On the underside of the felt it will be condensation.
If you are sure you have a good through=flow of ventilation (check that it has not been blocked by flopping insulation) then you need to look at where the humidity is coming from.
Some possibilities:
- Hot water in the loft tanks due to pumping-over or other plumbing fault
- No cover on water tanks in loft
- Steam rising from bathroom due to not using an extractor fan during and after baths and showers
- Excessive humidity in house due to leaving wet washing about the house or
draping it over radiators
- Bathroom ceiling cracked or gappy allowing steam to rise into loft (especially around holes for light fittings, pipe and switches)
- Incorrectly fitted bathroom ventilator or extractor venting into loft
- Badly fitting loft-hatch allowing excessive amounts of warm moist air to rise through gaps.
- Use of "condenser" tumble drier. Most of these allow a lot of water vapour to escape
- Use of vented tumble drier with no hose going outside; or faulty hose
- Use of paraffin heaters or gas fires that do not have a flue
Water vapour is lighter than air, so it rises through the house even more than warm air does. This is why clouds are above us.