We had cavity wall insulation & extra loft insulation installed in November which has made our 22 year old, 3 bed detached house warmer, but during the recent cold weather with still air days, the freezing cold loft space was running with water mainly dripping from the ridge and running down the rafters and bitumen felt. We have a ventilation slot along the full length of the underside of the soffit on either side of the roof, which has not been blocked with the extra lagging. Would lapvent help. I would be able to install them myself. How many would I need and what is the cost please. Many thanks. Andrew Ford.