Share 'Fix this leakage?'

What sort of leakage reduction could I expect if I try to plug all these tiny gaps? This is a typical cathedral ceiling around these parts, 2x6 tongue and groove over 4x beams. In this image we're looking at a big valley rafter running diagonally in front, with a beam in the background at right. There are a couple of can lights and a couple of skylights in there too. This is after about 30 minut…