Expert: Bruce A. Ryan II, RRO - 10/28/2009

Question
We are installing a new roof on our house which has tongue and groove ceilings. We scraped all  the old shingles and cylotex which exposes the T %26 G. We fitted 2 x 10 rafters 2'OC and havae installed kraft faced R30 bat insulation directly on the T&G. We have installed the insulation so the kraft paper is facing the outside toward the shingles. On top of that is the OSB plywood. On the roof, we figured we would get better ventilation with the kraft paper away from the house. Was this the right decision?

Answer
Mel,

I need to assume that your roof sheathing and not the ceiling is t%26g sheathing and that you have sistered 2x10s next to your existing smaller 2x rafters, or replaced them completely.

You generally want to face the Kraft paper toward the 'warm' side of the structure, which in most climates is toward the interior.  The paper is coated with an asphalt film that serves as a vapor retarder.  The integrity of the vapor retarder is usually questionable after the batts are installed, so this may or may not be a problem for you.  The possibility now exists that condensation can form on the Kraft paper on the insulation side of the batts, where it is not subject to the ventilation (drying) effects of your attic air space.  

The problem also is that there really can not be much of an air space between the T%26G and the insulation as the R-30 batts must nearly fill the entire joist space.  You need 1 but preferable 2 inches of clear space between the top of the insulation and the bottom of your roof sheathing for adequate ventilation.

Energy code, Building code, and practical construction often butt heads and we are sometimes forced to choose a winner.

I hope this helps,

Bruce Ryan II, RRO
Professional Roof Consultants, Inc.