The short answer
- For an existing single-skin metal or fibre cement warehouse, closed-cell spray foam applied to the underside of the sheets is usually the quickest and least disruptive way to insulate. It seals air leaks and stops condensation in the same layer, with no framing or fixings through the sheets.
- Rigid board gives a factory-controlled thickness and suits new or re-roofed buildings, or lined roofs. On an existing profiled roof it needs a support system and careful sealing of every joint.
- Mineral wool is often chosen where a non-combustible insulant is required by the fire strategy. It needs support, a liner and a well-sealed vapour control layer, and it must be kept dry.
- The target U-value decides the thickness. For an existing roof insulated on the slope, TGD L 2022 Table 10 gives 0.25 W/m²K, which is roughly 86 mm of Purios H foam before thermal bridging.
Why existing warehouses are hard to insulate
A large share of Ireland's warehouses, distribution sheds, workshops and industrial units were built with single-skin profiled steel or fibre cement roofs and walls. That was fine for cold storage of goods. It is not fine once the building is heated, re-let to a business with staff working inside, or used for something that dislikes damp and temperature swings.
These buildings share the same problems:
- Heat loss through the roof. The roof is usually the largest part of the envelope, and a bare sheet barely insulates at all.
- Air leakage. Laps, eaves, ridges, verges and shutter doors let heated air out and cold air in.
- Condensation. Moist air meets cold sheets and drips on stock, racking, machinery and people, especially on cold, clear nights.
- Overheating. In summer the same bare roof turns the building into an oven under the sun.
- Business in the building. The racking, stock and operations cannot stop for a long re-roof.
Any insulation choice has to be judged against all five, not just the U-value on a data sheet.
The three options explained
1. Closed-cell spray foam
Two-component polyurethane foam is sprayed onto the underside of the existing sheets, where it expands and self-bonds. It follows every corrugation and fills the laps. Duratite Ireland supplies Purios H (λ 0.0224 W/m·K, 35–40 kg/m³, at least 90% closed cell, compressive strength of at least 200 kPa, Euroclass E) and Enverge Nexseal LE (typical λ 0.027 W/m·K at 125 mm, μ 62, 78 kPa). Both are CE marked with published data sheets. No frame is needed, nothing is fixed through the sheet, and the insulation, air seal and condensation control are one layer.
2. Rigid board (typically PIR or phenolic)
Factory-made boards are fixed to a support frame or liner beneath the roof, or built into a new roof above a liner sheet during re-roofing. Boards give an accurate, consistent thickness and good thermal performance per millimetre. Under an existing profiled roof, the boards cannot follow the profile, so the voids at the corrugations and eaves must be closed separately. Every board joint must be taped or sealed to keep air and moisture out.
3. Mineral wool (glass or stone wool)
Quilt or slab wool is supported on a liner or mesh, usually with a vapour control layer on the warm side and a liner sheet to finish. Designers often choose mineral wool where the fire strategy calls for a non-combustible insulant. It needs more thickness than foam or board for the same U-value. It must be supported so it does not sag, and it must be kept dry, because wet wool performs poorly and can hold moisture against the steel.
Side-by-side comparison for an existing building
| Closed-cell spray foam | Rigid board | Mineral wool | |
|---|---|---|---|
| Installed on existing roof without re-roofing | Yes, directly to underside of sheets | Yes, but needs a support frame or liner | Yes, but needs support, liner and vapour control layer |
| Fixings through the existing sheet | None, self-bonding | Usually, for framing or liners | Usually, for support and liners |
| Follows profiled sheets | Yes, fills corrugations and laps | No, voids to close separately | Partly, voids to close separately |
| Airtightness | Seals laps and gaps as it insulates | Depends on taped joints and separate seals | Depends on vapour control layer and seals |
| Condensation on the sheet | Prevents it: no cold, ventilated void against the sheet | Controlled if vapour control is continuous | Controlled if vapour control is continuous; wool must stay dry |
| Thickness for a given U-value | Low (λ 0.0224–0.027 for our closed-cell grades) | Low | Higher |
| Adds stiffness to sheets | Yes | No | No |
| Fire | Combustible organic insulant; fire strategy decides if a lining is needed | Combustible; fire strategy decides | Often specified where a non-combustible insulant is required |
| Disruption | Low: no framing; area-by-area working | Medium: framing and fixing at height | Medium: support, liner and membrane at height |
| Future roof inspection | Underside is covered; inspection from above | Liner covers underside | Liner covers underside |
General comparison for insulating existing single-skin roofs from below. A new-build or full re-roof changes the picture, because insulated panels or built-up systems become straightforward options.
No insulant wins every row. Spray foam is strongest where the building is existing, profiled, leaky, damp and in use. Board and wool are strongest where a new roof is being built anyway, or where the fire strategy requires a non-combustible insulant.
Compare the options on your building
Tell us the roof type, the floor area and how the building is used. We will arrange a free survey and a fixed price.
Hitting the TGD L 2022 U-values
For existing buildings, TGD L 2022 Table 10 gives 0.25 W/m²K for a pitched roof insulated on the slope, 0.25 W/m²K for a flat roof and 0.35 W/m²K for other walls. Warehouses almost never have a ceiling, so the "on the slope" figure is the usual roof target. For a new building, Table 1 sets area-weighted averages of 0.16 for pitched roofs, 0.20 for flat roofs and 0.21 for walls.
| Target | Purios H (λ 0.0224) | Enverge Nexseal LE (λ 0.027) | Purios F open cell (λD 0.036) |
|---|---|---|---|
| Roof 0.25 W/m²K | about 86 mm | about 104 mm | about 139 mm |
| Walls 0.35 W/m²K | about 60 mm | about 73 mm | about 97 mm |
| New pitched roof 0.16 W/m²K | about 137 mm | about 165 mm | about 220 mm |
Indicative foam-layer thickness only, using surface resistances of 0.14 (roofs) and 0.17 (walls). The sheet, purlins, fixings and thermal bridging are excluded; your designer calculates the finished element.
Open-cell foam is included for comparison. It is lighter and more vapour-open, and it needs more thickness. On a single-skin metal roof, closed cell is normally the right choice because it also controls condensation at the sheet. Our open cell vs closed cell guide explains why.
Insulating a large roof can count as a major renovation if more than 25% of the building envelope is renovated. That brings in a requirement to upgrade the whole building to the cost-optimal level where feasible. Ask your designer before you fix the scope. Our TGD L 2022 guide covers the triggers in detail.
Fire safety: the question to settle first
Fire performance is the one area where the choice of insulant can be made for you. TGD B – Fire Safety sets out where materials of each reaction-to-fire class (to I.S. EN 13501-1) may be used, and when a combustible insulant must be protected by a lining or thermal barrier. The answer depends on:
- the use of the building: storage, industrial, retail, assembly
- the size and compartmentation of the building
- what is stored, and the fire load
- where the insulation is and whether it is exposed
- the requirements of your insurer
Settle this with your designer, fire consultant or insurer before choosing a product, and get the outcome in writing. If a lining is required over spray foam in some areas, it goes into the specification and the fixed price. It is not discovered after the job. We supply each product's declared classification and data sheets to support the decision. We will not tell you that foam can be left exposed without that review.
Five mistakes to avoid when insulating a warehouse
- Buying thickness instead of a U-value. "100 mm of insulation" is not a specification. Ask your designer for the U-value each element must reach, then compare quotes on that basis. The same thickness of two different products can give very different results.
- Insulating over a roof that leaks. Any insulation installed under active leaks gets wet. Foam, board and wool all suffer, and wet wool suffers most. Find and fix the leaks first, from above if the covering has reached the end of its life.
- Ignoring the eaves and the base of the walls. A beautifully insulated roof with open eaves gaps and a leaky wall base still loses heat and lets in draughts. Make sure the specification covers the junctions, not just the flat areas.
- Leaving the fire question until the end. If a lining is needed over the insulation in some areas, it changes the cost and the programme. Settle it with your advisers before you choose a product.
- Forgetting the paperwork. Without an installation record, product data sheets and Declarations of Performance, your BER assessor and certifier may have to fall back on defaults. Ask for the handover pack in the specification, so it is part of the price rather than an afterthought.
Doing the work while the business keeps running
Insulating an occupied warehouse is as much about logistics as insulation.
- Survey. An approved installer measures the roof and walls, including profile allowances, and inspects the sheets, fixings, rooflights, gutters and any asbestos cement. They also note racking heights, access routes and working hours.
- Specification and fixed price. Product, thickness per element, areas, protection and access method, and a programme, with one fixed price.
- Phasing. The building is worked in zones. Stock and racking in the zone are protected or moved, and the zone is kept clear during spraying and curing. The rest of the building keeps working.
- Access. Mobile elevating work platforms work from the floor, so there is no need to walk on fragile roofs.
- Handover. Installation record, thickness checks, product data sheets, Declarations of Performance and manufacturer-backed product warranty documents, ready for your BER assessor and certifier.
If the roof also leaks, fix that first. Foam on the underside is not a waterproofing layer for a roof with holes in it. Our roof leaks guide covers coating a roof from above, and our BER guide explains how the upgrade shows up in your rating.
We cover commercial buildings across the Republic, including business parks in Dublin, Cork, Limerick, Drogheda and Athlone.