The realm of modern building insulation has been revolutionised by spray foam, an innovative solution that offers exceptional thermal performance and air sealing capabilities. However, not all spray foams are created equal, with two primary types dominating the market: open cell and closed cell spray foam insulation. Understanding the fundamental differences between these two can be pivotal in selecting the optimal material for any construction or renovation project, ensuring long-term efficiency and comfort.
Spray foam insulation is typically a two-part liquid material that, when mixed, rapidly expands and hardens into a solid cellular plastic. This expansion allows it to fill cavities, crevices, and irregular spaces, creating a seamless, monolithic barrier against air leakage and heat transfer. While both open and closed cell variants achieve these core objectives, they do so through distinct cellular structures, leading to significant variations in their properties and ideal applications. The choice between them hinges on a careful assessment of density, R-value, vapour permeability, and structural characteristics, with closed cell foam insulation often being sought after for its superior strength and moisture resistance.
Open cell spray foam insulation is characterised by its light, airy, and flexible texture, resembling a sponge once cured. Its cells are intentionally left ‘open’, meaning they are not completely encapsulated. This allows air to fill the voids within the foam structure, contributing to its insulating properties primarily through trapping air, which is a poor conductor of heat. Being less dense than its counterpart, open cell foam expands significantly upon application, often up to 100 times its liquid volume, making it highly effective at filling large, complex cavities and reaching intricate spaces. Its forgiving nature allows for some movement in the building structure without cracking, making it a suitable choice for internal applications where such flexibility is an advantage.
In contrast, closed cell foam insulation presents a much denser, more rigid, and compact structure. As its name implies, the cells within this foam are completely enclosed and tightly packed together. Instead of simply trapping air, these closed cells encapsulate an insulating gas, usually a hydrofluoroolefin (HFO) or similar blowing agent, which possesses a much lower thermal conductivity than air. This gas plays a critical role in the superior R-value of closed cell foam insulation , offering a higher thermal resistance per unit of thickness compared to open cell. Its inherent rigidity also lends itself to providing additional structural support to walls and roofs, a significant benefit in certain construction scenarios.
One of the most distinguishing features when comparing the two types is their R-value, which measures thermal resistance. Open cell spray foam insulation typically offers an R-value ranging from 3.5 to 3.7 per inch of thickness. While this is still considerably better than traditional insulation materials like fibreglass or mineral wool, it is notably lower than that of closed cell foam insulation . The latter boasts an impressive R-value of approximately 6.0 to 7.0 per inch, sometimes even higher. This means that a thinner layer of closed cell foam insulation can achieve the same, or even better, thermal performance as a much thicker layer of open cell foam, which can be a crucial consideration in constructions with limited cavity space.
Beyond thermal resistance, air sealing is a primary advantage of both spray foam types. Both open and closed cell foam insulation create an effective monolithic air barrier that significantly reduces air leakage, a major contributor to heat loss and gain in buildings. By preventing uncontrolled air movement, spray foam enhances comfort, reduces draughts, and helps maintain consistent indoor temperatures. However, the denser structure of closed cell foam insulation makes it inherently more robust in forming an impenetrable barrier against air infiltration, often surpassing the performance of open cell in very windy or exposed environments.
Vapour permeability is another critical differentiator. Open cell spray foam is considered vapour permeable, meaning it allows moisture vapour to pass through it. This characteristic can be beneficial in certain applications where controlling moisture movement is desired through controlled ventilation. However, it also means that open cell foam cannot act as a standalone vapour barrier. Conversely, closed cell foam insulation is largely impermeable to moisture vapour, functioning as an effective vapour barrier when applied at sufficient thickness. This makes closed cell foam insulation an excellent choice for areas prone to moisture, such as basements, crawl spaces, and exterior walls, where preventing moisture intrusion into the building envelope is paramount to preventing mould growth and structural damage.
Water resistance is closely tied to vapour permeability. Due to its open cell structure, open cell foam can absorb and hold water, which can compromise its insulating performance and potentially lead to problems if moisture is not properly managed within the building assembly. This makes it unsuitable for direct contact with water or in areas with high risk of water exposure. In stark contrast, closed cell foam insulation is highly resistant to water absorption. Its impermeable, dense structure means that it sheds water rather than absorbing it, retaining its insulating properties even in damp conditions. This resilience against moisture makes closed cell foam insulation a preferred option for foundations, exterior sheathing, and other areas where water management is a primary concern.
The contribution to structural integrity is another area where closed cell foam insulation stands out. Its rigidity and density allow it to add significant racking strength to wall assemblies and roof decks, effectively gluing the components together. This can enhance a building’s resistance to wind uplift and other lateral forces, providing an additional layer of structural reinforcement. Open cell foam, being much softer and more flexible, offers negligible structural enhancement, performing solely as an insulation and air sealing material.
When considering sound attenuation, open cell foam generally performs better at absorbing airborne sound. Its softer, sponge-like matrix effectively traps and dissipates sound waves, making it an excellent choice for interior walls and floor cavities where noise reduction between rooms is desired. While closed cell foam insulation can block sound due to its density, it doesn’t absorb it as effectively as open cell, meaning it’s better for blocking sound transmission through a wall rather than deadening sound within a room.
Cost is often a significant factor in decision-making. Open cell spray foam insulation typically has a lower upfront material cost per square metre compared to closed cell foam insulation . This is due to its lower density and higher expansion rate, meaning less material is needed to cover a given area. However, it’s important to consider the total installed cost and the long-term benefits. The higher R-value per inch of closed cell foam insulation can mean that less material is required to achieve specific energy performance targets, potentially offsetting some of the initial cost difference, especially in projects with space limitations. Moreover, the superior long-term energy savings and resilience of closed cell foam insulation can offer a more favourable return on investment over the lifespan of the building.
From an application perspective, open cell spray foam is often recommended for interior applications such as wall cavities, attic rafters, and floor joists in residential settings, particularly where cost-effectiveness and sound dampening are priorities. Its vapour permeability means that moisture can dry out of the wall assembly, which can be advantageous in certain climates. On the other hand, closed cell foam insulation is the go-to choice for applications demanding high R-value in limited spaces, superior moisture resistance, and structural enhancement. This includes exterior walls, crawl spaces, basement walls, commercial buildings, and metal buildings. Its ability to create an impenetrable vapour barrier is particularly crucial in cold climates or regions with high humidity.
Environmental and health considerations also play a role. Both types of spray foam insulation contribute to reduced energy consumption, thereby lowering a building’s carbon footprint. Modern formulations increasingly utilise blowing agents with lower global warming potential. During the installation process, proper ventilation and personal protective equipment are essential, as the chemicals involved need to fully cure. Once cured, both open and closed cell foam insulation are considered inert. Reputable installers will ensure all safety protocols are meticulously followed, ensuring the long-term safety and health of occupants.
Ultimately, the choice between open cell and closed cell foam insulation is not about one being inherently ‘better’ than the other, but rather about selecting the most appropriate material for the specific needs of a project. Factors such as climate, building design, budget constraints, and desired performance characteristics (e.g., thermal resistance, moisture control, structural integrity, sound attenuation) must all be carefully weighed. For projects demanding maximum thermal performance, robust moisture protection, and structural reinforcement, closed cell foam insulation frequently emerges as the preferred option, offering unparalleled resilience and efficiency.
In conclusion, spray foam insulation offers a high-performance solution for modern building envelopes, significantly enhancing energy efficiency and indoor comfort. While open cell spray foam provides excellent air sealing and sound dampening at a competitive price point, closed cell foam insulation stands out for its superior R-value per inch, exceptional moisture resistance, and valuable structural contributions. Making an informed decision requires a thorough understanding of these differences and consulting with experienced insulation professionals who can assess the unique requirements of your property. By choosing wisely, property owners can invest in a solution that provides lasting benefits for decades to come, significantly impacting the building’s operational costs and overall liveability.