The short answer
- Your shed drips because warm, moist air inside meets the underside of a single-skin metal or fibre cement roof sheet that is colder than the air's dew point. On clear, cold nights the sheet loses heat to the sky and falls below that temperature, so water condenses and falls.
- Livestock, silage, wet grain and machinery all add moisture. An Irish climate with mild, damp air and cold clear nights makes it worse.
- Ventilation removes moist air but cannot warm the sheet. Keep it. Never block ventilation in a livestock building. On its own, though, it often does not stop dripping from a bare sheet.
- Closed-cell spray foam applied to the underside of the sheet keeps the inner surface warm and seals it from the moist air, which stops the condensation at its source. Self-bonding foam also stiffens the sheets and seals laps without drilling, which matters on asbestos cement roofs.
Why farm sheds rain on the inside
Condensation is simple physics. Air holds water vapour, and warmer air can hold more of it. When moist air touches a surface colder than its dew point, the temperature at which it can no longer hold its moisture, the vapour turns to liquid water on that surface.
A single-skin roof is the perfect condensing surface:
- It is thin and conductive. A steel or fibre cement sheet has almost no insulating value, so its underside sits at nearly the outside temperature.
- It loses heat to the night sky. On clear, still nights, a roof radiates heat upwards and can become colder than the outside air. That is why sheds often drip worst on frosty mornings after a clear night, even when the day was mild.
- The air under it is moist. Cattle and sheep breathe out water vapour. Slurry, wet bedding, silage, fresh grain and wet machinery add more. Mild, damp Irish air brings its own.
Put those together and water forms on the underside of the sheet, runs down the profile and drips from the lowest points: laps, fixings and purlins. It can look like a leaking roof, and many farmers replace sheets or reseal fixings only to find the "leak" is still there.
How to tell condensation from a leak
- Dripping that happens on dry, clear, cold nights, or after a cold night once the sun warms the roof, is condensation.
- Water that appears only during or after rain, in the same places each time, is more likely a leak.
- Condensation often shows as a wet sheen over the whole underside, or droplets hanging along every corrugation.
- Rust streaks along fixings and purlins and black mould on timber purlins suggest long-term condensation.
Many older sheds have both. A survey sorts out which is which before anyone quotes for a fix.
What condensation costs a farm
Livestock sheds
Drips onto animals and bedding keep lying areas wet. Wet bedding means more straw, more work and a damper environment for the animals. Water dripping into feed passages wets the feed. Farmers routinely tell us the drip line along the feed barrier is their first complaint.
Grain stores and feed stores
Condensation dripping onto stored grain creates wet patches in the top of the heap. Wet grain can heat, sprout or spoil, and a store that drips can damage the value of a crop that was dry when it went in.
Machinery sheds and workshops
Tractors, sprayers, balers and tools stored under a dripping roof rust faster, and electrics suffer. A workshop under a bare sheet is also cold and miserable to work in during winter.
The building itself
Constant wetting corrodes steel sheets from the inside, rusts fixings, rots timber purlins and grows mould. The roof fails from the underside long before the weather wears out the top.
Ventilation: essential, but not the whole answer
Good ventilation matters in every agricultural building, and in a livestock building it is essential for animal health. It removes moist, stale air and replaces it with drier outside air. Anyone who suggests sealing up a livestock shed to stop condensation is giving dangerous advice.
Ventilation has a limit, though. It lowers the moisture in the air, but it does not warm the underside of the sheet. On a clear, cold night, a bare sheet can radiate below the outside air temperature. At that point even well-ventilated air can condense on it. That is why sheds with generous open ridges and spaced boarding can still drip.
| Approach | What it does | Limits |
|---|---|---|
| Improve ventilation (open ridge, spaced boarding, outlets) | Removes moist air; essential for livestock | Does not warm the sheet; drips can continue on cold clear nights |
| Anti-condensation fleece on new sheets | Holds a limited amount of moisture on the sheet and releases it later | Factory-applied to new sheets only; can be overwhelmed; does not insulate |
| Board or quilt lining with a vapour control layer | Insulates and separates moist air from the sheet | Needs fixings and framing; joints and laps must be sealed perfectly; hard to clean; can trap moisture if damaged; fixings through asbestos cement are a problem |
| Replace the roof with insulated panels | Warm inner surface, new weathertight roof | Highest cost and disruption; asbestos removal if the old roof is asbestos cement |
| Closed-cell spray foam on the underside | Warms the inner surface, seals it from moist air, seals laps, stiffens sheets | Needs clean, dry sheets and correct application conditions; fire strategy must be considered (see below) |
Shed dripping this winter?
Send a photo of the underside of the roof and tell us what the building is used for. We will arrange a free survey and a fixed price.
How closed-cell spray foam stops condensation
Closed-cell spray foam is sprayed onto the underside of the roof sheets as a liquid. It expands and self-bonds to the sheet, following every corrugation and filling the laps. Once cured it does three jobs at once:
- It insulates the sheet. The foam's inner face stays close to the temperature of the air in the shed, not the cold sky outside, so it stays above the dew point in normal conditions.
- It seals the sheet from moist air. Closed-cell foam has a high resistance to water vapour. Enverge Nexseal LE has a declared water vapour resistance factor of μ 62, and Purios H is at least μ 46. Moist air cannot reach the cold metal, and there is no air gap behind the insulation for water to collect in, as there can be behind a lining.
- It seals and stiffens. Laps, gaps at fixings and small holes are sealed, which cuts draughts and wind-driven rain. The bonded foam also adds rigidity to lightweight sheets.
How thick?
For condensation control on an unheated farm building, the aim is to keep the inner surface warm enough under the building's worst normal conditions, not to reach an office-grade U-value. Typical build-ups are measured in tens of millimetres, applied in passes. Purios H is sprayed at 10–25 mm per pass. A workshop or building that will be heated needs more, and the thickness is then set by the U-value target. See our TGD L 2022 guide. Where the thickness is under 50 mm, Enverge Nexseal 2.0 is the closed-cell grade used; above 50 mm it is Nexseal LE. The survey sets the specification for your shed and the conditions inside it.
Asbestos cement farm roofs
Many older Irish farm buildings have asbestos cement roofs. Closed-cell foam on the underside is particularly suited to them, because it self-bonds, so nothing is drilled or fixed through the sheets. It encapsulates the underside while it insulates. Because it is work on an asbestos-containing material, it follows the asbestos survey, risk assessment and HSA permit process. Our asbestos cement roofs guide explains that process step by step.
By building type: what to expect
Cattle and sheep sheds
Foam goes to the underside of the roof sheets and, where drips come from them, to the upper wall sheets. Ventilation openings, open ridges and spaced boarding stay open. Before spraying, the building is cleared of animals in the work area, and surfaces are cleaned and dried. The installer plans access around your stock and workload, and the survey agrees the programme with you.
Grain and feed stores
Foam over the store area stops drips onto the crop. It is best done when the store is empty, so the floor and walls can be protected and the building ventilated during and after application.
Machinery sheds and farm workshops
These often benefit from a thicker build-up that makes a working space warmer as well as dry. Where a workshop is heated, the foam thickness becomes an energy decision as well as a condensation one.
Curved and older roofs
Spray foam follows curved, corrugated and irregular profiles that boards and panels struggle with. Nissen huts and arched stores are good candidates, provided the sheets are sound enough to take the foam.
Upgrading a pig or poultry house? Our farm shed insulation and TAMS 3 guide explains where roof and wall insulation appears in the TAMS 3 scheme documents, and what to confirm before you buy anything.
Fire safety, preparation and what happens on the day
Fire
Spray foam is a combustible organic insulant, like other plastic insulations. Keep it away from heat sources: no hot works near foam, and keep lamps, heaters and exhausts at a safe distance. Where a farm building is also a workplace or will be heated or occupied, the fire strategy under TGD B – Fire Safety and the product's I.S. EN 13501-1 classification must be considered. We supply the product data so your adviser or insurer can confirm whether a protective lining is needed in any area.
Preparation
- the work area is cleared of animals, feed, grain and anything that must not be oversprayed
- sheets must be clean and dry, with loose rust, dust and cobwebs removed
- walls, floors, windows and fittings below are masked
- the building is ventilated during application and cure, and people and animals stay out until the installer says it is safe to return
On the day
The installer builds the foam up in passes to the specified thickness and checks it as they go. You receive the installation record, product data and the manufacturer-backed product warranty documents at handover.
We cover farms across the Republic through approved installers in each county, including County Cavan, County Monaghan, County Tipperary, County Cork and County Mayo.