Walk into a UK warehouse, hangar or industrial unit on a cold January morning and the metal roof is dripping. Stock gets wet, lighting shorts, racking corrodes, slip risk on the floor — and it’s not a roof leak. It’s condensation, and on a profiled steel roof it can drip just as much water as a real leak.
Condensation isn’t fixed by re-roofing. It’s fixed by changing the temperature of the inside surface of the roof so it stays above the dew point of the air below. This article explains why metal roofs condensate worse than other constructions, what the BS 5250 risk classification looks like, and the single specification — closed-cell spray foam at the right thickness — that permanently solves it.
Why metal roofs drip
A profiled steel commercial roof has near-zero R-value on its own — typical 0.1 mm steel sheet contributes essentially nothing to thermal resistance. The internal surface temperature of the steel is therefore very close to the outside air temperature, not the inside.
When warm, moist internal air contacts the cold steel underside, the air at the steel surface cools below its dew point and water drops out as condensation. On a UK winter morning with:
- Outside air −2 °C
- Inside air 18 °C, 60 % RH (typical heated warehouse)
- Internal dew point: ~10 °C
- Internal steel surface temperature (uninsulated): ~−1 °C
The steel surface is 11 °C below the dew point of the inside air. Water condenses on every square metre of the steel and runs down the corrugations. On a 5,000 m² roof at this differential, you get liters per hour of water dropping onto stock and floor.
The fix is to raise the internal surface temperature of the roof above the dew point of the inside air. Below 5 °C of margin, you still get intermittent condensation in cold snaps; below 0 °C of margin you have permanent dripping.
The BS 5250 condensation risk classes
BS 5250 (control of moisture in buildings) classifies internal humidity into four bands:
| Class | Internal humidity | Building examples |
|---|---|---|
| 1 | Low (< 50 % RH winter) | Warehouses without people, dry storage |
| 2 | Moderate (50–60 %) | Most heated commercial premises, retail, light industrial |
| 3 | High (60–70 %) | Catering, food processing, swimming pools (low end), commercial kitchens |
| 4 | Very high (> 70 %) | Swimming pools, paper mills, breweries, wet processes |
For each class, BS 5250 defines a target internal-surface-temperature factor (f-factor) that the roof has to meet to keep mould-growth risk low. Higher humidity classes need higher f-factors and therefore better-insulated roofs.
A profiled steel roof with no insulation at all has an f-factor of essentially 0 — it always condenses except in summer. A roof with 140 mm of closed-cell spray foam has an f-factor approaching 0.99 — internal surface temperature within 0.5 °C of room temperature, no condensation regardless of humidity class.
What doesn’t fix it (common failed attempts)
Specifiers and FMs facing a condensating warehouse often try the following first — none of them are permanent solutions:
1. Anti-condensation paint
Sold as a “membrane that absorbs and releases moisture”, anti-condensation coatings provide a buffer of a few hours. In a heated warehouse with constant humidity load, the buffer saturates by mid-morning and the coating starts dripping like the bare steel beneath. Useful only in low-humidity, intermittently-heated buildings — not solving the underlying physics.
2. Improved ventilation
Adding ridge vents and eave vents can reduce humidity by exchanging warm moist internal air for cold outside air. Two problems:
- The cold outside air drops the internal temperature, often below comfort limits — defeats the purpose of heating
- In high-humidity occupations (food, manufacturing) the moisture load is constant and ventilation alone can’t keep up
Ventilation is supplementary, not a primary solution.
3. Dehumidification
Industrial dehumidifiers can lower internal RH below the dew point against the steel surface. Effective in small spaces but in a 5,000 m² warehouse the running cost (electricity + maintenance) over 10 years is typically multiples of the cost of insulating the roof properly.
4. Quilt insulation laid on top of suspended ceilings
A common 1980s spec — laying mineral wool quilt over a suspended ceiling. The roof void above the ceiling stays cold and condenses just as before. The dripping moves from “onto the floor” to “into the ceiling tiles” — out of sight but corroding the ceiling grid and supporting structure. Worse than nothing in many cases.
5. Drip trays / catch trays
A genuine engineering solution for some specialist environments (e.g. data centres) but expensive, ugly, and require ongoing maintenance. They don’t stop condensation, just collect it.
The permanent fix: closed-cell spray foam
The only specification that permanently eliminates condensation on a profiled steel commercial roof is to insulate the underside of the steel with closed-cell spray foam at sufficient thickness to keep the internal surface temperature above the dew point of the worst-case occupied conditions.
Why closed-cell specifically:
- High λ (low conductivity): 0.022 W/m·K means thin enough to fit under existing services
- Vapour-tight: the foam itself is the vapour barrier — no moisture migration through to the cold steel
- Bonds to substrate: no air gap behind the foam where condensation could form on the cold steel below
- Conforms to corrugations: fills every rib and trough completely, no cold spots
Mineral wool fails on three of these four. PIR fails on the conformability and the air-gap-behind-board issue. Open-cell foam fails on vapour permeability — it lets moisture through to the steel.
How thick to specify
The thickness is decided by the worst-case dew-point margin for the building’s humidity class.
| Humidity class | Internal conditions | Target spray foam thickness | Approx. internal surface temp at -2 °C external |
|---|---|---|---|
| Class 1 (low — dry storage) | 18 °C / 50 % RH, dew pt 7 °C | 50 mm | ~12 °C — 5 °C above dew point |
| Class 2 (moderate — heated commercial) | 18 °C / 60 % RH, dew pt 10 °C | 75 mm | ~14 °C — 4 °C above dew point |
| Class 3 (high — catering / food) | 20 °C / 65 % RH, dew pt 13 °C | 100 mm | ~16 °C — 3 °C above dew point |
| Class 4 (very high — pools / wet processes) | 22 °C / 75 % RH, dew pt 17 °C | 140 mm | ~19 °C — 2 °C above dew point |
For most UK warehouses the controlling factor isn’t condensation alone — it’s the U-value target from Approved Document L (typically 0.18 or 0.15). A roof insulated to those U-values uses 115–140 mm of closed-cell spray foam anyway, which is well above the condensation-control minimum for any UK humidity class. The condensation problem is solved as a by-product of meeting the energy regulations.
For unheated buildings (cold storage at 4 °C, agricultural sheds at 8 °C) the calculation is different — the inside is colder, dew points lower, and the condensation issue can be milder. Even so, 50 mm minimum of closed-cell foam is the rule of thumb for any UK metal roof to eliminate condensation reliably.
Hangar-specific notes
Aircraft hangars present extra challenges: large air volume, ventilation rates that change rapidly when doors open, occasional very high local humidity from refuelling and washing operations.
Specific hangar considerations:
- Spec at 100 mm minimum even where Approved Document L would allow less — hangar humidity swings are larger than typical warehouses
- Run the foam to the door soffit with a continuous bead — gaps at the door head are the most common condensation hot-spot
- Avoid vapour-permeable insulation — moisture from de-icing, refuelling and aircraft cleaning will saturate any quilt-type product
For a focused hangar guide see our Hangar insulation page.
Cold-storage and food-processing notes
For chilled and frozen warehouses, the condensation risk reverses — moisture comes from the warm outside air and condenses on the cold inside surfaces. Closed-cell spray foam is still the answer, but the application is to the outside of the cold envelope (or sandwich-panel construction with foam in the panel). For chilled-room conversions to existing buildings, internal foam application creates a thermal break and prevents the structure from cooling below dew point.
Application logistics
Spray foam application to an occupied commercial warehouse roof is the same logistics challenge as any other commercial spray foam install. See our Insulating a Warehouse While Operational guide for the full phasing playbook.
A 2,000 m² warehouse roof, condensation-cure spec (typically 75–100 mm closed-cell foam) installs in 2–3 days by a single crew, weekend windows feasible for 24/7 operations.
Frequently asked questions
Q: How thick does spray foam need to be to stop condensation?
A: For a typical UK heated warehouse (Class 2 humidity, 18 °C / 60 % RH internally), 75 mm of closed-cell spray foam stops dripping. For higher humidity uses (kitchens, food processing) increase to 100–140 mm. For most UK projects the U-value target from ADL2 drives the spec to 115–140 mm anyway, which fixes condensation as a by-product.
Q: Can I retrofit spray foam to an existing metal roof without re-roofing?
A: Yes — closed-cell spray foam is applied to the internal underside of the existing profiled steel sheet. No need to strip the outer roof. The application typically takes 2–3 days for a 2,000 m² warehouse, can be phased to keep operations running.
Q: What about condensation on the outside of the steel sheet?
A: External condensation (dew on the outer surface in the morning) is normal weather behaviour and not problematic — it evaporates as the sun rises. The damaging condensation is internal — water on the underside dripping onto stock — which is what the spray foam application eliminates.
Q: Can I use anti-condensation paint as a cheaper alternative?
A: Anti-condensation coatings provide a temporary moisture buffer but saturate within hours in any heated UK warehouse with ongoing humidity load. They’re a domestic / agricultural specialty, not a commercial solution. The cost difference vs spray foam is recouped within 1–2 winters of avoided stock damage and corrosion.
Q: Does spray foam work on asbestos-cement roofs as well as metal?
A: Yes — closed-cell spray foam encapsulates asbestos-cement profiled sheets while solving the condensation issue. Specialist application — see asbestos encapsulation for the full process.
Q: Will the foam cause the steel sheet to rust?
A: No. Closed-cell spray foam is hydrophobic and forms a vapour-tight bond against the steel. The sheet is protected from internal moisture, which actually extends its service life. The most common rust failure on uninsulated profiled steel is from internal condensation — the spray foam stops the cause.
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