I live in a 1½ story 1930s house. The slanted roof is insulated with sawdust. No air gap.
The roof boards are now so bad that they need to be replaced. The idea was to create an air gap so that the problem doesn't recur.
Now I found a diffusion-open underlay felt. If it works as they say, no air gap is needed.
http://www.monier.se/produkter/vaara-produkter/productcategory/underlagstak.html
The advantage is that I save some work with the air gap. And the insulation retains the old thickness. The disadvantage is that I don't quite trust "new" products.
Do you have experience with diffusion-open underlay felt? Feedback is greatly appreciated.
/Lars
