UPDATED on December 15, 2017 with information on the U-factor alternative.
A roof over a vented, unconditioned attic does not need to include any insulation. However, most cathedral ceilings and low-slope (flat) roofs are insulated roof assemblies: with this kind of roof, the insulation follows the slope of the roof.
Insulated roof assemblies can be vented or unvented. There are lots of different ways to insulate this type of roof, but one of the best methods calls for the installation of rigid foam insulation above the roof sheathing.
There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing:
- Rigid foam above the roof sheathing interrupts thermal bridging through the rafters.
- Rigid foam above the roof sheathing keeps the sheathing warmer and drier than it would be if all the insulation were on the interior side of the roof sheathing.
How much foam do I need?
If you plan to install rigid foam above your roof sheathing, you have two choices:
- Option 1: You can install all of the insulation above the roof sheathing (in which case the rigid foam has to meet minimum code requirements for ceiling R-value); or
- Option 2: You can install some of the insulation above the roof sheathing, and the rest of the insulation underneath the roof sheathing (and in direct contact with the roof sheathing).
If you choose Option 1, your rigid foam will be fairly thick: