The Headline Comparison
Rigid board insulation (PIR — polyisocyanurate — sold under brands like Kingspan, Celotex, and Recticel) has a lower laboratory thermal conductivity than spray foam: 0.022–0.025 W/mK compared to 0.027 W/mK for Nexseal LE closed-cell spray foam. On paper, that makes rigid board the better thermal insulator. In practice, the opposite is true — and understanding why is critical for making the right insulation choice for your commercial building.
| Property | Spray Foam (Nexseal LE) | Rigid Board (PIR) |
|---|---|---|
| Lambda (lab) | 0.027 W/mK | 0.022–0.025 W/mK |
| Lambda (installed) | 0.027 W/mK | 0.028–0.035 W/mK* |
| Thermal bridges | None | Every joint, fixing, and edge |
| Vapour barrier | Integral | Foil-faced (joints taped) |
| Airtightness | Excellent (seamless) | Poor (joints and gaps) |
| Condensation risk | Very low | Moderate (interstitial) |
| Installation speed | 200–400m²/day | 80–150m²/day |
| Mechanical fixings | None | Required (thermal bridges) |
| Complex geometry | Conforms to any shape | Must be cut and fitted |
| Testing standard | I.S. EN 14315-1 | Product-specific |
*Installed lambda values for rigid board reflect the real-world thermal bridging penalty documented in academic research.
Thermal Performance: Laboratory vs Installed
Laboratory testing of insulation materials measures thermal conductivity under ideal conditions — a uniform sample of material with no joints, no fixings, and no air gaps, tested between two temperature-controlled surfaces. This gives the lambda value printed on the product datasheet.
In a real building, the insulation is installed as a system — and the system always performs worse than the individual material. This "performance gap" is well-documented in academic literature and is the single most important factor in choosing between spray foam and rigid board:
Research by Leeds Beckett University found that the measured U-values of buildings insulated with rigid board were consistently 15–30% higher (worse) than the designed U-values. The reasons are entirely practical: gaps between boards, compression at fixings, inadequate taping of joints, and thermal bridges through the fixing system. These defects are not installation errors — they are inherent to the rigid board system. Even the most careful installation cannot eliminate every gap at every joint across a 5,000m² warehouse roof.
Spray foam, by contrast, is applied as a liquid and expands to fill every void. There are no joints, no fixings through the insulation layer, and no gaps. The installed thermal performance matches the laboratory performance. For Nexseal LE at 0.027 W/mK, what is measured in the laboratory is what is delivered on the building.
Thermal Bridging: The Performance Gap
A thermal bridge is any location where the insulation layer is interrupted by a more conductive material — typically a metal fixing, a timber batten, or a gap between insulation boards. Rigid board insulation systems create thermal bridges at:
• Board joints: Every joint between adjacent boards is a potential thermal bridge. Even with taped joints, the insulation is thinner at board edges and the tape itself has different thermal properties.
• Mechanical fixings: Each screw or bolt that penetrates the insulation to fix it to the substrate conducts heat directly through the insulation layer. A warehouse roof with rigid board insulation may have 2,000–5,000 fixings — each one a thermal bridge.
• Perimeter edges: Where insulation boards meet walls, soffits, eaves, and other building elements, gaps and compression are common.
• Service penetrations: Every pipe, duct, cable, and fitting that passes through the insulation board must be cut around, creating gaps that are difficult to seal completely.
Spray foam eliminates all of these thermal bridges. It is self-adhering (no fixings), seamless (no joints), and conforms around services (no cut-outs). The thermal bridge fraction in a spray foam system is effectively zero.
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Airtightness and Vapour Control
Airtightness and vapour control are closely related to thermal performance. Air leakage through the insulation layer carries heat and moisture — reducing thermal performance and creating condensation risk.
Closed-cell spray foam achieves excellent airtightness because the foam itself is the air barrier. There are no joints, no mechanical fixings, and no gaps for air to pass through. Buildings insulated with spray foam routinely achieve airtightness levels of 3–5 m³/h/m² at 50 Pa — within the reasonable upper limit of 5 m³/(h·m²) at 50 Pa set in TGD L 2022 for buildings other than dwellings.
Rigid board systems rely on careful taping of foil-faced board joints to create an air and vapour barrier. In practice, tape adhesion deteriorates over time, boards shift as buildings flex under wind and thermal loads, and any missed joint or poorly taped seam becomes an air leakage path. Rigid board systems typically achieve airtightness of 7–12 m³/h/m² — above that limit unless every joint is meticulously sealed, and significantly worse than spray foam.
The vapour control provided by closed-cell spray foam is integral — the foam cells are closed and contain no connected pathways for moisture vapour. Rigid board relies on foil facings and tape for vapour control, which are susceptible to damage during installation and degradation over time.
Installation Speed and Disruption
For commercial buildings, installation speed directly affects project cost and business disruption. Spray foam can be applied at 200–400m² per day in open warehouse and industrial environments. A 5,000m² warehouse roof can be insulated in 2–3 weeks.
Rigid board installation in the same environment covers 80–150m² per day — each board must be measured, cut to fit around obstructions, fixed with mechanical fasteners, and the joints taped. The same 5,000m² warehouse roof takes 5–8 weeks with rigid board — more than double the programme time and proportionally higher labour costs.
Spray foam also requires less access equipment time. The spray process is continuous and fast; rigid board installation involves constant repositioning of access platforms to measure, cut, fix, and tape each board.
Cost Comparison
At face value, rigid board insulation appears cheaper than spray foam on a material basis:
| Cost Element | Spray Foam (Nexseal LE) | Rigid Board (PIR) |
|---|---|---|
| Material cost (50mm) | Higher | Lower |
| Installation labour | Lower (fast, continuous) | Higher (measure, cut, fix, tape) |
| Fixings/adhesive | Included | Extra cost |
| Vapour control layer | Included (integral) | Extra cost |
| Tape for joints | Not required | Extra cost |
| Total installed cost | Comparable | Comparable — often higher on complex geometry |
When the full system cost is compared — including fixings, vapour control layer, joint tape, and the labour efficiency difference — the cost gap narrows considerably. For complex geometries (corrugated roofs, buildings with many obstructions), spray foam is frequently cheaper than rigid board because the fitting and cutting labour for rigid board escalates significantly.
The real cost advantage of spray foam, however, is in the energy savings. Because spray foam delivers its calculated U-value in practice (while rigid board delivers 15–30% less), the annual energy saving from spray foam is consistently higher. Over a 20-year period, this performance gap typically represents a substantial sum in additional energy costs for the rigid board system.
When to Use Each
Spray foam is the better choice when:
• The building has complex geometry (corrugated roofs, curved surfaces, many obstructions)
• Condensation control is critical (metal buildings, swimming pools, cold stores)
• Airtightness targets are demanding
• Installation speed is a priority
• The substrate is irregular or difficult to fix to mechanically
Rigid board is the better choice when:
• The surfaces are flat, regular, and unobstructed (new-build concrete walls, for example)
• A specific board-and-batten construction detail is required by the architect
• The insulation must be easily removable (for access to services behind)
• The building owner has a strong preference for rigid board based on existing specification
For most commercial and industrial retrofit applications in Ireland, spray foam is the more practical and cost-effective solution. Contact Duratite on info@duratite.ie to discuss which solution is right for your building.