Correct way to build an insulated and unventilated rafter space in a cold climate

[retire09]

If you have a rafter with the ceiling on the bottom and roof deck on top fully insulated with no ventilation in a very cold climate; should you have a vapor barrier on the bottom?

If you do have a vapor barrier on the bottom, can you also have foam insulation on the deck under the roof covering?

How do you build a "Hot" roof in a cold climate without creating a moisture problem?

 

[Span]

http://www.buildingscience.com/search?SearchableText=How+do+you+build+a+%22Hot%22+roof+in+a+cold+climate+without+creating+a+moisture+problem%3F

Might provide some helping tips.

 

[Francis Vineyard]

No interior vapor retarders are installed, the required minimum thickness of impermeable insulation against the roof deck acts as the vapor barrier; see 2009 IRC section R806.4

 

[Yankee]

No interior vapor retarders are installed, the required minimum thickness of impermeable insulation against the roof deck acts as the vapor barrier; see 2009 IRC section R806.4

Depends on the product though, they can have a variety of impermeability.

 

[tmurray]

Basically, you need to increase the insulation to a point where ice damming is no longer a concern. This all depends on orientation, temperature, wind, snow, rain ect. It is usually much easier to fur out the roof sheathing above the foam insulation to provide ventilation.

http://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam?topic=doctypes/insights