Extruded polystyrene (XPS) and expanded polystyrene (EPS), a comparison of the manufacturing process and the performance.
XPS and EPS, manufacturing-wise but also performance-wise represent two different groups of rigid thermal insulation materials. The two types of boards have different technical properties.
XPS stands out through: very low moisture absorbtion, high compressive strength, low thermal conductivity, etc. Let us compare these advantages so we get a more complete view of the board’s performance.
1. Thermal resistance (R) and conductivity (λ)
Thermal resistance (R) and conductivity (λ) are established by taking into account the aging process of the board. At least ten measurements are taken in the assessment process. The aging process is simulated using expanding gases with a lower thermal conductivity than air that remain inside the extruded polysterene for various periods of time, depending on the thickness of the tested board. A 25-year aging can be simulated using this technique. In order to establish their official values, the manufacturer of the board directly measures resistance and conductivity, in-depth as well as on its surface. The value that reflects the aging period is symbolized through the λaveragewhich stands for the average thermal conductivity of the extruded polystyrene once the aging process is complete.
The SR EN 13164 standard helps to precisely define the thermal resistance (R) and conductivity (λ) measuring methods. The SR EN 12667 is a special standard for measuring thin boards and SR EN 12939 deals with thick boards.
The resulting values are all stated by polysterene manufacturers and can be easily found in the product’s technical specification brochures, on the manufacturer’s websiteand also on the product label.The manufacturer will specify in the CE label the calculated (λ) value for the thermal conductivity, as specified in the SR EN 13164 method of measurement.
The thermal conductivity range for the XPS boards is 0.030, 0.040 and 0.045 W/mK.
The GIAS GrafitXPS board has a λ value ≥ 0.028 W/mK.
2. Water absorbtion
Water damages thermal insulation by permeating its structure. This leads both to the direct damage to the material as well as a loss of insulating qualities, as water conducts heat 25 times better than air. The spongy extruded polystyrene material on the other hand, due to its closed-cell structure, withstands water and humidity, being thus a reliable solution for the safe, long-term insulation of a building. The thermal insulation is not affected by water, the boards providing appropriate insulation during the lifetime of the building.
Due to this feature, the XPS boards provide an important advantage when their uses require a certain level of detail. For example, on a terraced roof, in the inverted terrace system, the XPS boards, due totheir waterproofing qualities, provide a long lifetime solution. When enveloping a building’s exterior wall the XPS boards are waterproofed for any conditions and are designed to withstand continuous inclement weather and freeze-defreeze cycles, maintaining their compressive strength for the entire lifetime of the building and their insulation capability throughout their lifetime.
3. Water vapour diffusion resistance (µ) – or the so-called ability to breathe.
The µ parameter defines the resistance to the diffusion of the water vapour as compared to the resistance to air and establishes the quantity of vapours that passes through a certain surface area in standardized time, temperature and humidity conditions and for a particular standardized width for the tested materials.
Diffusion occuring on the walls of a building, commonly called the ability of the wall to breathe, is defined by the µ parameter for every building material. Each has its own coefficient value for water diffusion. Extruded polystyrene materials, owing to their increased resistance to water vapour diffusion, have a parameter value that is optimally suited for their different uses.
See below for a comparison of µ values:
µair = 1
µmineral felt = 1
µEPS = 50-70,
µXPS = 50-250
µbitumen membrane = 20,000-50,000
µAluminium foil = 1,000,000
What is Sd?
Sd = µ x d
Sd is used in practice and indicates the thickness of a static layer of air that has the same water vapour resistance (µ)as the building material thickness (d). It is directly proportional with the diffusion resistance and the thickness of the building material t, measured in meters.
Sd= the equivalent thickness of a layer of air.
See below for a comparison of lengths that must be travelled by water vapours for various building materials:
Sd paramenter value for:
• 3 cm thick XPS boards (medium insulation):
100(µ) 0.03(d) = 3.0 m
• 4 cm thick EPS boards:
50(µ) 0.04(d) = 2.0 m
• 20 cm thick reinforced concrete wall:
100(µ) 0.02(d) = 20.0 m
• 2 mm thick bitumen membrane:
20,000(µ) 0.002(d) = 40.0 m
According to DIN 4108 the limit for the wall’s inability to breathe ( being diffusion-proof) is
Sd = 1500 m
4. Resistance to corrosive substances and solvents
The operation of a building must take into account the resistance to corrosion by various chemical substances as the thermal insulation comes in contact with: adhesives, paints, solvents, thinners, bitumen based products, concrete, etc. XPS boards can withstand corrosion from building materials like lime, cement, gypsum, bitumen, physiological serum, diluted acid, etc. XPS boards can not, however, withstand corrosion from flammable bitumen-based substances, varnish or solvent-based thinners.
5. FireproofingLike any other organic material, XPS boards can catch fire. Unless fire retardant substances are used, XPS, as a self-standing product, tests as an E class product under the European fire regulations.
Due to the XPS boards never being used on their own, that is, not directly exposed to a flame or covered in various other products, such as plasters, it makes sense that the testing is done with the covering material on.
The reaction to fire for a certain material is determined by its flammability (whether or not it can combust), the degree to which the flames persist and whether or not incandescent particles detach from it.
Fire resistance is the time value for which the material can hold its structure during a fire. It is influenced by the below parameters:
- the percentage of flame retardant additives used
- the flammability of the expanding gas
- the tested sample (the width)
- the material’s density
The GIAS GrafitXPS boards are class E products which means that the polystyrene will keep burning if exposed to fire. However, if the source of the fire is removed, the XPS board will extinguish.
6. Mechanical properties
One of the superior qualities of the XPS extruded polystyrene boards is its high compressive resistence for both short-term and long-term loads. A thermal insulator with high compressive strength will very slightly decrease its width when subjected to a load, maintaining intact calculations pertaining to the thermal insulation, the material’s insulation features and values being preserved.
The material’s thermal resistance is directly proportional to its width: R = d / λ.
Extruded polystyrene is among materials with high compressive strength.As a safety measure, loads that correspond to a 10% deformation of the material’s basic shape can be taken into account.
There is a direct proportionality between XPS’ density and one of its main features: its resistance to compression/elongation. As the density increases so will the resistance. The GIAS GrafitXPS graphite boards have a minimum density of 32 kg/m3, 20% higher than similar products.
The link between density and resistance to compression
The link between density and tensile strength
The testing method is regulated by SR EN 1606 that states that the maximum accepted elongation is 2%.
7. Dimensional stability
Thermal insulators, through their nature, will have to undergo temperature fluctuations. This will give rise to major temperature differences on their opposing sides while also being affected by the day-night cycles. Dimensional stability is the most important property that influences the behaviour of a thermal insulator during daily operation.
The spongy XPS extruded polystyrene material is assumed to have a three dimensional cell structure which is balanced, airtight and consistent.