Fucking lol!
Matey tried to fire up his wood burner last night an found a big draft coming down it. The house is so well sealed up that with the extract running the only way in for the air was down the bloody stove pipe through the burner!
So, I've just been over and put four 6" holes through the ceiling of the outer part of the room (ie, where the grow isn't) up into the loft. I stuck a 12" length of plastic sewer pipe into each one.
....
Venting into a loft can be dangerous.... not sure if it was a Viet grow with 50 odd lamps and associated output, but I am certain people have managed to mold everything and also rot all the beams in the roof with the extra humidity that builds up in there. Collapsing roof may be the earliest sign, if your ceilings do not collapse first. Builders/Roofers do not build roof spaces to take much dampness inside, the slates usually keep it outside