What is Compression Strength for Insulation?

Insulation materials are a critical component of the building fabric, regulating the thermal capabilities, enhancing energy efficiency, and even offering acoustic shielding. However, they also improve the durability of the building fabric. The primary contributory characteristic that provides this is compression strength. In today's blog, we'll discuss compression strength, why it's essential for insulation materials, and some of the best-performing materials.

What is compression strength?

Compression strength is the maximum load/capacity something can withstand before it reaches 10% relative deformation. Compression is a force that pushes the particles of a material closer together. This characteristic is essential because it impacts the material's ability to maintain its form, functionality, and effectiveness under pressure. For insulation, compression strength is critical because it influences how the material performs under a building's structural loads and during its lifespan.

Measuring compression strength

The measurement of compression strength is typically conducted through standardised tests where a sample of the material is subjected to increasing loads until it deforms or fails. This is quantified in terms of pressure, usually expressed in pascals (Pa) or sometimes in pounds per square inch (psi). For insulation materials, the testing standards might include:

• ASTM C165: This standard test method for measuring the compressive properties of thermal insulations determines the maximum compressive strength and deformation of the material under a compressive force. It is widely used for various insulation types.
• ISO 844: This test measures the compressive properties of rigid cellular plastics, including some foam insulation types.

What determines a material's compression strength?

• Material density: Generally, higher-density materials tend to have higher compression strength. For example, high-density spray foam or extruded polystyrene (XPS) foam typically exhibits greater compression strength than lower-density alternatives like expanded polystyrene (EPS).
• Material composition: The chemical and physical structure of the insulation material also plays a significant role. Fibreglass, for instance, consists of fine glass fibres that can offer moderate compression strength, whereas materials like cellulose might compress more readily under the same load.
• Moisture content: The presence of moisture can significantly reduce the compression strength of insulation materials. Moist materials are more prone to deformation under load, impacting their structural integrity and thermal performance.
• Age and condition: Over time, materials can degrade or settle, especially under continuous load or exposure to environmental factors, which can diminish their original compression strength.

Why is it important?

The significance of compression strength in insulation goes beyond the material's physical robustness; it directly impacts the insulation's efficacy, durability, and overall performance in various structural contexts.

Effective thermal performance

The compression strength of insulation is vital because if the material compresses under the weight of structural elements or due to external pressures, its thickness—and consequently its thermal resistance or R-value—diminishes. For instance, insulation compressed under a heavy load will lose not only its original thickness but also its ability to resist heat flow effectively. This reduction can lead to increased energy costs and reduced comfort levels within the building. Ensuring that insulation retains its designated thickness through adequate compression strength is, therefore, essential for achieving the desired thermal performance over the lifespan of the building.

Structural integrity and load distribution

In areas like roofing and flooring, where insulation might need to bear or distribute loads, its compression strength ensures it can withstand such stresses without deforming. Insulation materials with high compression strength help maintain the structural integrity of these areas by distributing loads evenly and reducing the risk of structural damage. This is particularly important in flat roofs or decks where insulation serves as an integral part of the load-bearing structure. The ability of insulation to handle these loads without significant compression ensures the longevity and durability of both the insulation and the structural elements it supports.

Longevity and material durability

This durability is crucial in environments where the material might be subjected to ongoing stress, such as in high-traffic areas or beneath mechanical equipment. Insulation with higher compression strength is also better at resisting damage during installation and maintenance activities, which can often compromise the material’s integrity. By choosing appropriate insulation, building owners can reduce the need for frequent replacements and maintenance, thereby lowering long-term costs and enhancing the building's performance.

Versatility in application

Different building areas and types of construction require varying levels of compression strength. Understanding the compression strength of insulation allows architects and builders to choose the right materials for each application, ensuring that the insulation performs well under the specific conditions of each site. For example, beneath a concrete floor slab, a high-compression strength material like extruded polystyrene (XPS) might be necessary to handle the substantial load, whereas, in wall cavities, lighter, less compressive materials might suffice. This tailored approach to selecting insulation based on its compression strength allows for optimised performance across different building parts.

Relationship between material density and compressive strength

Generally, as a material's density increases, so does its ability to withstand compressive forces. Density refers to the amount of mass per unit volume of a material and is a direct indicator of how compact the material's structure is. In denser materials, particles or fibres are packed more tightly together, contributing to their ability to resist deformation under compressive loads. The compact arrangement allows the material to distribute applied forces more effectively throughout its structure, thereby enhancing its strength and stiffness. Mineral wool is particularly interesting in composition:

"The thermal insulation products made of mineral wool depending on the orientation of fibres could be di vided into the chaotic (the case when fibres are distributed randomly and in different directions irregularly) and the directional (the case when fibres are distributed in a certain order) [1, 2] in terms of the structure. The mineral wool products which have the directional fibres structure are called “lamella” [3]. Moreover, some products of the combinative fibres structure are used for certain constructions [4]. [...] However, as construction practice shows, the contractors are often complaining about the irregular compressive stress (compressive strength) and deforma- tion of slabs used for facade and roof thermal insulation. The assumption may be made that the compressive strength and other mechanical properties of mineral wool products depend on fibre distribution (orientation) in the

product structure"

Buska, Andrius & Mačiulaitis, Romualdas. (2007). The compressive strength properties of mineral wool slabs: Influence of structure anisotropy and methodical factors. Journal of Civil Engineering and Management - J CIV ENG MANAG. 13. 10.1080/13923730.2007.9636425, accessed at https://www.researchgate.net/publication/228675436_The_compressive_strength_properties_of_mineral_wool_slabs_Influence_of_structure_anisotropy_and_methodical_factors

Higher density generally correlates with higher compressive strength for many materials, including metals, polymers, and insulation like foam or fibreglass. This is because the internal structure of denser materials can better support and distribute the stresses associated with compression. When a material is compressed, the individual particles or fibres resist the forces. The denser arrangement provides less space for these particles or fibres to move or buckle, thereby increasing the material's resistance to compression.

Compression strength of different insulation materials

Most materials' relative compressive strengths are listed in their technical documentation. Insulation is no different; we currently stock EPS, mineral wool, Kingspan K5, and XPS.

With XPS primarily insulating below the DPC, the most durable material we stock is Kingspan K5. For this reason, we call it a space-saving system, as it achieves exceptional durability and thermal performance at a much lower thickness. Building regulations dictate that retrofits must achieve a U-value of 0.3W/m²K, which Kingspan K5 achieves at a thickness of 60mm. The increased compressive strength also makes it suitable for urban areas where mechanical compression and damage can occur.