Blowfelt

Hi there, I am looking for advice on the amount of inslation I need to cover the I beams that run through my living room. We built a second storey on an existing first floor extension, but because of the 45 degree rule we could not build on the outside wall. Therefore the wall had to come in 900mm.The flat roof is a warm roof and the cavity is also insulated to building regs. I obviously want to stop condensation and cold bridging on the outer most beam and meet regs. Could someone please help me. Thanks in advance, Blowfelt.