Overview: Two Different Approaches
Spray foam and mineral wool represent fundamentally different approaches to commercial building insulation. Mineral wool (including glass wool and stone wool brands such as Rockwool, Knauf, and Isover) is a fibrous insulation material supplied in rolls, batts, or slabs, installed by cutting to size and fitting between structural members or mechanically fixing to surfaces. Spray foam (specifically closed-cell polyurethane foam such as Nexseal LE) is a liquid material applied by specialist equipment that expands and cures on contact with the substrate, forming a rigid, closed-cell insulation layer bonded directly to the surface.
Both materials have legitimate applications in commercial construction, and the correct choice depends on the specific building, application, performance requirements, and budget. This guide provides an objective comparison to help specifiers, building owners, and contractors make an informed decision.
Thermal Performance Comparison
The most important thermal performance metric for insulation is thermal conductivity (lambda value, measured in W/mK). Lower values indicate better insulating performance per unit thickness.
| Property | Closed-Cell Spray Foam | Mineral Wool (Stone Wool) |
|---|---|---|
| Thermal conductivity | 0.027 W/mK (Nexseal LE) | 0.034–0.040 W/mK |
| Thickness for U=0.25 (flat roof renovation) | ~105mm | ~130–155mm |
| Thickness for U=0.20 (new flat roof) | ~130mm | ~165–195mm |
| Thermal bridging at fixings | None (self-adhering) | Yes (mechanical fixings) |
| Installed vs calculated performance | Equal to or better than calculated | 15–25% worse than calculated |
| Air barrier function | Yes (inherent at 25mm+) | No (air-permeable) |
The practical significance of these differences is substantial. In a steel portal frame building with purlins at 1.5m centres, 100mm of mineral wool between purlins delivers an effective U-value of approximately 0.45 W/m²K (after accounting for thermal bridges at purlins). 80mm of spray foam over the same purlins delivers an effective U-value of approximately 0.28 W/m²K — significantly better performance from a thinner installation because the thermal bridges are eliminated.
Acoustic Properties
Mineral wool has a clear advantage in acoustic absorption. Dense stone wool (80–140 kg/m³) is one of the most effective acoustic absorbers available, with a Noise Reduction Coefficient (NRC) of 0.90–1.05. It is widely used in acoustic partitions, studio walls, and noise-sensitive applications.
Closed-cell spray foam is a poor acoustic absorber (NRC 0.10–0.25) due to its rigid, closed-cell structure. However, open-cell spray foam achieves NRC values of 0.70–0.85 — competitive with many mineral wool products, though not matching the highest-performing dense stone wool slabs.
For applications where acoustic performance is the primary requirement — recording studios, plant rooms, acoustic enclosures — mineral wool is the stronger choice. For applications where thermal performance is primary and acoustic performance is secondary — warehouses, factories, data centres — spray foam is preferred.
Fire Rating Comparison
Fire performance is a critical consideration in commercial insulation specification:
Mineral wool is non-combustible (Euroclass A1 or A2-s1,d0 to I.S. EN 13501-1, depending on facing and binder content). This is mineral wool’s greatest single advantage: it does not burn, does not contribute to fire spread, and does not produce significant smoke. In applications where fire regulations demand non-combustible insulation — such as certain escape routes, tall buildings where the fire safety design under TGD B calls for it, and some insurance-specified requirements — mineral wool is the default and often the only compliant option.
Spray foam is combustible (Euroclass E or D when untreated). With DC315 intumescent coating, it achieves Euroclass B-s1,d0 — a “limited combustibility” rating that is acceptable for most commercial applications including roofs, walls, soffits, and general industrial areas. However, it does not achieve the non-combustible A1/A2 rating of mineral wool, and this distinction matters in fire-engineered solutions for high-risk buildings.
For most commercial and industrial buildings — warehouses, factories, retail, offices, schools — spray foam with DC315 meets fire safety requirements. For high-rise, healthcare, and some public buildings where non-combustible materials are mandated, mineral wool is required.
Moisture and Condensation Resistance
This is where the two materials diverge most significantly:
Mineral wool is highly absorbent. If the vapour barrier is breached — through poor installation, damage, or degradation over time — mineral wool absorbs moisture from condensation, rain ingress, or internal humidity. Wet mineral wool loses virtually all of its insulating properties: a 100mm mineral wool batt at 5% moisture content by volume has an effective thermal conductivity approximately 50% worse than its dry value. At 10% moisture content, it is essentially useless as insulation.
Closed-cell spray foam has a closed-cell content exceeding 90% and absorbs less than 2% moisture by volume even under prolonged immersion. Its thermal performance is essentially unaffected by moisture exposure. Furthermore, at thicknesses above 50mm, closed-cell spray foam acts as its own vapour barrier — meaning there is no separate membrane to breach, damage, or degrade.
In commercial buildings where moisture is present — food processing, swimming pools, laundries, car washes, agricultural buildings, marine environments — spray foam’s moisture resistance is decisive. In dry, well-controlled environments — offices, retail, schools — mineral wool performs adequately provided the vapour barrier remains intact throughout the building’s life.
Installation Speed and Disruption
Spray foam installation rates are typically 200–400 m² per day for a two-person crew, depending on the specified thickness and application complexity. A 2,000m² warehouse roof can be insulated in 5–10 working days.
Mineral wool installation rates for the same application are typically 50–100 m² per day per two-person crew (for between-purlin fitting with liner sheet), meaning the same 2,000m² roof would take 20–40 working days.
The speed advantage of spray foam reduces programme duration, site preliminaries costs, scaffold/access equipment hire periods, and disruption to building occupants.
10-Year Cost Comparison
The upfront cost of spray foam is typically higher than mineral wool per square metre. However, the total cost picture over 10 years includes energy savings, maintenance, and performance degradation:
| Cost Factor | Spray Foam | Mineral Wool |
|---|---|---|
| Material + installation | Higher upfront | Lower upfront |
| DC315 fire coating | Additional cost where the fire safety design requires it | N/A (non-combustible) |
| 10-year energy saving vs uninsulated | Greater (no bridging at purlins, airtight) | Lower (bridging and air leakage) |
| Maintenance/repair (10 years) | Negligible | Periodic repairs where quilt sags or gets wet |
| Performance degradation | Negligible | 10–20% (sagging, moisture) |
When energy savings are included, the total cost of ownership over 10 years is frequently comparable or favourable for spray foam — despite the higher upfront cost — because of its superior real-world thermal performance and zero maintenance requirements. Duratite prices every project individually, with a fixed quote after a free site survey.
When to Choose Each Material
Choose spray foam when:
- Thermal performance and energy efficiency are the primary objectives
- Condensation control is critical (food processing, cold storage, marine, agricultural)
- The substrate has complex geometry (profiled metal, Metsec, irregular surfaces)
- Installation speed and minimal disruption are important
- The building has steel framing with thermal bridging at purlins/rails
- Moisture resistance is required (wet environments, roofs with leak history)
Choose mineral wool when:
- Non-combustible insulation is mandated by fire regulations or insurance
- Acoustic performance is the primary requirement
- Budget is severely constrained and energy savings are not a priority
- The application is in a tall building where the fire safety design under TGD B requires non-combustible materials
Duratite supplies Nexseal LE closed-cell spray foam (0.027 W/mK, declared to I.S. EN 14315-1) for commercial and industrial insulation across Ireland and Northern Ireland. Contact us on info@duratite.ie for a specification comparison and quotation.