One of the best ways to protect your mansard roof from the problems of ventilation is to use a vapor barrier on the.
Insulating a manserd roof.
If you are insulating with air permeable insulation material something like cellulose on the attic floor then my recommendation is to vent the attic.
Through insulating during a conversion to a mansard roof you can lower your heating regularly and enjoy extra usable space through the house extension.
This works everywhere no matter what kind of roof you have what kind of attic you have or where you live.
Step 2 add caulking.
Step 1 add a vapor barrier.
One strategy would be to incorporate the attic into the conditioned space and then use a sandwich of barriers and insulation under a furred vented over roof to control ice damming.
It also has the advantage that you have an attic that s now more than suitable for being turned into a semi finished or finished instead of unfinished space.
Assuming that the attic you describe has a low slope roof you should follow the advice in this article.
Once that is done then venting can be addressed.
One possible improvement could be a 45 soffit on the inside to provide both better insulation and venting to the upper zone.
If you have an existing house with this style then dense pack cellulose or pour in place open.
Insulating low slope residential roofs.
Insulating a mansard roof.
During the process framing you can incorporate the details of installation into the style of roof for quality heat savings.
Achieving a u value of 0 20w m meaning your roof will be better insulated than your walls will need 120mm thickness of rigid foam or 200mm of mineral wool or natural insulation.
Step 3 add foam insulation.
Metal exterior shingles and metal gutters result in a long roof.
An option on a mansard style roof would be a cupola on top for venting.
If you are building a new home with this style of roof then either spray foam either closed or open.