Loft Humidity

During the really cold weather in early December 2023, the relative humidity in my loft was 100% and condensation was literally dripping from the roofing felt onto the loft insulation and onto the odds and ends I have stored up there.

I wondered about improving the soffit ventilation but a local roofing firm said it seemed fine and suggested that I fit some felt-lap vents. I’d never heard of them before but I ordered some from Amazon and they arrived next day.

Videos on YouTube recommend fitting some close to loft “floor” (just above the insulation) and some close to the top, near the ridge tiles. The videos recommend scattering them randomly rather than fitting them in straight lines facing each other so as to create a bit of turbulance and avoid air just entering via one vent and leaving by the one closest. I ended up fitting 20 throughout the loft. I ordered 10 each of slightly different designs in case one works better than the other.

I’ve fitted one of my hobby project temperature/humidity transmitters up there so I can monitor the humidity. Although the temperature has been relatively high the last week or two, the humidity seems to be holding better at around 78 – 80%. Without the vents, the humidity was more-or-less stuck at 100% regardless of the temperature. It has been quite windy too which probably helps so time will tell if the felt-lap vents have improved things.

Click graph for larger image.