hope someone can give advice, had new roof with breathable felt fitted and need to insulate 1st b4 vapour barrier etc. roof battens are 4"x2" but insulation is 4". need to make battens deeper so there would be a 2"air gap between roof and insulation. whats the best way to fit new battens... poss another 4"x2" screwed directly onto old ones or "sister" it at side??? any advice. ps i know kingspan would of been better that roll out insulation but found out after