Understanding Thickness and U-Values
The thickness of spray foam insulation required for any application is determined by a simple calculation: the target U-value divided into the thermal conductivity of the insulation material, with adjustments for the thermal resistance of other elements in the construction (substrate, air gaps, surface resistances).
The key values for this calculation are:
- Nexseal LE closed-cell spray foam thermal conductivity: 0.027 W/mK (declared to I.S. EN 14315-1)
- TGD L 2022 target U-values (Republic of Ireland) vary by element type (roof, wall, floor) and by whether the building is new or being renovated; dwellings follow TGD L – Dwellings, and Northern Ireland follows the Technical Booklets under the Building Regulations (Northern Ireland) 2012
The simplified thickness formula is: Thickness (mm) = lambda value × (1 / target U-value − other resistances) × 1,000, where the other resistances are the internal and external surface resistances (about 0.14 m²K/W for a roof and 0.17 m²K/W for a wall) plus any substrate. In practice, a 200mm concrete wall or a steel deck provides minimal thermal resistance, so the spray foam thickness accounts for almost all of the insulation requirement. The figures below are indicative — every specification should be confirmed by a U-value calculation.
Roof Insulation Thickness
| Application | Target U-Value | Spray Foam Thickness | Notes |
|---|---|---|---|
| New-build commercial flat roof | 0.20 W/m²K | ~130mm | Underside of deck; TGD L 2022 Table 1 |
| Commercial flat roof renovation | 0.25 W/m²K | ~105mm | Existing roof retained; Table 10 |
| New pitched roof (between/over rafters) | 0.16 W/m²K | ~165mm | Depends on rafter depth |
| Pitched roof renovation (at slope) | 0.25 W/m²K | ~105mm | Table 10; insulation at rafter level |
| Portal frame roof (over purlins) | 0.16–0.20 new / 0.25 renovation | 105–165mm | Encapsulates purlins; target depends on roof type |
Wall Insulation Thickness
| Application | Target U-Value | Spray Foam Thickness | Notes |
|---|---|---|---|
| New-build commercial walls | 0.21 W/m²K | ~125mm | Internal or external face |
| Commercial wall renovation | 0.35 W/m²K | ~75mm | Internal application typical; Table 10 |
| Portal frame wall (over side rails) | 0.21 W/m²K (new) | ~125mm | Encapsulates side rails |
| Masonry wall (internal lining) | 0.35 W/m²K (renovation) | ~65mm | Masonry provides some resistance |
| Shipping container walls | Comfort target (about 0.40–0.50 W/m²K) | 50–65mm | Applied to corrugated walls |
Floor Insulation Thickness
| Application | Target U-Value | Spray Foam Thickness | Notes |
|---|---|---|---|
| Ground floor (new-build) | 0.21 W/m²K | 80–120mm | Ground and slab contribute; calculate per floor |
| Ground floor (renovation) | Agreed with the designer | 60–100mm | Ground provides some resistance |
| Suspended timber floor | 0.21 W/m²K | ~115–120mm | Applied from underside |
| Car park soffit | 0.21 W/m²K | ~120mm | Requires DC315 coating |
Soffit Insulation Thickness
Soffits — the underside of floors over unheated spaces — are treated as exposed floors under TGD L 2022, with the same 0.21 W/m²K maximum U-value as other floors in new buildings other than dwellings. Common soffit applications include car park soffits (covered in our dedicated guide), soffits over loading bays, and ground floor soffits over ventilated crawl spaces.
Typical soffit insulation thickness is around 120mm for new buildings (0.21 W/m²K on a concrete soffit), and less for renovation work, depending on the target U-value agreed with the building designer.
Marine and Specialist Applications
Marine, agricultural, and specialist applications have different thickness requirements driven by condensation control rather than Building Regulations U-value targets:
| Application | Typical Thickness | Primary Driver |
|---|---|---|
| Steel boat hull / marine cabin | 50–75mm | Condensation control |
| Shipping container | 50–65mm | Thermal comfort + condensation |
| Agricultural building | 35–80mm | Condensation on livestock buildings |
| Cold store | 120–180mm | Temperature maintenance |
Factors That Affect Required Thickness
Several factors can increase or decrease the required spray foam thickness:
- Existing insulation — if the construction already has some insulation (even if degraded), less spray foam may be needed to achieve the target U-value. A site survey will assess the contribution of existing insulation.
- Substrate thermal resistance — a 215mm brick wall provides approximately 0.3 m²K/W of thermal resistance, reducing the spray foam thickness required. A steel wall provides negligible thermal resistance.
- Internal temperature — heated spaces at higher temperatures require more insulation to achieve the same U-value. A warehouse at 15°C needs less insulation than a clean room at 22°C for the same comfort outcome.
- External exposure — buildings in exposed, windy locations, common along Ireland’s Atlantic coast and on upland sites, experience greater wind-driven heat loss and may benefit from thicker insulation than sheltered buildings.
- Condensation risk — in high-humidity environments, the insulation thickness may need to exceed the U-value minimum to maintain internal surface temperatures above the dew point and prevent condensation.
Duratite supplies Nexseal LE closed-cell spray foam (thermal conductivity 0.027 W/mK, declared to I.S. EN 14315-1) and provides free specification support including U-value calculations and thickness recommendations for any application. Contact us on info@duratite.ie.