Understanding Steel Portal Frame Construction
Steel portal frame construction accounts for the majority of commercial and industrial buildings erected across Ireland over the past 50 years. The system uses hot-rolled or cold-formed steel columns and rafters connected by moment-resisting joints to create clear-span structures from 10m to 60m+ in width. The roof and wall cladding is typically supported on secondary steel members — purlins (roof) and side rails (walls) — which span between the primary portal frames at 1.2–1.8m centres.
The standard cladding system consists of profiled steel sheeting fixed to the purlins and side rails, with insulation fitted between the outer cladding and an inner liner sheet. This “built-up” or “liner and quilt” system has been the industry standard for decades — but it has fundamental thermal performance limitations that spray foam insulation eliminates.
The Thermal Bridging Problem
The greatest weakness of traditional portal frame insulation is thermal bridging through the steel purlins and side rails. Steel has a thermal conductivity of approximately 50 W/mK — nearly 2,000 times more conductive than spray foam insulation at 0.027 W/mK. Every purlin that passes through the insulation layer acts as a thermal short circuit, conducting heat directly from the warm interior to the cold exterior.
In a typical built-up cladding system with 100mm mineral wool quilt draped between purlins, the purlins create thermal bridges at 1.5m centres across the entire roof and wall area. The effect on overall thermal performance is dramatic:
| Insulation System | Nominal R-Value | Effective U-Value (inc. bridges) |
|---|---|---|
| 100mm mineral wool between purlins | 2.86 m²K/W | 0.50–0.60 W/m²K |
| 150mm mineral wool between purlins | 4.29 m²K/W | 0.35–0.45 W/m²K |
| 80mm spray foam over purlins | 2.96 m²K/W | 0.28–0.32 W/m²K |
| 100mm spray foam over purlins | 3.70 m²K/W | 0.22–0.26 W/m²K |
The key insight is that 80mm of spray foam applied over the purlins (encapsulating them within the insulation envelope) delivers a better effective U-value than 150mm of mineral wool between the purlins. The spray foam achieves this by eliminating the thermal bridge entirely — the purlins sit within the warm side of the insulation, surrounded by foam on all sides.
Purlin-to-Purlin Spray Foam Application
The spray foam is applied directly to the underside of the profiled steel roof sheeting, building up in passes to encapsulate the purlins within the foam layer. The application method is:
- First pass (15–20mm) — an initial “flash coat” is applied to the steel sheeting and purlin flanges. This bonds the foam to the steel substrate and provides the foundation for subsequent passes.
- Build-up passes (20–25mm each) — additional passes build the foam thickness to the specified depth, typically 80–100mm. Between each pass, a cooling period of 10–15 minutes allows the exothermic reaction to dissipate.
- Purlin encapsulation — as the foam builds up, it progressively encapsulates the purlin flanges and web, eliminating the thermal bridge. The foam bonds to all exposed steel surfaces.
- Surface finish — the final pass creates a relatively smooth surface that can be left exposed, coated with DC315 (if fire rating is required), or plastered for a finished appearance.
The result is a continuous insulation layer with no joints, no fixings, and no thermal bridges — bonded directly to the structural steelwork and cladding.
Comparison with Traditional Liner Systems
Traditional built-up cladding systems (also known as “liner and quilt” or “twin-skin” systems) consist of an outer profiled sheet, mineral wool insulation draped over the purlins, and an inner liner sheet that supports the insulation and provides a finished ceiling. While this system has been the industry default for decades, it has significant limitations:
- Thermal bridges at every purlin — as detailed above, purlins create systematic thermal bridges that reduce effective thermal performance by 30–50% compared to nominal insulation values.
- Compression at purlin tops — mineral wool quilt is compressed to near-zero thickness where it passes over the top of each purlin, providing no insulation at the point where the steel structure is closest to the external environment.
- Sagging between purlins — over time, mineral wool quilt sags between purlins under its own weight, creating air gaps between the insulation and the outer sheeting. These air gaps allow convective heat loss that further degrades thermal performance.
- Moisture retention — mineral wool absorbs moisture from condensation, rain ingress through leaking fixings, and internal humidity. Wet mineral wool loses almost all of its insulating properties and adds dead weight to the liner system.
- Vapour barrier failure — the vapour barrier in a built-up system relies on sealed laps at every liner sheet joint and every purlin. In practice, achieving and maintaining an airtight vapour barrier across thousands of linear metres of laps is extremely difficult. Any breach allows moisture to enter the insulation layer.
Spray foam eliminates every one of these issues. There are no thermal bridges (purlins are encapsulated), no compression points, no sagging, no moisture absorption (closed-cell structure), and no need for a separate vapour barrier (the foam is its own vapour barrier at 50mm+).
U-Value Calculations for Portal Frames
In the Republic of Ireland, Technical Guidance Document L 2022 – Buildings other than Dwellings sets maximum area-weighted average U-values for new buildings of 0.20 W/m²K for flat roofs, 0.16 W/m²K for pitched roofs, and 0.21 W/m²K for walls. Where an existing building is renovated, Table 10 applies: 0.25 W/m²K for flat roofs and for pitched roofs insulated at the slope, and 0.35 W/m²K for walls. In Northern Ireland, Technical Booklet F2 sets the equivalent targets.
With Nexseal LE closed-cell spray foam at 0.027 W/mK (declared to I.S. EN 14315-1):
- Roof U-value of 0.25 W/m²K (renovation) — requires approximately 105mm of spray foam applied over purlins
- Roof U-value of 0.20 W/m²K (new flat roof) — requires approximately 130mm over purlins; 0.16 W/m²K (new pitched roof) requires approximately 165mm
- Wall U-value of 0.21 W/m²K (new build) — requires approximately 125mm over side rails; 0.35 W/m²K (renovation) requires approximately 75mm
These thicknesses account for the thermal bridging correction factor, which is minimal for spray foam (typically less than 5%) compared to built-up systems (30–50% correction required).
Condensation Control
Condensation is the most common building defect in steel portal frame buildings. The combination of a highly conductive steel structure, temperature cycling between day and night, and internal moisture from processes, vehicles, or occupants creates conditions where condensation forms on the underside of the steel cladding, drips onto stock, equipment, and personnel, and causes progressive corrosion of the steel structure.
Spray foam eliminates condensation in portal frame buildings by:
- Raising the internal surface temperature of the cladding above the dew point
- Encapsulating all steel surfaces (purlins, rafter flanges, side rails, gutter brackets) that would otherwise be cold bridges and condensation nucleation points
- Creating an inherent air and vapour barrier that prevents moist internal air from reaching the cold steel surfaces
Installation Considerations
Key considerations for spray foam application in steel portal frame buildings:
- Access — spray foam application to roof undersides requires access equipment (mobile elevating work platforms, scissor lifts, or scaffolding). Clear eaves height, structural loading limitations, and floor surface condition affect access equipment selection.
- Temperature and moisture — spray foam should be applied when the steel surface is dry and above 5°C. In winter, and after the damp spells common in the Irish climate, the building may need to be heated and dried out before application begins.
- Ventilation — during application, adequate ventilation must be provided for worker safety. The building should be unoccupied during spraying and for a minimum of 24 hours after completion.
- Existing insulation — if the building has existing mineral wool insulation, this should ideally be removed before spray foam is applied. Spraying over wet or compressed mineral wool creates a moisture trap. However, in some cases where the existing insulation is dry and well-adhered, spray foam can be applied directly over the liner sheet.
Duratite supplies Nexseal LE closed-cell spray foam (0.027 W/mK, declared to I.S. EN 14315-1) for steel portal frame building applications. Our approved installer network includes contractors experienced in industrial building insulation across Ireland and Northern Ireland. Contact us on info@duratite.ie for a specification consultation.