Insulation ability to resist deformation or maintain shape is often called compression strength
The specific compression strength values needed must be determined by the architect or engineer.
Strength is used to compare different products or brands
Compressive strength declarations are made by insulation manufacturers, based on recognised test standards.Compressive failure of insulation materials can cause deterioration of the insulation system and lead to damage of mechanical systems and equipment.
When identifying the compressive capabilities of an insulation material, it undergoes two tests:
1.Short term testing (Compressive Behaviour BS EN 826:2013),
2.Long term testing (Compressive Creep BS EN 1606:2013)
The compressive strength of thermal insulation is measured by a method described in the formal test standard EN 826 “Thermal insulating products for building applications – determination of compression behaviour.”
A load is applied to the insulation sample being tested.
A result is declared when either the material reaches the limit of elastic behaviour (i.e. it fails), in which case the result is the load ‘at yield’, or the insulation compresses by 10% relative to the original sample thickness.
When the material fails before it has been compressed by 10% then the declared load at yield is the compressive strength of the material.No thermal insulation material would ever be used in a situation where it would be subjected to the tested loads. The amount of compression and the risk of failure would be inappropriate. EN 826 is clear that its test method is employed to obtain ‘reference values’ that allow appropriate loading calculations to be carried out by a structural engineer.
In terms off compressive strength, contractors tend to be most familiar with extruded polystyrene (XPS). Expanded polystyrene (EPS) performs as well or better than XPS, and at a substantially lower cost. Important factor need to be consider when comparing these two insulations for any belowgrade or under-slab application is compressive strength
Using an unnecessarily high-strength insulation, you end up paying for something you don’t really need. Since EPS is lower cost per inch than XPS, and is available in a range of compressive strengths , using it belowgrade and under slabs can save on insulation costs.
Grade insulation must be strong enough to withstand the pressure of the loads above it.
The compressive strength of EPS is worse than XPS. For EPS to meet the same compressive strengths as XPS, the density of the foam would need to be a lot higher.
Both phenolic and PIR foams derive some of their long-term thermal performance from facing materials that restrict the loss of the gas in the foam structure. Protecting those facings is important to ensure they perform for the life of the building.
That means keeping water away from the insulation boards, regardless of them being closed cell materials with low rates of water absorption. They should always be installed above the damp proof membrane (DPM), and never laid exposed directly to the ground.
Similarly, although EPS insulation has no facings that are susceptible to damage from alkalis or moisture, its capacity for moisture absorption means it must still be installed above the DPM.
As the table shows, EPS is capable of much greater loadbearing capacity than phenolic and PIR foams – but is also made available in lower compressive strengths. Even with compressive strengths exceeding 120 or 140 kPa, manufacturers of phenolic and PIR products can be extremely cautious about offering their insulation for anything more than light commercial applications.
Different types of polystyrene insulation, price tends to be the common differentiator between EPS and XPS. However, the significantly increased compressive strength offered by higher grades of XPS makes it the only choice for certain applications – particularly those involving vehicular traffic.
Like EPS boards, XPS products are typically unfaced. But they offer another important advantage: lower moisture absorption. They are more tolerant of wet conditions, so can be installed against the ground without affecting performance over time.
