i brought my tiled loft of a fancier who packed up he had it felted and tiles on top, the felt was at least half inch thick of dust as it had no escape, the loft had only been erected 3 years earlier. when we took the tiles off the top it was like a fire had been started there was dust everywhere, when i put it back up at my garden i did not bother felting it i just put the tiles straight to the battens screwing about 20% of them through the small holes at the top of the tile. my ventilation now works better than ever, i have seen the new breathable felt usually blue in colour and would think that is liable to clog up as well. my recommendation would be to go without the felt saving you both money and time, ironically the fancier i purchased it off packed up due to pigeon lung. hope this info is of use.