

3. The difference between declared λ and λ “in place”: experimental data

Porzuczek’s study (2024) tested ten samples of commercial pipe sheathing (mineral wool, PUR, PEF, FEF, EPS) mounted on a nominal 20 mm pipe-typical geometry of split refrigeration lines-according to ISO 8497, maintaining the air gap that is generated in the actual installation. Main results:

Material	λ at 10 °C declared [W/(m-K)]	λ at 10 °C measured [W/(m-K)]	Strain
Mineral wool (MW-1)	0,033	0,033	within the limits
Mineral wool (MW-2)	0,033	0,033	within the limits
Polyurethane (PUR-1)	0,032	0,034	+6%
Polyurethane (PUR-2)	0,032	0,035	+9%, up to +10% at 100 °C
Polyethylene foam (PEF-1)	0,038	0.036 at 10 °C, up to +16.4% at 80 °C after cycles	out of limit
Flexible Elastomer (FEF-1)	0,037	0,039	+5%
Flexible Elastomer (FEF-2)	0,037	0,041	+11%
EPS	0,036	0,036	within the limits

Source: Porzuczek 2024, Tab. 3.

Five out of ten samples-all of which are regularly on the market and declared to comply with product standards-showed conductivity measured in situ higher than declared, in two cases exceeding the +10% limit stipulated by ISO 13787.

For PEF in particular, the study found a cumulative phenomenon: in five successive measurements at 80 °C, λ increased progressively from +9.6 percent to +16.4 percent over the stated value. Porzuczek hypothesizes cellular structure alterations at the allowable temperature limit, but the operative point is clear: under severe operating conditions, PEF performance can degrade over time.

On PUR, the same study identifies a specific cause: convection in the radial air gap increases heat loss, especially at higher temperatures. Due to the stiffness of the material, the radial air gap cannot be completely eliminated by compression. This is the inherent limitation of any rigid insulation strung after the fact.

To these data should be added a less recent but still cited result in the literature: ASTM published under C335 a collection of tests (more than 150 tests over two years) that found, on a 305-mm section of pipe with a longitudinal gap of only 6.4 mm at a butt joint, a 15% deterioration in thermal performance.

4. Design implications: insulation as an economic variable

Insulation sizing is not just a regulatory compliance issue: it directly affects the operating costs of the plant and the payback of the initial investment.

Yıldız and Ersöz, in two papers in 2015 and 2016 published in Energy and Renewable and Sustainable Energy Reviews, calculated the optimal economic thickness for gas and liquid lines of R-410A VRF systems. Their results indicate that, for the same refrigerant, the optimum thickness varies between 9 and 12 mm for the gas line and between 6 and 9 mm for the liquid line, with payback times of less than a year for the most unfavorable situations.

The study by Daşdemir, Ertürk, Keçebaş and Demircan (2017), published in Energy, extended the analysis to include the effect of the air gap. The conclusion is that, on small-diameter pipes such as those typical of split refrigeration lines (Ø < 1″), the air gap weighs more than the thickness of the insulation itself on the energy balance, while on large-diameter pipes it is the thickness that dominates.

Abujab and Abusafa, in a case study of a VRF system published in Energy Reports in 2022, quantified the reduction in energy losses with insulation at the optimal thickness between 78.5 percent and 81.6 percent compared to an uninsulated system. The figure itself is expected, but it is important to note that those calculations assume perfectly adhered insulation: each cm² of air gap subtracts percentage points from that reduction.

5. The preinsulated system: what changes technically

A system in which the sheath is applied at the production stage-by direct extrusion onto copper, or by thermal processes that adhere a pre-formed closed-cell PE foam sheath to the outer diameter of the pipe-solves the problem at its root. The difference is not cosmetic:

Elimination of radial air gap. The closed-cell PE foam, applied in-line, follows the actual diameter of the copper without mating tolerances. The measured thermal conductivity coincides with the actual operating thermal conductivity.
Continuity of insulation at bends. Sheathing applied retrospectively tends to deform and open microvoids in curved sections. A sheath integral to the tube elastically deforms with it.
Eccentricity of the insulator reduced. On a nominally uniform pipe, the eccentricity of the sheathing applied retrospectively-combined with the mating clearance-can cause the actual thickness of insulation in some places to be significantly less than nominal.
Intrinsic vapor barrier. Pre-insulated systems with closed-cell PE foam and outer PE film achieve μ factors of resistance to water vapor diffusion > 15,000 (EN 13469 standard), a value that prevents the formation of interstitial condensation-which, in addition to being a health and structural problem, further degrades the thermal conductivity of the insulation itself (the presence of water in the pores can increase λ by a factor of 3-8 depending on the material).

The reference standard for closed-cell PE foam applied to ACR pipes is EN ISO 15758, while copper must comply with EN 12735-1 for air conditioning and refrigeration applications (R32, R410A, R407C). The minimum fire classification required for indoor installation is BL-s1-d0 according to EN 13501-1.

6. What to specify in specifications and what to check on site

Based on the above, some operational guidance for those designing or installing ACR systems:

Under specification:

Specify not only λ stated at 10 °C, but λ in place or, in the alternative, request test certification according to ISO 8497 / ASTM C335 (test on pipe, not flat plate).
Indicate the vapor diffusion resistance factor μ (EN 13469) as a binding parameter, not optional.
For outdoor installation, request UV protection film with certified resistance (e.g., ASTM G-155, accelerated aging tests of significant duration).
Require explicit manufacturer’s statement on sheath-to-pipe adhesion and application process.

In the process of laying:

Avoid sectioning the insulation and subsequent reassembly with tape: the longitudinal cut is a permanent thermal weak point.
Treat the transition sections between insulated and bare (connections to valves, cartridges) with special care: this is where statistically the greatest losses are concentrated.
If there are tight bends, visually check that the sheathing has not deformed creating gaps.

Under review:

Infrared thermography on significant sections of the line, with the machine running, to identify hot spots (in heat pump) or cold spots (in cooling) that indicate discontinuities in the insulation.

7. Conclusions

The difference between a pre-insulated system with sheathing integral to the pipe and a system with insulation strung on site is not a marketing issue. It is a measurable thermophysical fact, documented in independent literature and quantifiable in percentage points of system efficiency.

In design calculations there is a tendency to use the stated λ as the input data-it is established practice and in many cases the only data available. Porzuczek’s 2024 study, along with previous literature on the effect of air gap, shows that this approach systematically underestimates actual leakage, in some cases by 10-16%. On a medium-to-large VRF system, over a useful life horizon of 10-15 years, those percentage points translate into non-negligible operating costs and generator sizing that may prove insufficient under peak conditions.

For those designing and installing ACR systems, the operational message is simple: treat sheathing adhesion as a technical parameter, not a product detail. The difference between a correct data sheet in the lab and an efficient system in place goes from there.

Bibliographical references

Porzuczek, J. (2024). Comparative Study on Selected Insulating Materials for Industrial Piping. Materials, 17(7), 1601. https://doi.org/10.3390/ma17071601
Daşdemir, A.; Ertürk, M.; Keçebaş, A.; Demircan, C. (2017). Effects of air gap on insulation thickness and life cycle costs for different pipe diameters in pipeline. Energy, 122, 492-504.
Yıldız, A.; Ersöz, M.A. (2015). Determination of the economical optimum insulation thickness for VRF (variable refrigerant flow) systems. Energy, 89, 835-844.
Yıldız, A.; Ersöz, M.A. (2016). The effect of wind speed on the economical optimum insulation thickness for HVAC duct applications. Renewable and Sustainable Energy Reviews, 55, 1289-1300.
Abujab, M.; Abusafa, A. (2022). Optimal Insulation’s Thickness of Pipes in Variable Refrigerant Flow (VRF) System – An-Najah Child Institute as a Case Study. Energy Reports, 8, 321-330.
ASTM International – Thermal Performance of Insulated Pipe Systems. STP29487S.
ISO 8497 – Thermal insulation – Determination of steady-state thermal transmission properties of thermal insulation for circular pipes.
ASTM C335 – Standard Test Method for Steady-State Heat Transfer Properties of Horizontal Pipe Insulations.
ISO 13787 – Thermal insulation products for building equipment and industrial installations – Determination of declared thermal conductivity.
EN 12735-1 – Copper and copper alloys – Seamless, round tubes for air conditioning and refrigeration – Part 1: Tubes for piping systems.
EN ISO 15758 – Hygrothermal performance of building equipment and industrial installations – Calculation of water vapour diffusion – Cold pipe insulation systems.
EN 13469 – Thermal insulating products for building equipment and industrial installations – Determination of water vapour transmission properties of preformed pipe insulation.
EN 13501-1 – Fire classification of construction products and building elements.