I have recently built a new second storey extension to my house , previously a flat roofed property .The pitched roof has 200mm. x 75mm. rafters ,bird mouthed onto new steel purlins , which will allow insulation to a depth of 150mm. , with a space of 50mm. for ventilation through permeable roof felt .My question is this ? Is the new type , bubble silver backed foil , better than insulation board ?