condensation in loft?

[Newby2]

In Jan. 2014 when I got into the loft, I noticed that the underside felt along the two valleys of my roof were wet (with droplets). At that point I thought it was rain driven underneath the valley through the mortar cracks by strong winds (we had some extreme weather last winter). Then I have monitored the situation continuously. For the past 10 months the felt is dry even after some heavy rains.

Yesterday morning we had pretty heavy rain and when I checked around noon the felt was dry. The rain died down since yesterday afternoon. However when I checked again this morning, the felt along the two valleys were wet again (1st time in 10 months, and yet no heavy rain last night)! I did notice this morning that there were lots of water on bedroom windows due to condensation so I wonder if the wet felt is also due to condensation? But in the loft the felt is only wet near the valleys (about 1 meter along the valley rafter near the top) and felt in other parts are dry.

Are valleys the coldest part in the roof? And warm air gets into the loft via hatch and condensate on the underside felt along the valley? Anyone can shed some lights on this? Thanks a lot.

 

[Alastairreid]

Almost certainly condensation! the underside of the valleys will be colder than the rest of the roof as there is no air gap between the valley and felt.

What type of ventilation do you have in your loft? it should be airy and cold.

 

[Newby2]

thanks a lot for your reply. I don't know what kind of ventilation it is, but just see there is a big silver coloured vent pipe there. the house was built in early 90s. The wet patch of the felt is about 1 meter long (along the valley rafter) and 1 or 2 cm wide. the loft is indeed cold in winter and very warm in summer.