Condensation on underside of roof

[smt]

Our house is 19th century and so does not have the modern soffit/guttering arrangement. We had the roof replaced a couple of years ago and breathable roofing felt was used.
Each winter since then, we have experienced condensation on the breathable felt surface – not enough to soak anything beneath, but enough to make the roof timbers in contact with it, wet.
As there is no other form of loft ventilation, it would seem that is the problem, however, we have been advised that tile vents may not be the answer, but rather, it has been suggested that an old-fashioned air vent in our gable end ("as old houses used to have") is the way to go.
It seems to me this isn't an exact science and everyone seems to have different opinions, but I am really keen to discover the definitive solution to our problem.
Any help would be gratefully received.

 

[datarebal]

air in gables are a good idea if you want to avoid vent tiles/slates.. you could also bump the insulation up at ceiling level if it's not up to standard.

 

[smt]

Thanks for your response and I understand your advice.
However, I have also read accounts where, following an increase in insulation, some people have experienced more condensation problems. Who's right? There's only one way to find out, etc.

Will the addition of a gable end vent, create enough ventilation back down at eaves level in your humble opinion?

The reason I am slightly reluctant to commit to this option, is that the building is a three storey old flax mill, which means a significant scaffolding cost, as there is a ground/first floor extension, which means this is the only way of getting to that point in the gable end.

 

[datarebal]

Increasing the insulation should lower the amount of warm air into the loft.. not rocket science really.
However, care should be taken not to block any air flow from eaves area should there be any.

You mention breathable underlay, do you know the brand?

another trick to try would be putting a spacer between felt laps allowing air that has blown by the tiles into the loft, You will be amazed how much draught there is even on a slate roof.

The end of the day you need to increase ventilation be it by Air bricks, vent ridge, vent tiles, over fascia vents (combination of all..)

 

[smt]

I'm afraid I don't know the brand of the underlay – just white on the inside and green on the other (probably not very helpful, sorry).

I will look at the possibility of opening-up the overlaps to increase airflow, but at the same time, am a little concerned about creating a gap for other 'things' to get in. I suppose with it being slate, it is quite flat though.

If I had discreet/flat slate vents fitted in place of some slates, do they need to be fitted in pairs on opposite sides of the roof and is it OK then to pierce the underlay membrane?

Once again, thanks very much for your advice – much appreciated.