The Modular Building Insulation Challenge
Modular buildings — prefabricated structures manufactured off-site and delivered to location — are an increasingly important part of construction across Ireland. From temporary site welfare cabins and portable classrooms to permanent modular healthcare facilities and residential units, the sector covers a wide range of building types and permanence levels. However, thermal performance remains a persistent weakness across the modular building sector, with many units delivering insulation performance well below current Building Regulations standards.
The fundamental challenge is that modular buildings are typically constructed with thin wall, floor, and roof panels — 50–100mm total panel thickness — that limit the depth available for insulation. Combined with the thin steel or aluminium skins that form the structural panels, the result is a building with minimal thermal mass, significant cold bridging at panel frames and joints, and insulation that is often compressed, poorly fitted, or degraded from repeated transport and relocation.
Types of Modular Buildings
Site welfare units — the most common modular buildings on construction sites across Ireland. Drying rooms, canteens, offices, and toilet blocks provided as portable cabins, typically on a short-term hire basis. Insulation quality varies enormously: some hire fleet units have adequate factory-fitted insulation, while others — particularly older units — have minimal or degraded insulation that results in high heating costs, condensation, and poor comfort.
Modular classrooms — used by schools to provide temporary or semi-permanent additional teaching space. These units may remain on site for years or decades. Because they often stay in use for many years, schools reasonably expect them to provide the same comfort, ventilation, and acoustic conditions as permanent classrooms — but many modular classrooms fall short of current Part L insulation standards.
Healthcare pods — modular clinical rooms, consulting rooms, and diagnostic units deployed in hospital grounds or community settings. These require precise temperature control, acoustic privacy, and infection control — all of which are compromised by poor insulation.
Container-based modular — ISO shipping containers converted into offices, retail units, studios, workshops, and residential units. Container-based modular is growing rapidly, but the steel container shell has no inherent thermal performance and requires comprehensive insulation to be habitable.
Why Factory-Fitted Insulation Falls Short
Most modular buildings are factory-insulated with rigid board or mineral wool fitted between the panel framing members. This factory insulation has several limitations:
- Thermal bridges at framing — steel or timber framing members within the panels create thermal bridges at every stud, rail, and nogging position. In a typical modular panel with 50mm studs at 600mm centres, the framing accounts for 10–15% of the panel area — all of which is uninsulated.
- Thin panels — modular panel depths of 50–100mm limit the insulation thickness achievable. Even with high-performance PIR insulation at 0.022 W/mK, a 50mm deep panel can only achieve a U-value of approximately 0.40 W/m²K — far short of the 0.21 W/m²K maximum set by TGD L 2022 for walls in new buildings other than dwellings.
- Transport damage — insulation within modular panels can be compressed, displaced, or water-damaged during transport to site, particularly on multi-use hire fleet units that are relocated repeatedly. The panels are not typically opened for inspection after each move.
- Joint sealing — the joints between modular panels are sealed with mastic and cover strips. Over time, mastic degrades, cover strips loosen, and air gaps develop at panel joints — creating draughts and condensation paths that the original insulation cannot address.
Spray Foam for Modular Upgrades
Spray foam provides a rapid, effective solution for upgrading the thermal performance of existing modular buildings:
Internal wall upgrade: 25–50mm of closed-cell spray foam applied to the internal face of modular panel walls. The foam bonds directly to the panel surface, fills all gaps at framing members, seals panel joints, and provides an additional insulation layer that eliminates the thermal bridges in the factory-fitted insulation. A 25mm upgrade reduces the wall U-value by approximately 40%; a 50mm upgrade by approximately 55%.
Roof upgrade: spray foam applied to the underside of the modular roof panel. This is the highest-priority upgrade because roof heat loss is typically the largest contributor to modular building energy consumption. 50–75mm of spray foam on the roof underside can halve the roof U-value.
Floor upgrade: spray foam applied to the underside of the floor panel (from outside, with the unit raised on jacks or blocks). This is particularly effective for units sited on open ground or raised on stilts, where the floor is exposed to wind-driven cooling.
Container-Based Modular Buildings
Shipping container conversions are one of the fastest-growing segments of the modular building market. The steel container shell has essentially zero insulation value (steel thermal conductivity: 50 W/mK), and the corrugated wall profile makes traditional insulation installation difficult and wasteful of the already limited internal space.
Spray foam is the ideal insulation solution for container conversions because:
- It conforms precisely to the corrugated wall profile, filling every trough and coating every surface — rigid board can only be fixed to the inner face of the corrugations, leaving the troughs uninsulated
- It bonds directly to the steel, providing corrosion protection for the internal surfaces
- At 50–65mm thickness, it achieves wall U-values of around 0.40–0.50 W/m²K, with greater thickness specified where a unit must meet Building Regulations targets
- It provides an inherent vapour barrier, eliminating the condensation that plagues uninsulated or poorly insulated container buildings
For a standard 20ft container, spray foam insulation (walls, ceiling, and end walls) typically requires 1–2 days of application. A 40ft container requires 2–3 days. See our shipping container insulation page for more detail.
Building Regulations for Modular Structures
Whether the Building Regulations apply to a temporary or relocatable modular unit depends on its use and how long it will remain in place, so owners should take advice from their designer or the local Building Control Authority. Modular buildings intended for medium- or long-term use — modular classrooms, healthcare facilities, and permanent offices — should be treated like any other building and designed to meet Part L.
For new buildings other than dwellings, TGD L 2022 sets the same maximum U-values for modular buildings as for any other building type: walls 0.21 W/m²K, flat roofs 0.20 W/m²K, pitched roofs 0.16 W/m²K, and ground floors 0.21 W/m²K. For existing buildings being upgraded, the renovation values in Table 10 are more relaxed (for example 0.35 W/m²K for walls and 0.25 W/m²K for flat roofs) but still require significant improvement over the typical uninsulated or factory-insulated condition. In Northern Ireland, Technical Booklet F2 applies.
Speed Advantage
The speed of spray foam installation is a particular advantage for modular buildings, which are often deployed on tight timescales:
- A single portacabin (6m × 3m) can be fully insulated (walls, roof, floor) in 1 day
- A modular classroom block (four linked units) in 2–3 days
- A 20ft shipping container in 1–2 days
- A modular office complex (10+ units) in 5–8 days
This speed means that insulation upgrades can be completed during weekend or holiday shutdowns, site mobilisation periods, or between unit relocations — without impacting the primary use of the building.
Duratite supplies Nexseal LE closed-cell spray foam (0.027 W/mK, declared to I.S. EN 14315-1) for modular building insulation across Ireland and Northern Ireland. Contact us on info@duratite.ie for a quotation.