Demonstrating how water vapour diffusion can greatly reduce the effectiveness of a material, Dr Laurentiu Pestritu underscores the importance of selecting the right kind of insulation in the interest of installation longevity and energy savings.

Energy saving is becoming increasingly important in cold applications. Whilst so far low-temperature insulation has mainly served to prevent condensation, in future, minimising energy losses over the entire service life of a system will have to become a primary objective of all low-temperature insulation. In light of this, insulation materials must be protected against moisture penetration.

Factors to be considered while selecting insulation material

In building physics, the key characteristic for assessing insulation materials is the thermal conductivity. It describes the ability of a material to conduct heat. The thermal conductivity indicates the amount of heat which is conducted through a layer of a material which is 1 m² in area and 1 m in thickness in one second when the difference in temperature between the two surfaces is 1K. The lower the thermal conductivity value, the better the insulating ability of a material and the less energy is lost. The unit of thermal conductivity is Watt per metre and per Kelvin [W/(m · K)]; the symbol is the lower case Greek letter lambda (λ).

However, the insulation effect of a material can be greatly reduced by moisture. When selecting and determining the thickness for low-temperature insulation, it is necessary to bear in mind that over the service life energy losses can increase dramatically as a result of moisture penetration. With every vol.-% of moisture content the thermal conductivity increases and the insulation effect deteriorates. The results are not only higher energy losses but also a drop in the surface temperature. If this falls below the dew-point temperature, condensation occurs. Only if the thermal conductivity of the insulation material does not increase significantly as a result of moisture penetration, is it possible to guarantee that the surface temperature will remain above the dew-point even after many years of operation.

A reliable insulation system must, therefore, also be protected against undue moisture penetration. How much moisture is able to penetrate the insulation as a result of vapour diffusion depends on the water vapour diffusion resistance (µ-factor) of the insulation material. The µ-factor indicates by what factor a material’s resistance to water vapour is greater than that of a static layer of air of the same thickness and temperature. µ is a dimensionless parameter. The lower the µ-factor of an insulation material, the greater the rise in moisture content – and, thus, energy losses – over the years. It is essential to bear this in mind when selecting insulation material.

Only externally tested values provide certainty

The thermal conductivity λ and the water-vapour diffusion resistance µ are of key importance. If no quality assurance according to regulations is available for various insulation materials, the client’s express approval is required before these insulation materials may be used.

Without external testing, the user is faced with the risk of the actual product values deviating from those published. This can have extremely unpleasant consequences as far as the reliability and functioning of the insulated installation are concerned and may lead to complaints. Therefore, responsible manufacturers of insulation materials ensure compliance with the key technical values of quality testing. Only when the λ-value and µ-factor of an insulation material are subject to systematic external testing is it possible to compare the product’s properties which determine the quality.

Risk factor condensation

The consequences of water vapour diffusion usually remain invisible initially – until they become apparent as condensation and lead to building damage and the disruption of operational processes. The formation of condensation can be prevented by dimensioning the insulation to ensure that its surface temperature is at least equal to the dew-point temperature of the ambient air. The thermal conductivity is the value that is taken as a starting point for determining the insulation thicknesses needed to prevent condensation.

Reliability due to “sealed” cells

As even small amounts of moisture are enough to cause corrosion, no matter how good a water vapour barrier is, it cannot replace effective corrosion protection. A high diffusion resistance reduces the risk of corrosion but does not, of course, mean that effective corrosion protection can be dispensed with.

The advantages of a material with a high diffusion resistance lie in the fact that the amount of moisture which might condense on the cold side is greatly restricted. Thus, it is not possible for larger amounts of water to accumulate and cause damage anywhere in the system. Apart from this, it is ensured that even over a longer period of time hardly any moisture penetrates the insulation material, allowing it to maintain its insulation properties.

In the case of closed-cell, foamed insulation materials with a high diffusion resistance and flexibility, the risks of moisture penetration are far lower than for other materials (eg, open cell materials). In closed cell insulation materials, the diffusion resistance is not applied in a thin layer (which as such is susceptible to damage), but built up continually – cell by cell – throughout the entire thickness of the foam. This is achieved in the production process by “sealing” the individual cell walls against water vapour diffusion.

Long-term prevention of energy losses

Nowadays, Glaser’s two-zone model is used to assess the risk of moisture penetrating an insulation construction as a result of condensation. The main problem of the two-zone model is determining the “condensation temperature” ϑc at which the partial pressure curve coming from the outside merges with the saturated vapour pressure curve and separates the “dry zone” from the “wet zone”.

Some of the moisture already condenses in the inner zone of the insulation material, whilst the rest advances to the cold surface of the installation. For our purposes this means: the lower the µ-factor of an insulation material, the greater the rise in the moisture content, ie, the “wet” zone becomes larger. As a result, the insulation properties deteriorate over the years and the energy losses rise. The appropriate equations/calculation formulae can be taken from the German VDI directive 2055, sheet 1 [1; 2]. In general, they require iterations which rule out manual calculation. These formulae are reflected in European standards.

Over the entire service life, the rise in energy losses is all the more serious, the higher the thermal conductivity and the lower the water vapour diffusion resistance. The energy loss from the installations which are insulated with a material with a thermal conductivity of λ24°C ≤ 0.034 W/(m·K) and a water vapour diffusion resistance of µ ≥ 7,900 may rise slightly in the course of the service life. However, after 10 years, it is still much lower than the initial energy loss (“dry” initial value) of an insulation material with a higher λ-value and lower µ-factor. The simultaneous improvement of the λ-value and µ-factor has con¬siderable effects on the long-term behaviour of the insulation system.

Summary

In low-temperature insulations there is a danger of moisture penetrating the insulation material. If the risk of moisture penetration is not eliminated, water and/or ice can form at spots in the insulation sys¬tem where the temperature lies below the dew point. As a consequence of moisture penetration, the thermal conductivity and, thus, also the energy losses from the installations rise.

Only low-temperature insulation materials with a low (initial) thermal conductivity and high resistance to water vapour diffusion are protected against undue moisture penetration in the long term and protect the installations against energy losses throughout their service life. The results described above clearly show how great the influence of even slight deviations in the technical values can be on the energy losses. Therefore, only insulation materials whose values can be guaranteed should be used.

The product properties which are relevant to the function must be subject to continuous internal and external monitoring and, thus, reliably ensured. The alarmingly high costs arising from moist insulation and corrosion damage every year reveal just how important it is that an insulation system functions properly in the long term.

Energy aspects are also becoming increasingly important in air conditioning and refrigeration technology. Therefore, it makes sense to plan low-temperature insulation in such a way that it not only fulfils the minimum requirement of condensation control, but also enables optimal energy savings to be achieved.

The writer is Insulation Product Manager, Hira Industries, Dubai. He can be contacted at: laurentiu@rhira.com

References:

1 VDI 2055, Part 1, Draft, 2007-02
Thermal insulation of heated and refrigerated operational installations in the industry and the building services – Calculation rules

2 Dipl. Ing. Michaela Störkmann: Langzeitverhalten von elastomeren Dämmstoffen in der Kältetechnik – Physikalische Grundlagen der Feuchtigkeitsaufnahme durch Dampfdiffusion (Long-term behaviour of elastomeric insulation materials in refrigeration technology – Fundamental physics of moisture absorption through vapour diffusion); Isoliertechnik 2-2006

Aerofoam® XLPE is composed by cross-linked closed cell polyethylene foam, alupet foil and adhesive backing. The polyethylene foam has a density of 30 Kg/m3 ± 3 Kg/m3 and has an in built water vapor barrier. The thermal conductivity value measured for 24° C is 0.034 W/m · K and the water vapor diffusion resistance factor is 7,917.