Why Is There Condensation on the Roofing Felt in My Loft?
For droplets or a wet sheen forming on the underside of roofing felt or membrane inside a cold loft, especially during winter or after cold nights.
What this usually means
Condensation on the underside of loft roofing felt usually happens when warm, moisture-laden air from the home reaches the cold roof covering and cools below its dew point. Poor roof-space ventilation can make it worse, but so can moisture leaking into the loft through an unsealed hatch, gaps around pipes or bathroom extract ducting. NHBC guidance says roof spaces need adequate ventilation and warns against blocking eaves ventilation with insulation or stored items. A roof leak is still possible, so the first job is to separate weather-related water entry from widespread condensation.
What may be causing this
Warm humid air is leaking from the rooms below
Clues: Condensation is widespread rather than tied to one roof defect, and the loft hatch, pipe penetrations or ceiling gaps are poorly sealed.
What to do: Reduce moisture transfer from the home by sealing obvious air-leak paths appropriately and using kitchen/bathroom extraction correctly.
The roof space is not ventilating effectively
Clues: Eaves vents are blocked by insulation or storage, or there is little evidence of cross-ventilation through the loft.
What to do: Restore the intended ventilation path without randomly adding vents. The correct solution depends on the roof construction.
A bathroom or other extractor is discharging into the loft
Clues: A flexible duct ends in the roof space or is disconnected, and condensation is worse near that area.
What to do: Extract air should terminate outside through a properly installed duct and external vent, not dump moist air into the loft.
It is a roof leak rather than condensation
Clues: Wetness is concentrated around one penetration, chimney, valley or damaged section and gets worse with rain from a particular direction.
What to do: Repair the roof defect before treating the problem as condensation.
How to check what is happening
Widespread condensation is more likely than one local roof leak.
Temperature-driven condensation is strongly indicated.
Restricted airflow may be allowing moisture to build up in the roof space.
Moist indoor air is being delivered directly into the cold roof space.
Can you fix this yourself?
You can inspect the moisture pattern from a safe boarded area, keep storage away from eaves vents, close the loft hatch properly and check whether extract ducts visibly terminate outside. Changes to roof ventilation, wet insulation, damaged roof coverings or inaccessible ducts should be handled professionally.
- Torch
- Phone camera
- Humidity meter if available
- Safe boarded loft access
What you can safely try
- 1
Decide whether the moisture is widespread or local
Condensation often appears as droplets across broad areas of cold felt. A leak is more likely to create a concentrated wet trail around a defect or penetration.
- 2
Look for warm air getting into the loft
Check whether the loft hatch closes properly, whether obvious ceiling gaps are open and whether bathroom or kitchen extract ducts are intact and actually terminate outside.
- 3
Make sure ventilation paths are not buried
From a safe boarded position, look towards the eaves and make sure insulation or stored items have not blocked the designed airflow route.
- 4
Use extraction properly downstairs
Run kitchen and bathroom extractors during moisture-producing activities and afterwards, and keep internal doors closed where appropriate so less humid air reaches the loft.
- 5
Get persistent or severe condensation assessed
If the felt is dripping heavily, timbers stay wet or ventilation is unclear, a roofer or building professional can check the roof design and distinguish condensation from weather ingress.
Call a roofer if…
- Roof timbers are saturated, mouldy or showing signs of decay.
- Insulation is soaked or water is dripping onto electrical equipment.
- Wetness is concentrated around a chimney, valley, rooflight or damaged covering.
- Inspection requires stepping off safe boarding or climbing onto the roof.
- The condensation remains severe after obvious moisture and ventilation issues are corrected.
Get the right professional.
This problem can have more than one cause. Start with the checks above: a roofer is the usual first route, while a electrician may be more appropriate when the clues point that way.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- Condensation in your roof spaceNHBC
- Ventilation of rooms and roofsBath & North East Somerset Council Building Control
Last reviewed: 31 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.